1.1. Why this document was written

Well, I found a big empty space in the HOWTO's out there lacking in information about the iptables and Netfilter functions in the new Linux 2.4.x kernels. Among other things, I'm going to try to answer questions that some might have about the new possibilities like state matching. Most of this will be illustrated with an example rc.firewall.txt file that you can use in your /etc/rc.d/ scripts. Yes, this file was originally based upon the masquerading HOWTO for those of you who recognize it.

Also, there's a small script that I wrote just in case you screw up as much as I did during the configuration available as rc.flush-iptables.txt.

1.2. How it was written

I originally wrote this as a very small tutorial for boingworld.com, which was an Amiga/Linux/General newssite that a small group of people, including me, ran a couple of years back. Due to the fantastic amount of readers and comments that I got from it, I continued to write on it. The original version was approximately 10-15 A4 pages in printed version and has since been growing slowly but steadily. A huge amount of people has helped me out, spellchecking, bug corrections, etc. At the time of writing this, the http://iptables-tutorial.frozentux.net/ site has had over 600.000 unique hits alone.

This document was written to guide you through the setup process step by step and hopefully help you to understand some more about the iptables package. I have based most of the stuff here on the example rc.firewall file, since I found that example to be a good way to learn how to use iptables. I decided to just follow the basic chain structure and from there walk through each and one of the chains traversed and explain how the script works. That way the tutorial is a little bit harder to follow, though this way is more logical. Whenever you find something that's hard to understand, just come back to this tutorial.

1.3. Terms used in this document

This document contains a few terms that may need more detailed explanations before you read them. This section will try to cover the most obvious ones and how I have chosen to use them within this document.

Connection - This is generally referred to in this document as a series of packets relating to each other. These packets refer to each other as an established kind of connection. A connection is in another word a series of exchanged packets. In TCP, this mainly means establishing a connection via the 3-way handshake, and then this is considered a connection until the release handshake.

DNAT - Destination Network Address Translation. DNAT refers to the technique of translating the Destination IP address of a packet, or to change it simply put. This is used together with SNAT to allow several hosts to share a single Internet routable IP address, and to still provide Server Services. This is normally done by assigning different ports with an Internet routable IP address, and then tell the Linux router where to send the traffic.

Kernel space - This is more or less the opposite of User space. This implies the actions that take place within the kernel, and not outside of the kernel.

Stream - This term refers to a connection that sends and receives packets that are related to each other in some fashion. Basically, I have used this term for any kind of connection that sends two or more packets in both directions. In TCP this may mean a connection that sends a SYN and then replies with an SYN/ACK, but it may also mean a connection that sends a SYN and then replies with an ICMP Host unreachable. In other words, I use this term very loosely.

SNAT - Source Network Address Translation. This refers to the techniques used to translate one source address to another in a packet. This is used to make it possible for several hosts to share a single Internet routable IP address, since there is currently a shortage of available IP addresses in IPv4 (IPv6 will solve this).

State - This term refers to which state the packet is in, either according to RFC 793 - Transmission Control Protocol or according to userside states used in Netfilter/iptables. Note that the used states internally, and externally, do not fully follow the RFC 793 specification fully. The main reason is that Netfilter has to make several assumptions about the connections and packets.

User space - With this term I mean everything and anything that takes place outside the kernel. For example, invoking iptables -h takes place outside the kernel, while iptables -A FORWARD -p tcp -j ACCEPT takes place (partially) within the kernel, since a new rule is added to the ruleset.

Userland - See User space.

Packet - A singular unit sent over a network, containing a header and a data portion. For example, an IP packet or an TCP packet. In Request For Comments (RFC's) a packet isn't so generalized, instead IP packets are called datagrams, while TCP packets are called segments. I have chosen to call pretty much everything packets in this document for simplicity.

Segment - A TCP segment is pretty much the same as an packet, but a formalized word for a TCP packet.

2.1. TCP/IP Layers

TCP/IP is, as already stated, multi-layered. This means that we have one functionality running at one depth, and another one at another level, etcetera. The reason that we have all of these layers is actually very simple.

The biggest reason is that the whole architecture is very extensible. We can add new functionality to the application layers, for example, without having to reimplement the whole TCP/IP stack code, or to include a complete TCP/IP stack into the actual application. Just the same way as we don't need to rewrite every single program, every time that we make a new network interface card. Each layer should need to know as little as possible about each other, to keep them separated.

When we are talking about the programming code of TCP/IP which resides inside the kernel, we are often talking about the TCP/IP stack. The TCP/IP stack simply means all of the sublayers used, from the Network access layer and all the way up to the Application layer.

There are two basic architectures to follow when talking about layers. One of them is the OSI (Open Systems Interconnect) Reference Model and consists of 7 layers. We will only look at it superficially here since we are more interested in the TCP/IP layers. However, from an historical point, this is interesting to know about, especially if you are working with lots of different types of networks. The layers are as follows in the OSI Reference Model list.

1. Application layer

2. Presentation layer

3. Session layer

4. Transport layer

5. Network layer

6. Data Link layer

7. Physical layer

A packet that is sent by us, goes from the top and to the bottom of this list, each layer adding its own set of headers to the packet in what we call the encapsulation phase. When the packet finally reaches it's destination the packet goes backwards through the list and the headers are stripped out of the packet, one by one, each header giving the destination host all of the needed information for the packet data to finally reach the application or program that it was destined for.

The second and more interesting layering standard that we are more interested in is the TCP/IP protocol architecture, as shown in the TCP/IP architecture list. There is no universal agreement among people on just how many layers there are in the TCP/IP architecture. However, it is generally considered that there are 3 through 5 layers available, and in most pictures and explanations, there will be 4 layers discussed. We will, for simplicities sake, only consider those four layers that are generally discussed.

1. Application layer

2. Transport layer

3. Internet layer

4. Network Access layer

As you can see, the architecture of the TCP/IP protocol set is very much like the OSI Reference Model, but yet not. Just the same as with the OSI Reference Model, we add and subtract headers for each layer that we enter or leave.

For example, lets use one of the most common analogies to modern computer networking, the snail-mail letter. Everything is done in steps, just as is everything in TCP/IP.

You want to send a letter to someone asking how they are, and what they are doing. To do this, you must first create the data, or questions. The actual data would be located inside the Application layer.

After this we would put the data written on a sheet of paper inside an envelope and write on it to whom the letter is destined for within a specific company or household. Perhaps something like the example below:

Attn: John Doe
      

This is equivalent to the the Transport layer, as it is known in TCP/IP. In the Transport layer, if we were dealing with TCP, this would have been equivalent to some port most likely though.

At this point we write the address on the envelope of the recipient, such as this:

V. Andersgardsgatan 2
41715 Gothenburg
      

This would in the analogy be the same as the Internet layer. The internet layer contains information telling us where to reach the recipient, or host, in a TCP/IP network. Just the same way as the recipient on an envelope.

The final step is to put the whole letter in a postbox. Doing this would approximately equal to putting a packet into the Network Access Layer. The network access layer contains the functions and routines for accessing the actual physical network that the packet should be transported over.

When the receiver finally receives the letter, he will open the whole letter from the envelope and address etc (decapsulate it). The letter he receives may either require a reply or not. In either case, the letter may be replied upon by the receiver, by reversing the receiver and transmitter addresses on the original letter he received, so that receiver becomes transmitter, and transmitter becomes receiver.

It is very important to understand that iptables was and is specifically built to work inside the headers of the Internet and the Transport layers. It is possible to do some very basic filtering with iptables in the Application and Network access layers as well, but it was not designed for this, nor is it very suitable for those purposes. For example, if we use a string match and match for a specific string inside the packet, lets say get /index.html. Will that work? Normally, yes. However, if the packet size is very small, it will not. The reason is that iptables is built to work on a per packet basis, which means that if the string is split into several separate packets, iptables will not see that whole string. For this reason, you are much, much better off using a proxy of some sort for filtering in the application layer. We will discuss these problems in more detail later on in the IP filtering introduction.

As iptables and netfilter mainly operate in the Internet and Transport layers, that is the layers that we will put our main focus in, in the upcoming sections of this chapter. Under the Internet layer, we will almost exclusively see the IP protocol. There are a few additions to this, such as, for example, the GRE protocol, but they are very rare. Also, iptables is (as the name implies) not focused around these protocols very well either. Because of all these factors we will mainly focus around the IP protocol of the Internet layer, and TCP, UDP and ICMP of the Transport layer.

The ICMP protocol is actually sort of a mix between the two layers. It runs in the Internet layer, but it has the exact same headers as the IP protocol, but also a few extra headers, and then directly inside that encapsulation, the data. We will discuss this in more detail further on, in the ICMP characteristics.

2.2. IP characteristics

The IP protocol resides in the Internet layer, as we have already said. The IP protocol is the protocol in the TCP/IP stack that is responsible for letting your machine, routers, switches and etcetera, know where a specific packet is going. This protocol is the very heart of the whole TCP/IP stack, and makes up the very foundation of everything in the Internet.

The IP protocol encapsulates the Transport layer packet with information about which Transport layer protocol it came from, what host it is going to, and where it came from, and a little bit of other useful information. All of this is, of course, extremely precisely standardized, down to every single bit. The same applies to every single protocol that we will discuss in this chapter.

The IP protocol has a couple of basic functionalities that it must be able to handle. It must be able to define the datagram, which is the next building block created by the transport layer (this may in other words be TCP, UDP or ICMP for example. The IP protocol also defines the Internet addressing system that we use today. This means that the IP protocol is what defines how to reach between hosts, and this also affects how we are able to route packets, of course. The addresses we are talking about are what we generally call an IP address. Usually when we talk about IP addresses, we talk about dotted quad numbers (e.g., 127.0.0.1). This is mostly to make the IP addresses more readable for the human eye, since the IP address is actually just a 32 bit field of 1's and 0's (127.0.0.1 would hence be read as 01111111000000000000000000000001 within the actual IP header).

The IP protocol has even more magic it must perform up it's sleeve. It must also be able to decapsulate and encapsulate the IP datagram (IP data) and send or receive the datagram from either the Network access layer, or the transport layer. This may seem obvious, but sometimes it is not. On top of all this, it has two big functions it must perform as well, that will be of quite interest for the firewalling and routing community. The IP protocol is responsible for routing packets from one host to another, as well as packets that we may receive from one host destined for another. Most of the time on single network access host, this is a very simple process. You have two different options, either the packet is destined for our locally attached network, or possibly through a default gateway. but once you start working with firewalls or security policies together with multiple network interfaces and different routes, it may cause quite some headache for many network administrators. The last of the responsibilities for the IP protocol is that it must fragment and reassemble any datagram that has previously been fragmented, or that needs to be fragmented to fit in to the packetsize of this specific network hardware topology that we are connected to. If these packet fragments are sufficiently small, they may cause a horribly annoying headache for firewall administrators as well. The problem is, that once they are fragmented to small enough chunks, we will start having problems to read even the headers of the packet, not to mention the actual data.

As of Linux kernel 2.4 series, and iptables, this should no longer be a problem for most linux firewalls. The connection tracking system used by iptables for state matching and NAT'ing etc must be able to read the packet defragmented. Because of this, conntrack automatically defragments all packets before they reach the netfilter/iptables structure in the kernel.

The IP protocol is also a connectionless protocol, which in turn means that IP does not "negotiate" a connection. a connection-oriented protocol on the other hand negotiates a "connection" (called a handshake) and then when all data has been sent, tears it down. TCP is an example of this kind of protocol, however, it is implemented on top of the IP protocol. The reason for not being connection-oriented just yet are several, but among others, a handshake is not required at this time yet since there are other protocols that this would add an unnecessarily high overhead to, and that is made up in such a way that if we don't get a reply, we know the packet was lost somewhere in transit anyways, and resend the original request. As you can see, sending the request and then waiting for a specified amount of time for the reply in this case, is much preferred over first sending one packet to say that we want to open a connection, then receive a packet letting us know it was opened, and finally acknowledge that we know that the whole connection is actually open, and then actually send the request, and after that send another packet to tear the connection down and wait for another reply.

IP is also known as an unreliable protocol, or simply put it does not know if a packet was received or not. It simply receives a packet from the transport layer and does its thing, and then passes it on to the network access layer, and then nothing more to it. It may receive a return packet, which traverses from network access layer to the IP protocol which does it's thing again, and then passes it on upwards to the Transport layer. However, it doesn't care if it gets a reply packet, or if the packet was received at the other end. Same thing applies for the unreliability of IP as for the connectionless-ness, since unreliability would require adding an extra reply packet to each packet that is sent. For example, let us consider a DNS lookup. As it is, we send a DNS request for servername.com. If we never receive a reply, we know something went wrong and re-request the lookup, but during normal use we would send out one request, and get one reply back. Adding reliability to this protocol would mean that the request would require two packets (one request, and one confirmation that the packet was received) and then two packets for the reply (one reply, and one reply to acknowledge the reply was received). In other words, we just doubled the amount of packets needed to send, and almost doubled the amount of data needed to be transmitted.

2.3. IP headers

The IP packet contains several different parts in the header as you have understood from the previous introduction to the IP protocol. The whole header is meticuluously divided into different parts, and each part of the header is allocated as small of a piece as possible to do it's work, just to give the protocol as little overhead as possible. You will see the exact configuration of the IP headers in the IP headers image.

Understand that the explanations of the different headers are very brief and that we will only discuss the absolute basics of them. For each type of header that we discuss, we will also list the proper RFC's that you should read for further understanding and technical explanations of the protocol in question. As a sidenote to this note, RFC stands for Request For Comments, but these days, they have a totally different meaning to the Internet community. They are what defines and standardises the whole Internet, compared to what they were when the researchers started writing RFC's to each other. Back then, they were simply requests for comments and a way of asking other researchers about their opinions.

The IP protocol is mainly described in RFC 791 - Internet Protocol. However, this RFC is also updated by RFC 1349 - Type of Service in the Internet Protocol Suite, which was obsoleted by RFC 2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers, and which was updated by RFC 3168 - The Addition of Explicit Congestion Notification (ECN) to IP and RFC 3260 - New Terminology and Clarifications for Diffserv.

As you can see, all of these standards can get a little bit hard to follow at times. One tip for finding the different RFC's that are related to each other is to use the search functions available at RFC-editor.org. In the case of IP, consider that the RFC 791 is the basic RFC, and all of the other are simply updates and changes to that standard. We will discuss these more in detail when we get to the specific headers that are changed by these newer RFC's. One thing to remember is, that sometimes, an RFC can be obsoleted (not used at all). Normally this means that the RFC has been so drastically updated and that it is better to simply replace the whole thing. It may also become obsolete for other reasons as well. When an RFC becomes obsoleted, a field is added to the original RFC that points to the new RFC instead.

Version - bits 0-3. This is a version number of the IP protocol in binary. IPv4 iscalled 0100, while IPv6 is called 0110. This field is generally not used for filtering very much. The version described in RFC 791 is IPv4.

IHL (Internet Header Length) - bits 4-7. This field tells us how long the IP header is in 32 bit words. As you can see, we have split the header up in this way (32 bits per line) in the image as well. Since the Options field is of optional length, we can never be absolutely sure of how long the whole header is, without this field. The minimum length of this of the header is 5 words.

Type of Service, DSCP, ECN - bits 8-15. This is one of the most complex areas of the IP header for the simple reason that it has been updated 3 times. It has always had the same basic usage, but the implementation has changed several times. First the field was called the Type of Service field. Bit 0-2 of the field was called the Precedence field. Bit 3 was Normal/Low delay, Bit 4 was Normal/High throughput, Bit 5 was Normal/High reliability and bit 6-7 was reserved for future usage. This is still used in a lot of places with older hardware, and it still causes some problems for the Internet. Among other things, bit 6-7 are specified to be set to 0. In the ECN updates (RFC , we start using these reserved bits and hence set other values than 0 to these bits. But a lot of old firewalls and routers have built in checks looking if these bits are set to 1, and if the packets do, the packet is discarded. Today, this is clearly a violation of RFC's, but there is not much you can do about it, except to complain.

The second iteration of this field was when the field was changed into the DS field as defined in RFC 2474. DS stands for Differentiated Services. According to this standard bits 8-14 is Differentiated Services Code Point (DSCP) and the remaining two bits (15-16) are still unused. The DSCP field is pretty much used the same as in how the ToS field was used before, to mark what kind of service this packet should be treated like if the router in question makes any difference between them. One big change is that a device must ignore the unused bits to be fully RFC 2474 compliant, which means we get rid of the previous hassle as explained previously, as long as the device creators follow this RFC.

The third, and almost last, change of the ToS field was when the two, previously, unused bits were used for ECN (Explicit Congestion Notification), as defined in RFC 3168. ECN is used to let the end nodes know about a routers congestion, before it actually starts dropping packets, so that the end nodes will be able to slow down their data transmissions, before the router actually needs to start dropping data. Previously, dropping data was the only way that a router had to tell that it was overloaded, and the end nodes had to do a slow restart for each dropped packet, and then slowly gather up speed again. The two bits are named ECT (ECNCapable Transport) and CE (Congestion Experienced) codepoints.

The final iteration of the whole mess is RFC 3260 which gives some new terminology and clarifications to the usage of the DiffServ system. It doesn't involve too many new updates or changes, except in the terminology. The RFC is also used to clarify some points that were discussed between developers.

Total Length - bits 16 - 31. This field tells us how large the packet is in octets, including headers and everything. The maximum size is 65535 octets, or bytes, for a single packet. The minimum packet size is 576 bytes, not caring if the packet arrives in fragments or not. It is only recommended to send larger packets than this limit if it can be guaranteed that the host can receive it, according to RFC 791. However, these days most networks runs at 1500 byte packet size. This includes almost all ethernet connections, and most Internet connections.

Identification - bits 32 - 46. This field is used in aiding the reassembly of fragmented packets.

Flags - bits 47 - 49. This field contains a few miscellaneous flags pertaining to fragmentation. The first bit is reserved, but still not used, and must be set to 0. The second bit is set to 0 if the packet may be fragmented, and to 1 if it may not be fragmented. The third and last bit can be set to 0 if this was the last fragment, and 1 if there are more fragments of this same packet.

Fragment Offset - bits 50 - 63. The fragment offset field shows where in the datagram that this packet belongs. The fragments are calculated in 64 bits, and the first fragment has offset zero.

Time to live - bits 64 - 72. The TTL field tells us how long the packet may live, or rather how many "hops" it may take over the Internet. Every process that touches the packet must remove one point from the TTL field, and if the TTL reaches zero, the whole packet must be destroyed and discarded. This is basically used as a safety trigger so that a packet may not end up in an uncontrollable loop between one or several hosts. Upon destruction the host should return an ICMP Destination Unreachable message to the sender.

Protocol - bits 73 - 80. In this field the protocol of the next level layer is indicated. For example, this may be TCP, UDP or ICMP among others. All of these numbers are defined by the Internet Assigned Numbers Authority. All numbers can befound on their homepage Internet Assigned Numbers Authority.

Header checksum - bits 81 - 96. This is a checksum of the IP header of the packet.This field is recomputed at every host that changes the header, which means pretty much every host that the packet traverses over, since they most often change the packets TTL field or some other.

Source address - bits 97 - 128. This is the source address field. It is generally written in 4 octets, translated from binary to decimal numbers with dots in between. That is for example, 127.0.0.1. The field lets the receiver know where the packet came from.

Destination address - bits 129 - 160. The destination address field contains the destination address, and what a surprise, it is formatted the same way as the source address.

Options - bits 161 - 192 <> 478. The options field is not optional, as it may sound. Actually, this is one of the more complex fields in the IP header. The options field contains different optional settings within the header, such as Internet timestamps, SACK or record route options. Since these options are all optional, the Options field can have different lengths, and hence the whole IP header. However, since we always calculate the IP header in 32 bit words, we must always end the header on an even number, that is the multiple of 32. The field may contain zero or more options.

The options field starts with a brief 8 bit field that lets us know which options are used in the packet. The options are all listed in the TCP Options table, in the TCP options appendix. For more information about the different options, read the proper RFC's. For an updated listing of the IP options, check at Internet Assigned Numbers Authority.

Padding - bits variable. This is a padding field that is used to make the header end at an even 32 bit boundary. The field must always be set to zeroes straight through to the end.

2.4. TCP characteristics

The TCP protocol resides on top of the IP protocol. It is a stateful protocol and has built-in functions to see that the data was received properly by the other end host. The main goals of the TCP protocol is to see that data is reliably received and sent, that the data is transported between the Internet layer and Application layer correctly, and that the packet data reaches the proper program in the application layer, and that the data reaches the program in the right order. All of this is possible through the TCP headers of the packet.

The TCP protocol looks at data as an continuous data stream with a start and a stop signal. The signal that indicates that a new stream is waiting to be opened is called a SYN three-way handshake in TCP, and consists of one packet sent with the SYN bit set. The other end then either answers with SYN/ACK or SYN/RST to let the client know if the connection was accepted or denied, respectively. If the client receives an SYN/ACK packet, it once again replies, this time with an ACK packet. At this point, the whole connection is established and data can be sent. During this initial handshake, all of the specific options that will be used throughout the rest of the TCP connection is also negotiated, such as ECN, SACK, etcetera.

While the datastream is alive, we have further mechanisms to see that the packets are actually received properly by the other end. This is the reliability part of TCP. This is done in a simple way, using a Sequence number in the packet. Every time we send a packet, we give a new value to the Sequence number, and when the other end receives the packet, it sends an ACK packet back to the data sender. The ACK packet acknowledges that the packet was received properly. The sequence number also sees to it that the packet is inserted into the data stream in a good order.

Once the connection is closed, this is done by sending a FIN packet from either end-point. The other end then responds by sending a FIN/ACK packet. The FIN sending end can then no longer send any data, but the other end-point can still finish sending data. Once the second end-point wishes to close the connection totally, it sends a FIN packet back to the originally closing end-point, and the other end-point replies with a FIN/ACK packet. Once this whole procedure is done, the connection is torn down properly.

As you will also later see, the TCP headers contain a checksum as well. The checksum consists of a simple hash of the packet. With this hash, we can with rather high accuracy see if a packet has been corrupted in any way during transit between the hosts.

2.5. TCP headers

The TCP headers must be able to perform all of the tasks above. We have already explained when and where some of the headers are used, but there are still other areas that we haven't touched very deeply at. Below you see an image of the complete set of TCP headers. It is formatted in 32 bit words per row, as you can see.

Source port - bit 0 - 15. This is the source port of the packet. The source port was originally bound directly to a process on the sending system. Today, we use a hash between the IP addresses, and both the destination and source ports to achieve this uniqueness that we can bind to a single application or program.

Destination port - bit 16 - 31. This is the destination port of the TCP packet. Just as with the source port, this was originally bound directly to a process on the receiving system. Today, a hash is used instead, which allows us to have more open connections at the same time. When a packet is received, the destination and source ports are reversed in the reply back to the originally sending host, so that destination port is now source port, and source port is destination port.

Sequence Number - bit 32 - 63. The sequence number field is used to set a number on each TCP packet so that the TCP stream can be properly sequenced (e.g., the packets winds up in the correct order). The Sequence number is then returned in the ACK field to ackonowledge that the packet was properly received.

Acknowledgment Number - bit 64 - 95. This field is used when we acknowledge a specific packet a host has received. For example, we receive a packet with one Sequence number set, and if everything is okey with the packet, we reply with an ACK packet with the Acknowledgment number set to the same as the original Sequence number.

Data Offset - bit 96 - 99. This field indicates how long the TCP header is, and where the Data part of the packet actually starts. It is set with 4 bits, and measures the TCP header in 32 bit words. The header should always end at an even 32 bit boundary, even with different options set. This is possible thanks to the Padding field at the very end of the TCP header.

Reserved - bit 100 - 103. These bits are reserved for future usage. In RFC 793 this also included the CWR and ECE bits. According to RFC 793 bit 100-105 (i.e., this and the CWR and ECE fields) must be set to zero to be fully compliant. Later on, when we started introducing ECN, this caused a lot of troubles because a lot of Internet appliances such as firewalls and routers dropped packets with them set. This is still true as of writing this.

CWR - bit 104. This bit was added in RFC 3268 and is used by ECN. CWR stands for Congestion Window Reduced, and is used by the data sending part to inform the receiving part that the congestion window has been reduced. When the congestion window is reduced, we send less data per timeunit, to be able to cope with the total network load.

ECE - bit 105. This bit was also added with RFC 3268 and is used by ECN. ECE stands for ECN Echo. It is used by the TCP/IP stack on the receiver host to let the sending host know that it has received an CE packet. The same thing applies here, as for the CWR bit, it was originally a part of the reserved field and because of this, some networking appliances will simply drop the packet if these fields contain anything else than zeroes. This is actually still true for a lot of appliances unfortunately.

URG - bit 106. This field tells us if we should use the Urgent Pointer field or not. If set to 0, do not use Urgent Pointer, if set to 1, do use Urgent pointer.

ACK - bit 107. This bit is set to a packet to indicate that this is in reply to another packet that we received, and that contained data. An Acknowledgment packet is always sent to indicate that we have actually received a packet, and that it contained no errors. If this bit is set, the original data sender will check the Acknowledgment Number to see which packet is actually acknowledged, and then dump it from the buffers.

PSH - bit 108. The PUSH flag is used to tell the TCP protocol on any intermediate hosts to send the data on to the actual user, including the TCP implementation on the receiving host. This will push all data through, unregardless of where or how much of the TCP Window that has been pushed through yet.

RST - bit 109. The RESET flag is set to tell the other end to tear down the TCP connection. This is done in a couple of different scenarios, the main reasons being that the connection has crashed for some reason, if the connection does not exist, or if the packet is wrong in some way.

SYN - bit 110. The SYN (or Synchronize sequence numbers) is used during the initial establishment of a connection. It is set in two instances of the connection, the initial packet that opens the connection, and the reply SYN/ACK packet. It should never be used outside of those instances.

FIN - bit 111. The FIN bit indicates that the host that sent the FIN bit has no more data to send. When the other end sees the FIN bit, it will reply with a FIN/ACK. Once this is done, the host that originally sent the FIN bit can no longer send any data. However, the other end can continue to send data until it is finished, and will then send a FIN packet back, and wait for the final FIN/ACK, after which the connection is sent to a CLOSED state.

Window - bit 112 - 127. The Window field is used by the receiving host to tell the sender how much data the receiver permits at the moment. This is done by sending an ACK back, which contains the Sequence number that we want to acknowledge, and the Window field then contains the maximum accepted sequence numbers that the sending host can use before he receives the next ACK packet. The next ACK packetwill update accepted Window which the sender may use.

Checksum - bit 128 - 143. This field contains the checksum of the whole TCP header. It is a one's complement of the one's complement sum of each 16 bit word in the header. If the header does not end on a 16 bit boundary, the additional bits are set to zero. While the checksum is calculated, the checksum field is set to zero. The checksum also covers a 96 bit pseudoheader containing the Destination-, Source-address, protocol, and TCP length. This is for extra security.

Urgent Pointer - bit 144 - 159. This is a pointer that points to the end of the data which is considered urgent. If the connection has important data that should be processed as soon as possible by the receiving end, the sender can set the URG flag and set the Urgent pointer to indicate where the urgent data ends.

Options - bit 160 - **. The Options field is a variable length field and contains optional headers that we may want to use. Basically, this field contains 3 subfields at all times. An initial field tells us the length of the Options field, a second field tells us which options are used, and then we have the actual options. A complete listing of all the TCP Options can be found in TCP options.

Padding - bit **. The padding field pads the TCP header until the whole header ends at a 32-bit boundary. This ensures that the data part of the packet begins on a 32-bit boundary, and no data is lost in the packet. The padding always consists of only zeros.

2.6. UDP characteristics

The User Datagram Protocol (UDP protocol) is a very basic and simple protocol on top of the IP protocol. It was developed to allow for very simple data transmission without any error detection of any kind, and it is stateless. However, it is very well fit for query/response kind of applications, such as for example DNS, et cetera, since we know that unless we get a reply from the DNS server, the query was lost somewhere. Sometimes it may also be worth using the UDP protocol instead of TCP, such as when we want only error/loss detection but don't care about sequencing of the packets. This removes some overhead that comes from the TCP protocol. We may also do the other thing around, make our own protocol on top of UDP that only contains sequencing, but no error or loss detection.

The UDP protocol is specified in RFC 768 - User Datagram Protocol. It is a very short and brief RFC, which fits a simple protocol like this very well.

2.7. UDP headers

The UDP header can be said to contain a very basic and simplified TCP header. It contains destination-, source-ports, header length and a checksum as seen in the image below.

Source port - bit 0-15. This is the source port of the packet, describing where a reply packet should be sent. This can actually be set to zero if it doesn't apply. For example, sometimes we don't require a reply packet, and the packet can then be set to source port zero. In most implementations, it is set to some port number.

Destination port - bit 16-31. The destination port of the packet. This is required for all packets, as opposed to the source port of a packet.

Length - bit 32-47. The length field specifies the length of the whole packet in octets, including header and data portions. The shortest possible packet can be 8 octets long.

Checksum - bit 48-63. The checksum is the same kind of checksum as used in the TCP header, except that it contains a different set of data. In other words, it is a one's complement of the one's complement sum of parts of the IP header, the whole UDP header, the UDP data and padded with zeroes at the end when necessary.

2.8. ICMP characteristics

ICMP messages are used for a basic kind of error reporting between host to host, or host to gateway. Between gateway to gateway, a protocol called Gateway to Gateway protocol (GGP) should normally be used for error reporting. As we have already discussed, the IP protocol is not designed for perfect error handling, but ICMP messages solves some parts of these problems. The big problem from one standpoint is that the headers of the ICMP messages are rather complicated, and differ a little bit from message to message. However, this will not be a big problem from a filtering standpoint most of the time.

The basic form is that the message contains the standard IP header, type, code and a checksum. All ICMP messages contains these fields. The type specifies what kind of error or reply message this packet is, such as for example destination unreachable, echo, echo reply, or redirect message. The code field specifies more information, if necessary. If the packet is of type destination unreachable, there are several possible values on this code field such as network unreachable, host unreachable, or port unreachable. The checksum is simply a checksum for the whole packet.

As you may have noticed, I mentioned the IP header explicitly for the ICMP packet. This was done since the actual IP header is an integral part of the ICMP packet, and the ICMP protocol lives on the same level as the IP protocol in a sense. ICMP does use the IP protocol as if it where a higher level protocol, but at the same time not. ICMP is an integral part of IP, and ICMP must be implemented in every IP implementation.

2.9. ICMP headers

As already explained, the headers differs a little bit from ICMP type to ICMP type. Most of the ICMP types are possible to group by their headers. Because of this, we will discuss the basic header form first, and then look at the specifics for each group of types that should be discussed.

All packets contain some basic values from the IP headers discussed previously in this chapter. The headers have previously been discussed at some length, so this is just a short listing of the headers, with a few notes about them.

• Version - This should always be set to 4.

• Internet Header Length - The length of the header in 32 bit words.

• Type of Service - See above. This should be set to 0, as this is the only legit setting according to RFC 792 - Internet Control Message Protocol.

• Total Length - Total length of the header and data portion of the packet, counted in octets.

• Identification, Flags and Fragment offsets - Ripped from the IP protocol.

• Time To Live - How many hops this packet will survive.

• Protocol - which version of ICMP is being used (should always be 1).

• Header Checksum - See the IP explanation.

• Source Address - The source address from whom the packet was sent. This is not entirely true, since the packet can have another source address, than that which is located on the machine in question. The ICMP types that can have this effect will be noted if so.

• Destination Address - The destination address of the packet.

There are also a couple of new headers that are used by all of the ICMP types. The new headers are as follows, this time with a few more notes about them:

• Type - The type field contains the ICMP type of the packet. This is always different from ICMP type to type. For example ICMP Destination Unreachable packets will have a type 3 set to it. For a complete listing of the different ICMP types, see the ICMP types appendix. This field contains 8 bits total.

• Code - All ICMP types can contain different codes as well. Some types only have a single code, while others have several codes that they can use. For example, the ICMP Destination Unreachable (type 3) can have at least code 0, 1, 2, 3, 4 or 5 set. Each code has a different meaning in that context then. For a complete listing of the different codes, see the ICMP types appendix. This field is 8 bits in length, total. We will discuss the different codes a little bit more in detail for each type later on in this section.

• Checksum - The Checksum is a 16 bit field containing a one's complement of the ones complement of the headers starting with the ICMP type and down. While calculating the checksum, the checksum field should be set to zero.

At this point the headers for the different packets start to look different also. We will describe the most common ICMP Types one by one, with a brief discussion of its headers and different codes.

---

2.9.3. Source Quench

A source quench packet can be sent to tell the originating source of a packet or stream of packets to slow down when continuing to send data. Note that gateway or destination host that the packets traverses can also be quiet and silently discard the packets, instead of sending any source quench packets.

This packet contains no extra header except the data portion, which contains the internet header plus 64 bits of the original data datagram. This is used to match the source quench message to the correct process, which is currently sending data through the gateway or to the destination host.

All source quench packets have their ICMP types set to 4. They have no codes except 0.

Today, there are a couple of new possible ways of notifying the sending and receiving host that a gateway or destination host is overloaded. One way for example is the ECN (Explicit Congestion Notification) system.

2.10. TCP/IP destination driven routing

TCP/IP has grown in complexity quite a lot when it comes to the routing part. In the beginning, most people thought it would be enough with destination driven routing. The last few years, this has become more and more complex however. Today, Linux can route on basically every single field or bit in the IP header, and even based on TCP, UDP or ICMP headers as well. This is called policy based routing, or advanced routing.

This is simply a brief discussion on how the destination driven routing is performed. When we send a packet from a sending host, the packet is created. After this, the computer looks at the packet destination address and compares it to the routing table that it has. If the destination address is local, the packet is sent directly to that address via its hardware MAC address. If the packet is on the other side of a gateway, the packet is sent to the MAC address of the gateway. The gateway will then look at the IP headers and see the destination address of the packet. The destination address is looked up in the routing table again, and the packet is sent to the next gateway, et cetera, until the packet finally reaches the local network of the destination.

As you can see, this routing is very basic and simple. With the advanced routing and policy based routing, this gets quite a bit more complex. We can route packets differently based on their source address for example, or their TOS value, et cetera.

2.11. What's next?

This chapter has brought you up to date to fully understand the subsequent chapters. The following has been gone through thoroughly:

• TCP/IP structure

• IP protocol functionality and headers.

• TCP protocol functionality and headers.

• UDP protocol functionality and headers.

• ICMP protocol functionality and headers.

• TCP/IP destination driven routing.

All of this will come in very handy later on when you start to work with the actual firewall rulesets. All of this information are pieces that fit together, and will lead to a better firewall design.

3.1. What is an IP filter

It is important to fully understand what an IP filter is. Iptables is an IP filter, and if you don't fully understand this, you will get serious problems when designing your firewalls in the future.

An IP filter operates mainly in layer 2, of the TCP/IP reference stack. Iptables however has the ability to also work in layer 3, which actually most IP filters of today have. But per definition an IP filter works in the second layer.

If the IP filter implementation is strictly following the definition, it would in other words only be able to filter packets based on their IP headers (Source and Destionation address, TOS/DSCP/ECN, TTL, Protocol, etcetera. Things that are actually in the IP header. However, since the Iptables implementation is not perfectly strict around this definition, it is also able to filter packets based on other headers that lie deeper into the packet (TCP, UDP, etc), and shallower (MAC source address).

There is one thing however, that iptables is very strict about in these days. It does not "follow" streams or puzzle data together. This would simply be too timeconsuming. The implications of this will be discussed a little bit more just further on. It does keep track of packets and see if they are of the same stream (via sequence numbers, port numbers, etc.) almost exactly the same way as the real TCP/IP stack. This is called connection tracking, and thanks to this we can do things such as Destination and Source Network Address Translation (generally called DNAT and SNAT), as well as state matching of packets.

As I implied above, iptables can not connect data from different packets to each other, and hence you can never be fully certain that you will see the complete data at all times. I am specifically mentioning this since there are constantly at least a couple of questions about this on the different mailing lists pertaining to netfilter and iptables and how to do things that are generally considered a really bad idea. For example, every time there is a new windows based virus, there are a couple of different persons asking how to drop all streams containing a specific string. The bad idea about this is that it is so easily circumvented. For example if we match for something like this:

cmd.exe

Now, what happens if the virus/exploit writer is smart enough to make the packet size so small that cmd winds up in one packet, and .exe winds up in the next packet? Or what if the packet has to travel through a network that has this small a packet size on its own? Yes, since these string matching functions is unable to work across packet boundaries, the packet will get through anyway.

Some of you may now be asking yourself, why don't we simply make it possible for the string matches, etcetera to read across packet boundaries? It is actually fairly simple. It would be too costly on processor time. Connection tracking is already taking way to much processor time to be totally comforting. To add another extra layer of complexity to connection tracking, such as this, would probably kill more firewalls than anyone of us could expect. Not to think of how much memory would be used for this simple task on each machine.

There is also a second reason for this functionality not being developed. There is a technology called proxies. Proxies were developed to handle traffic in the higher layers, and are hence much better at fullfilling these requirements. Proxies were originally developed to handle downloads and often used pages and to help you get the most out of slow Internet connections. For example, Squid is a webproxy. A person who wants to download a page sends the request, the proxy either grabs the request or receives the request and opens the connection to the web browser, and then connects to the webserver and downloads the file, and when it has downloaded the file or page, it sends it to the client. Now, if a second browser wants to read the same page again, the file or page is already downloaded to the proxy, and can be sent directly, and saves bandwidth for us.

As you may understand, proxies also have quite a lot of functionality to go in and look at the actual content of the files that it downloads. Because of this, they are much better at looking inside the whole streams, files, pages etc.

3.2. IP filtering terms and expressions

To fully understand the upcoming chapters there are a few general terms and expressions that one must understand, including a lot of details regarding the TCP/IP chapter. This is a listing of the most common terms used in IP filtering.

• Drop/Deny - When a packet is dropped or denied, it is simply deleted, and no further actions are taken. No reply to tell the host it was dropped, nor is the receiving host of the packet notified in any way. The packet simply disappears.

• Reject - This is basically the same as a drop or deny target or policy, except that we also send a reply to the host sending the packet that was dropped. The reply may be specified, or automatically calculated to some value. (To this date, there is unfortunately no iptables functionality to also send a packet notifying the receiving host of the rejected packet what happened (ie, doing the reverse of the Reject target). This would be very good in certain circumstances, since the receiving host has no ability to stop Denial of Service attacks from happening.)

• State - A specific state of a packet in comparison to a whole stream of packets. For example, if the packet is the first that the firewall sees or knows about, it is considered new (the SYN packet in a TCP connection), or if it is part of an already established connection that the firewall knows about, it is considered to be established. States are known through the connection tracking system, which keeps track of all the sessions.

• Chain - A chain contains a ruleset of rules that are applied on packets that traverses the chain. Each chain has a specific purpose (e.g., which table it is connected to, which specifies what this chain is able to do), as well as a specific application area (e.g., only forwarded packets, or only packets destined for this host). In iptables, there are several different chains, which will be discussed in depth in later chapters.

• Table - Each table has a specific purpose, and in iptables there are 3 tables. The nat, mangle and filter tables. For example, the filter table is specifically designed to filter packets, while the nat table is specifically designed to NAT (Network Address Translation) packets.

• Match - This word can have two different meanings when it comes to IP filtering. The first meaning would be a single match that tells a rule that this header must contain this and this information. For example, the --source match tells us that the source address must be a specific network range or host address. The second meaning is if a whole rule is a match. If the packet matches the whole rule, the jump or target instructions will be carried out (e.g., the packet will be dropped.)

• Target - There is generally a target set for each rule in a ruleset. If the rule has matched fully, the target specification tells us what to do with the packet. For example, if we should drop or accept it, or NAT it, etc. There is also something called a jump specification, for more information see the jump description in this list. As a last note, there might not be a target or jump for each rule, but there may be.

• Rule - A rule is a set of a match or several matches together with a single target in most implementations of IP filters, including the iptables implementation. There are some implementations which let you use several targets/actions per rule.

• Ruleset - A ruleset is the complete set of rules that are put into a whole IP filter implementation. In the case of iptables, this includes all of the rules set in the filter, nat and mangle tables, and in all of the subsequent chains. Most of the time, they are written down in a configuration file of some sort.

• Jump - The jump instruction is closely related to a target. A jump instruction is written exactly the same as a target in iptables, with the exception that instead of writing a target name, you write the name of another chain. If the rule matches, the packet will hence be sent to this second chain and be processed as usual in that chain.

• Connection tracking - A firewall which implements connection tracking is able to track connections/streams simply put. The ability to do so is often done at the impact of lots of processor and memory usage. This is unfortunately true in iptables as well, but much work has been done to work on this. However, the good side is that the firewall will be much more secure with connection tracking properly used by the implementer of the firewall policies.

• Accept - To accept a packet and to let it through the firewall rules. This is the opposite of the drop or deny targets, as well as the reject target.

• Policy - There are two kinds of policies that we speak about most of the time when implementing a firewall. First we have the chain policies, which tells the firewall implementation the default behaviour to take on a packet if there was no rule that matched it. This is the main usage of the word that we will use in this book. The second type of policy is the security policy that we may have written documentation on, for example for the whole company or for this specific network segment. Security policies are very good documents to have thought through properly and to study properly before starting to actually implement the firewall.

3.3. How to plan an IP filter

One of the first steps to think about when planning the firewall is their placement. This should be a fairly simple step since mostly your networks should be fairly well segmented anyway. One of the first places that comes to mind is the gateway between your local network(s) and the Internet. This is a place where there should be fairly tight security. Also, in larger networks it may be a good idea to separate different divisions from each other via firewalls. For example, why should the development team have access to the human resources network, or why not protect the economic department from other networks? Simply put, you don't want an angry employee with the pink slip tampering with the salary databases.

Simply put, the above means that you should plan your networks as well as possible, and plan them to be segregated. Especially if the network is medium- to big-sized (100 workstations or more, based on different aspects of the network). In between these smaller networks, try to put firewalls that will only allow the kind of traffic that you would like.

It may also be a good idea to create a De-Militarized Zone (DMZ) in your network in case you have servers that are reached from the Internet. A DMZ is a small physical network with servers, which is closed down to the extreme. This lessens the risk of anyone actually getting in to the machines in the DMZ, and it lessens the risk of anyone actually getting in and downloading any trojans etc. from the outside. The reason that they are called de-militarized zones is that they must be reachable from both the inside and the outside, and hence they are a kind of grey zone (DMZ simply put).

There are a couple of ways to set up the policies and default behaviours in a firewall, and this section will discuss the actual theory that you should think about before actually starting to implement your firewall, and helping you to think through your decisions to the fullest extent.

Before we start, you should understand that most firewalls have default behaviours. For example, if no rule in a specific chain matches, it can be either dropped or accepted per default. Unfortunately, there is only one policy per chain, but this is often easy to get around if we want to have different policies per network interface etc.

There are two basic policies that we normally use. Either we drop everything except that which we specify, or we accept everything except that which we specifically drop. Most of the time, we are mostly interested in the drop policy, and then accepting everything that we want to allow specifically. This means that the firewall is more secure per default, but it may also mean that you will have much more work in front of you to simply get the firewall to operate properly.

Your first decision to make is to simply figure out which type of firewall you should use. How big are the security concerns? What kind of applications must be able to get through the firewall? Certain applications are horrible to firewalls for the simple reason that they negotiate ports to use for data streams inside a control session. This makes it extremely hard for the firewall to know which ports to open up. The most common applications works with iptables, but the more rare ones do not work to this day, unfortunately.

There are also some applications that work partially, such as ICQ. Normal ICQ usage works perfectly, but not the chat or file sending functions, since they require specific code to handle the protocol. Since the ICQ protocols are not standardized (they are proprietary and may be changed at any time) most IP filters have chosen to either keep the ICQ protocol handlers out, or as patches that can be applied to the firewalls. Iptables have chosen to keep them as separate patches.

It may also be a good idea to apply layered security measures, which we have actually already discussed partially so far. What we mean with this, is that you should use as many security measures as possible at the same time, and don't rely on any one single security concept. Having this as a basic concept for your security will increase security tenfold at least. For an example, let's look at this.

As you can see, in this example I have in this example chosen to place a Cisco PIX firewall at the perimeter of all three network connections. It may NAT the internal LAN, as well as the DMZ if necessary. It will also block all outgoing traffic except http return traffic as well as ftp and ssh traffic. It will allow incoming http traffic from both the LAN and the Internet, and ftp and ssh traffic from the LAN. On top of this, we note that each webserver is based on Linux, hence we throw iptables and netfilter on each of the machines as well and add the same basic policies on these.

On top of this, we may add Snort on each of the machines. Snort is an excellent open source "network intrusion detection system" (NIDS) which looks for signatures in the packets that it sees, and if it sees a signature of some kind of attack or breakin it can either e-mail the administrator and notify him about it, or even make active responses to the attack such as blocking the IP from which the attack originated. It should be noted that active responses should not be used lightly since snort has a bad behaviour of reporting lots of false positives (e.g., reporting an attack which is not really an attack).

It could also be a good idea to throw in an proxy in front of the webservers to catch some of the bad packets as well, which could also be a possibility to throw in for all of the locally generated webconnections. With a webproxy you can narrow down on traffic used by webtraffic from your employees, as well as restrict their webusage to some extent. As for a webproxy to your own webservers, you can use it to block some of the most obvious connections to get through. A good proxy that may be worth using is the Squid.

Another precaution that one can take is to install Tripwire. This is an excellent last line of defense kind of application. What it does is to make checksums of all the files specified in a configuration file, and then it is run from cron once in a while to see that all of the specified files are the same as before, or have not changed in an illegit way. This program will in other words be able to find out if anyone has actually been able to get through and tampered with the system. A suggestion is to run this on all of the webservers.

One last thing to note is that it is always a good thing to follow standards, as we know. As you have already seen with the ICQ example, if you don't use standardized systems, things can go terribly wrong. For your own environments, this can be ignored to some extent, but if you are running a broadband service or modempool, it gets all the more important. People who connect through you must always be able to rely on your standardization, and you can't expect everyone to run the specific operating system of your choice. Some people want to run Windows, some want to run Linux or even VMS and so on. If you base your security on proprietary systems, you are in for some trouble.

A good example of this is certain broadband services that have popped up in Sweden who base lots of security on Microsoft network logon. This may sound like a great idea to begin with, but once we start considering other operating systems and so on, this is no longer such a good idea. How will someone running Linux get online? Or VAX/VMS? Or HP/UX? With Linux it can be done of course, if it wasn't for the fact that the network administrator refuses anyone to use the broadband service if they are running linux by simply blocking them in such case. However, this book is not a theological discussion of what is best, so let's leave it as an example of why it is a bad idea to use non-standards.

3.4. Whats next?

This chapter has gone through several of the basic IP filtering and security measures that you can take to secure your networks, workstations and servers. The following subjects have been brought up:

• IP filtering usage

• IP filtering policies

• Network planning

• Firewall planning

• Layered security techniques

• Network segmentation

In the next chapter we will take a quick look at what Network Address Translation (NAT) is, and after that we will start looking closer at Iptables and it's functionality and actually start getting hands on with the beast.

4.1. What NAT is used for and basic terms and expressions

Basically, NAT allows a host or several hosts to share the same IP address in a way. For example, let's say we have a local network consisting of 5-10 clients. We set their default gateways to point through the NAT server. Normally the packet would simply be forwarded by the gateway machine, but in the case of an NAT server it is a little bit different.

NAT servers translates the source and destination addresses of packets as we already said to different addresses. The NAT server receives the packet, rewrites the source and/or destination address and then recalculates the checksum of the packet. One of the most common usages of NAT is the SNAT (Source Network Address Translation) function. Basically, this is used in the above example if we can't afford or see any real idea in having a real public IP for each and every one of the clients. In that case, we use one of the private IP ranges for our local network (for example, 192.168.1.0/24), and then we turn on SNAT for our local network. SNAT will then turn all 192.168.1.0 addresses into it's own public IP (for example, 217.115.95.34). This way, there will be 5-10 clients or many many more using the same shared IP address.

There is also something called DNAT, which can be extremely helpful when it comes to setting up servers etc. First of all, you can help the greater good when it comes to saving IP space, second, you can get an more or less totally impenetrable firewall in between your server and the real server in an easy fashion, or simply share an IP for several servers that are separated into several physically different servers. For example, we may run a small company server farm containing a webserver and ftp server on the same machine, while there is a physically separated machine containing a couple of different chat services that the employees working from home or on the road can use to keep in touch with the employees that are on-site. We may then run all of these services on the same IP from the outside via DNAT.

The above example is also based on separate port NAT'ing, or often called PNAT. We don't refer to this very often throughout this book, since it is covered by the DNAT and SNAT functionality in netfilter.

In Linux, there are actually two separate types of NAT that can be used, either Fast-NAT or Netfilter-NAT. Fast-NAT is implemented inside the IP routing code of the Linux kernel, while Netfilter-NAT is also implemented in the Linux kernel, but inside the netfilter code. Since this book won't touch the IP routing code too closely, we will pretty much leave it here, except for a few notes. Fast-NAT is generally called by this name since it is much faster than the netfilter NAT code. It doesn't keep track of connections, and this is both its main pro and con. Connection tracking takes a lot of processor power, and hence it is slower, which is one of the main reasons that the Fast-NAT is faster than Netfilter-NAT. As we also said, the bad thing about Fast-NAT doesn't track connections, which means it will not be able to do SNAT very well for whole networks, neither will it be able to NAT complex protocols such as FTP, IRC and other protocols that Netfilter-NAT is able to handle very well. It is possible, but it will take much, much more work than would be expected from the Netfilter implementation.

There is also a final word that is basically a synonym to SNAT, which is the Masquerade word. In Netfilter, masquerade is pretty much the same as SNAT with the exception that masquerading will automatically set the new source IP to the default IP address of the outgoing network interface.

4.2. Caveats using NAT

As we have already explained to some extent, there are quite a lot of minor caveats with using NAT. The main problem is certain protocols and applications which may not work at all. Hopefully, these applications are not too common in the networks that you administer, and in such case, it should cause no huge problems.

The second and smaller problem is applications and protocols which will only work partially. These protocols are more common than the ones that will not work at all, which is quite unfortunate, but there isn't very much we can do about it as it seems. If complex protocols continue to be built, this is a problem we will have to continue living with. Especially if the protocols aren't standardized.

The third, and largest problem, in my point of view, is the fact that the user who sits behind a NAT server to get out on the internet will not be able to run his own server. It could be done, of course, but it takes a lot more time and work to set this up. In companies, this is probably preferred over having tons of servers run by different employees that are reachable from the Internet, without any supervision. However, when it comes to home users, this should be avoided to the very last. You should never as an Internet service provider NAT your customers from a private IP range to a public IP. It will cause you more trouble than it is worth having to deal with, and there will always be one or another client which will want this or that protocol to work flawlessly. When it doesn't, you will be called down upon.

As one last note on the caveats of NAT, it should be mentioned that NAT is actually just a hack more or less. NAT was a solution that was worked out while the IANA and other organisations noted that the Internet grew exponentially, and that the IP addresses would soon be in shortage. NAT was and is a short term solution to the problem of the IPv4 (Yes, IP which we have talked about before is a short version of IPv4 which stands for Internet Protocol version 4). The long term solution to the IPv4 address shortage is the IPv6 protocol, which also solves a ton of other problems. IPv6 has 128 bits assigned to their addresses, while IPv4 only have 32 bits used for IP addresses. This is an incredible increase in address space. It may seem like ridiculous to have enough IP addresses to set one IP address for every atom in our planet, but on the other hand, noone expected the IPv4 address range to be too small either.

4.3. Example NAT machine in theory

This is a small theoretical scenario where we want a NAT server between 2 different networks and an Internet connection. What we want to do is to connect 2 networks to each other, and both networks should have access to each other and the Internet. We will discuss the hardware questions you should take into consideration, as well as other theory you should think about before actually starting to implement the NAT machine.

4.4. What's next?

We have in this chapter in detail explained NAT and the theory around it. In special we have discussed a couple of different angles to use, and some of the normal problems that may arise from using NAT together with proxies. The following areas

• NAT usage

• NAT components

• NAT history

• Terms and words used about NAT

• Hardware discussions regarding NAT

• Problems with NAT

All of this will always be of use when you are working with netfilter and iptables. NAT is very widely used in today's networks, even though it is only an intermediary solution for a very unfortunate and unexpected problem. NAT will of course be discussed more in depth later on when we start looking at the Linux netfilter and iptables implementations in more depth.

5.1. Where to get iptables

The iptables user-space package can be downloaded from the http://www.netfilter.org/. The iptables package also makes use of kernel space facilities which can be configured into the kernel during make configure. The necessary steps will be discussed a bit further down in this document.

5.2. Kernel setup

To run the pure basics of iptables you need to configure the following options into the kernel while doing make config or one of its related commands:

CONFIG_PACKET - This option allows applications and utilities that need to work directly with various network devices. Examples of such utilities are tcpdump or snort.

CONFIG_PACKET is strictly speaking not needed for iptables to work, but since it contains so many uses, I have chosen to include it here. If you do not want it, don't include it.

CONFIG_NETFILTER - This option is required if you're going to use your computer as a firewall or gateway to the Internet. In other words, this is most definitely required for anything in this tutorial to work at all. I assume you will want this, since you are reading this.

And of course you need to add the proper drivers for your interfaces to work properly, i.e. Ethernet adapter, PPP and SLIP interfaces. The above will only add some of the pure basics in iptables. You won't be able to do anything productive to be honest, it just adds the framework to the kernel. If you want to use the more advanced options in Iptables, you need to set up the proper configuration options in your kernel. Here we will show you the options available in a basic 2.4.9 kernel and a brief explanation :

CONFIG_IP_NF_CONNTRACK - This module is needed to make connection tracking. Connection tracking is used by, among other things, NAT and Masquerading. If you need to firewall machines on a LAN you most definitely should mark this option. For example, this module is required by the rc.firewall.txt script to work.

CONFIG_IP_NF_FTP - This module is required if you want to do connection tracking on FTP connections. Since FTP connections are quite hard to do connection tracking on in normal cases, conntrack needs a so called helper; this option compiles the helper. If you do not add this module you won't be able to FTP through a firewall or gateway properly.

CONFIG_IP_NF_IPTABLES - This option is required if you want do any kind of filtering, masquerading or NAT. It adds the whole iptables identification framework to the kernel. Without this you won't be able to do anything at all with iptables.

CONFIG_IP_NF_MATCH_LIMIT - This module isn't exactly required but it's used in the example rc.firewall.txt. This option provides the LIMIT match, that adds the possibility to control how many packets per minute that are to be matched, governed by an appropriate rule. For example, -m limit --limit 3/minute would match a maximum of 3 packets per minute. This module can also be used to avoid certain Denial of Service attacks.

CONFIG_IP_NF_MATCH_MAC - This allows us to match packets based on MAC addresses. Every Ethernet adapter has its own MAC address. We could for instance block packets based on what MAC address is used and block a certain computer pretty well since the MAC address very seldom changes. We don't use this option in the rc.firewall.txt example or anywhere else.

CONFIG_IP_NF_MATCH_MARK - This allows us to use a MARK match. For example, if we use the target MARK we could mark a packet and then depending on if this packet is marked further on in the table, we can match based on this mark. This option is the actual match MARK, and further down we will describe the actual target MARK.

CONFIG_IP_NF_MATCH_MULTIPORT - This module allows us to match packets with a whole range of destination ports or source ports. Normally this wouldn't be possible, but with this match it is.

CONFIG_IP_NF_MATCH_TOS - With this match we can match packets based on their TOS field. TOS stands for Type Of Service. TOS can also be set by certain rules in the mangle table and via the ip/tc commands.

CONFIG_IP_NF_MATCH_TCPMSS - This option adds the possibility for us to match TCP packets based on their MSS field.

CONFIG_IP_NF_MATCH_STATE - This is one of the biggest news in comparison to ipchains. With this module we can do stateful matching on packets. For example, if we have already seen traffic in two directions in a TCP connection, this packet will be counted as ESTABLISHED. This module is used extensively in the rc.firewall.txt example.

CONFIG_IP_NF_MATCH_UNCLEAN - This module will add the possibility for us to match IP, TCP, UDP and ICMP packets that don't conform to type or are invalid. We could for example drop these packets, but we never know if they are legitimate or not. Note that this match is still experimental and might not work perfectly in all cases.

CONFIG_IP_NF_MATCH_OWNER - This option will add the possibility for us to do matching based on the owner of a socket. For example, we can allow only the user root to have Internet access. This module was originally just written as an example on what could be done with the new iptables. Note that this match is still experimental and might not work for everyone.

CONFIG_IP_NF_FILTER - This module will add the basic filter table which will enable you to do IP filtering at all. In the filter table you'll find the INPUT, FORWARD and OUTPUT chains. This module is required if you plan to do any kind of filtering on packets that you receive and send.

CONFIG_IP_NF_TARGET_REJECT - This target allows us to specify that an ICMP error message should be sent in reply to incoming packets, instead of plainly dropping them dead to the floor. Keep in mind that TCP connections, as opposed to ICMP and UDP, are always reset or refused with a TCP RST packet.

CONFIG_IP_NF_TARGET_MIRROR - This allows packets to be bounced back to the sender of the packet. For example, if we set up a MIRROR target on destination port HTTP on our INPUT chain and someone tries to access this port, we would bounce his packets back to him and finally he would probably see his own homepage.

The MIRROR target is not to be used lightly. It was originally built as a test and example module, and will most probably be very dangerous to the person setting it up (resulting in serious DDoS if among other things).

CONFIG_IP_NF_NAT - This module allows network address translation, or NAT, in its different forms. This option gives us access to the nat table in iptables. This option is required if we want to do port forwarding, masquerading, etc. Note that this option is not required for firewalling and masquerading of a LAN, but you should have it present unless you are able to provide unique IP addresses for all hosts. Hence, this option is required for the example rc.firewall.txt script to work properly, and most definitely on your network if you do not have the ability to add unique IP addresses as specified above.

CONFIG_IP_NF_TARGET_MASQUERADE - This module adds the MASQUERADE target. For instance if we don't know what IP we have to the Internet this would be the preferred way of getting the IP instead of using DNAT or SNAT. In other words, if we use DHCP, PPP, SLIP or some other connection that assigns us an IP, we need to use this target instead of SNAT. Masquerading gives a slightly higher load on the computer than NAT, but will work without us knowing the IP address in advance.

CONFIG_IP_NF_TARGET_REDIRECT - This target is useful together with application proxies, for example. Instead of letting a packet pass right through, we remap them to go to our local box instead. In other words, we have the possibility to make a transparent proxy this way.

CONFIG_IP_NF_TARGET_LOG - This adds the LOG target and its functionality to iptables. We can use this module to log certain packets to syslogd and hence see what is happening to the packet. This is invaluable for security audits, forensics or debugging a script you are writing.

CONFIG_IP_NF_TARGET_TCPMSS - This option can be used to counter Internet Service Providers and servers who block ICMP Fragmentation Needed packets. This can result in web-pages not getting through, small mails getting through while larger mails don't, ssh works but scp dies after handshake, etc. We can then use the TCPMSS target to overcome this by clamping our MSS (Maximum Segment Size) to the PMTU (Path Maximum Transmit Unit). This way, we'll be able to handle what the authors of Netfilter themselves call "criminally brain-dead ISPs or servers" in the kernel configuration help.

CONFIG_IP_NF_COMPAT_IPCHAINS - Adds a compatibility mode with the obsolescent ipchains. Do not look to this as any real long term solution for solving migration from Linux 2.2 kernels to 2.4 kernels, since it may well be gone with kernel 2.6.

CONFIG_IP_NF_COMPAT_IPFWADM - Compatibility mode with obsolescent ipfwadm. Definitely don't look to this as a real long term solution.

As you can see, there is a heap of options. I have briefly explained here what kind of extra behaviors you can expect from each module. These are only the options available in a vanilla Linux 2.4.9 kernel. If you would like to take a look at more options, I suggest you look at the patch-o-matic (POM) functions in Netfilter user-land which will add heaps of other options in the kernel. POM fixes are additions that are supposed to be added in the kernel in the future but have not quite reached the kernel yet. These functions should be added in the future, but have not quite made it in yet. This may be for various reasons - such as the patch not being stable yet, to Linus Torvalds being unable to keep up, or not wanting to let the patch in to the mainstream kernel yet since it is still experimental.

You will need the following options compiled into your kernel, or as modules, for the rc.firewall.txt script to work. If you need help with the options that the other scripts need, look at the example firewall scripts section.

• CONFIG_PACKET

• CONFIG_NETFILTER

• CONFIG_IP_NF_CONNTRACK

• CONFIG_IP_NF_FTP

• CONFIG_IP_NF_IRC

• CONFIG_IP_NF_IPTABLES

• CONFIG_IP_NF_FILTER

• CONFIG_IP_NF_NAT

• CONFIG_IP_NF_MATCH_STATE

• CONFIG_IP_NF_TARGET_LOG

• CONFIG_IP_NF_MATCH_LIMIT

• CONFIG_IP_NF_TARGET_MASQUERADE

At the very least the above will be required for the rc.firewall.txt script. In the other example scripts I will explain what requirements they have in their respective sections. For now, let's try to stay focused on the main script which you should be studying now.

5.3. User-land setup

First of all, let's look at how we compile the iptables package. It's important to realize that for the most part configuration and compilation of iptables goes hand in hand with the kernel configuration and compilation. Certain distributions come with the iptables package preinstalled, one of these is Red Hat. However, in Red Hat it is disabled per default. We will check closer on how to enable it and take a look at other distributions further on in this chapter.

---

5.3.1. Compiling the user-land applications

First of all unpack the iptables package. Here, we have used the iptables 1.2.6a package and a vanilla 2.4 kernel. Unpack as usual, using bzip2 -cd iptables-1.2.6a.tar.bz2 | tar -xvf - (this can also be accomplished with the tar -xjvf iptables-1.2.6a.tar.bz2, which should do pretty much the same as the first command. However, this may not work with older versions of tar). The package should now be unpacked properly into a directory named iptables-1.2.6a. For more information read the iptables-1.2.6a/INSTALL file which contains pretty good information on compiling and getting the program to run.

After this, there you have the option of configuring and installing extra modules and options etcetera for the kernel.The step described here will only check and install standard patches that are pending for inclusion to the kernel, there are some even more experimental patches further along, which may only be available when you carry out other steps.

Some of these patches are highly experimental and may not be such a good idea to install them. However, there are heaps of extremely interesting matches and targets in this installation step so don't be afraid of at least looking at them. To carry out this step we do something like this from the root of the iptables package:

make pending-patches KERNEL_DIR=/usr/src/linux/

The variable KERNEL_DIR should point to the actual place that your kernel source is located at. Normally this should be /usr/src/linux/ but this may vary, and most probably you will know yourself where the kernel source is available.

The above command only asks about certain patches that are just about to enter the kernel anyway. There might be more patches and additions that the developers of Netfilter are about to add to the kernel, but is a bit further away from actually getting there. One way to install these is by doing the following:

make most-of-pom KERNEL_DIR=/usr/src/linux/

The above command would ask about installing parts of what in Netfilter world is called patch-o-matic, but still skip the most extreme patches that might cause havoc in your kernel. Note that we say ask, because that's what these commands actually do. They ask you before anything is changed in the kernel source. To be able to install all of the patch-o-matic stuff you will need to run the following command:

make patch-o-matic KERNEL_DIR=/usr/src/linux/

Don't forget to read the help for each patch thoroughly before doing anything. Some patches will destroy other patches while others may destroy your kernel if used together with some patches from patch-o-matic etc.

You may totally ignore the above steps if you don't want to patch your kernel, it is in other words not necessary to do the above. However, there are some really interesting things in the patch-o-matic that you may want to look at so there's nothing bad in just running the commands and see what they contain.

After this you are finished doing the patch-o-matic parts of installation, you may now compile a new kernel making use of the new patches that you have added to the source. Don't forget to configure the kernel again since the new patches probably are not added to the configured options. You may wait with the kernel compilation until after the compilation of the user-land program iptables if you feel like it, though.

Continue by compiling the iptables user-land application. To compile iptables you issue a simple command that looks like this:

make KERNEL_DIR=/usr/src/linux/

The user-land application should now compile properly. If not, you are on your own, or you could subscribe to the Netfilter mailing list, where you have the chance of asking for help with your problems. There are a few things that might go wrong with the installation of iptables, so don't panic if it won't work. Try to think logically about it and find out what's wrong, or get someone to help you.

If everything has worked smoothly, you're ready to install the binaries by now. To do this, you would issue the following command to install them:

make install KERNEL_DIR=/usr/src/linux/

Hopefully everything should work in the program now. To use any of the changes in the iptables user-land applications you should now recompile and reinstall your kernel and modules, if you hadn't done so before. For more information about installing the user-land applications from source, check the INSTALL file in the source which contains excellent information on the subject of installation.

---

5.3.2. Installation on Red Hat 7.1

Red Hat 7.1 comes preinstalled with a 2.4.x kernel that has Netfilter and iptables compiled in. It also contains all the basic user-land programs and configuration files that are needed to run it. However, the Red Hat people have disabled the whole thing by using the backward compatible ipchains module. Annoying to say the least, and a lot of people keep asking different mailing lists why iptables doesn't work. So, let's take a brief look at how to turn the ipchains module off and how to install iptables instead.

The default Red Hat 7.1 installation today comes with a hopelessly old version of the user-space applications, so you might want to compile a new version of the applications as well as install a new and custom compiled kernel before fully exploiting iptables.

First of all you will need to turn off the ipchains modules so it won't start in the future. To do this, you will need to change some filenames in the /etc/rc.d/ directory-structure. The following command should do it:

chkconfig --level 0123456 ipchains off

By doing this we move all the soft links that points to the /etc/rc.d/init.d/ipchains script to K92ipchains. The first letter which per default would be S, tells the initscripts to start the script. By changing this to K we tell it to Kill the service instead, or to not run it if it was not previously started. Now the service won't be started in the future.

However, to stop the service from actually running right now we need to run another command. This is the service command which can be used to work on currently running services. We would then issue the following command to stop the ipchains service:

service ipchains stop

Finally, to start the iptables service. First of all, we need to know which run-levels we want it to run in. Normally this would be in run-level 2, 3 and 5. These run-levels are used for the following things:

• 2. Multiuser without NFS or the same as 3 if there is no networking.

• 3. Full multiuser mode, i.e. the normal run-level to run in.

• 5. X11. This is used if you automatically boot into Xwindows.

To make iptables run in these run-levels we would do the following commands:

chkconfig --level 235 iptables on

The above commands would in other words make the iptables service run in run-level 2, 3 and 5. If you'd like the iptables service to run in some other run-level you would have to issue the same command in those. However, none of the other run-levels should be used, so you should not really need to activate it for those run-levels. Level 1 is for single user mode, i.e, when you need to fix a screwed up box. Level 4 should be unused, and level 6 is for shutting the computer down.

To activate the iptables service, we just run the following command:

service iptables start

There are no rules in the iptables script. To add rules to an Red Hat 7.1 box, there is two common ways. Firstly, you could edit the /etc/rc.d/init.d/iptables script. This would have the undesired effect of deleting all the rules if you updated the iptables package by RPM. The other way would be to load the rule-set and then save it with the iptables-save command and then have it loaded automatically by the rc.d scripts.

First we will describe the how to set up iptables by cutting and pasting to the iptables init.d script. To add rules that are to be run when the computer starts the service, you add them under the start) section, or in the start() function. Note, if you add the rules under the start) section don't forget to stop the start() function in the start) section from running. Also, don't forget to edit a the stop) section either which tells the script what to do when the computer is going down for example, or when we are entering a run-level that doesn't require iptables. Also, don't forget to check out the restart section and condrestart. Note that all this work will probably be trashed if you have, for example, Red Hat Network automatically update your packages. It may also be trashed by updating from the iptables RPM package.

The second way of doing the set up would require the following: First of all, make and write a rule-set in a shell script file, or directly with iptables, that will meet your requirements, and don't forget to experiment a bit. When you find a set up that works without problems, or as you can see without bugs, use the iptables-save command. You could either use it normally, i.e. iptables-save > /etc/sysconfig/iptables, which would save the rule-set to the file /etc/sysconfig/iptables. This file is automatically used by the iptables rc.d script to restore the rule-set in the future. The other way is to save the script by doing service iptables save, which would save the script automatically to /etc/sysconfig/iptables. The next time you reboot the computer, the iptables rc.d script will use the command iptables-restore to restore the rule-set from the save-file /etc/sysconfig/iptables. Do not intermix these two methods, since they may heavily damage each other and render your firewall configuration useless.

When all of these steps are finished, you can deinstall the currently installed ipchains and iptables packages. This because we don't want the system to mix up the new iptables user-land application with the old preinstalled iptables applications. This step is only necessary if you are going to install iptables from the source package. It's not unusual for the new and the old package to get mixed up, since the rpm based installation installs the package in non-standard places and won't get overwritten by the installation for the new iptables package. To carry out the deinstallation, do as follows:

rpm -e iptables

And why keep ipchains lying around if you won't be using it any more? Removing it is done the same way as with the old iptables binaries, etc:

rpm -e ipchains

After all this has been completed, you will have finished with the update of the iptables package from source, having followed the source installation instructions. None of the old binaries, libraries or include files etc should be lying around any more.

6.1. General

When a packet first enters the firewall, it hits the hardware and then gets passed on to the proper device driver in the kernel. Then the packet starts to go through a series of steps in the kernel, before it is either sent to the correct application (locally), or forwarded to another host - or whatever happens to it.

First, let us have a look at a packet that is destined for our own local host. It would pass through the following steps before actually being delivered to our application that receives it:

Note that this time the packet was passed through the INPUT chain instead of the FORWARD chain. Quite logical. Most probably the only thing that's really logical about the traversing of tables and chains in your eyes in the beginning, but if you continue to think about it, you'll find it will get clearer in time.

Now we look at the outgoing packets from our own local host and what steps they go through.

In this example, we're assuming that the packet is destined for another host on another network. The packet goes through the different steps in the following fashion:

As you can see, there are quite a lot of steps to pass through. The packet can be stopped at any of the iptables chains, or anywhere else if it is malformed; however, we are mainly interested in the iptables aspect of this lot. Do note that there are no specific chains or tables for different interfaces or anything like that. FORWARD is always passed by all packets that are forwarded over this firewall/router.

Do not use the INPUT chain to filter on in the previous scenario! INPUT is meant solely for packets to our local host that do not get routed to any other destination.

We have now seen how the different chains are traversed in three separate scenarios. If we were to figure out a good map of all this, it would look something like this:

To clarify this image, consider this. If we get a packet into the first routing decision that is not destined for the local machine itself, it will be routed through the FORWARD chain. If the packet is, on the other hand, destined for an IP address that the local machine is listening to, we would send the packet through the INPUT chain and to the local machine.

Also worth a note, is the fact that packets may be destined for the local machine, but the destination address may be changed within the PREROUTING chain by doing NAT. Since this takes place before the first routing decision, the packet will be looked upon after this change. Because of this, the routing may be changed before the routing decision is done. Do note, that all packets will be going through one or the other path in this image. If you DNAT a packet back to the same network that it came from, it will still travel through the rest of the chains until it is back out on the network.

If you feel that you want more information, you could use the rc.test-iptables.txt script. This test script should give you the necessary rules to test how the tables and chains are traversed.

6.2. mangle table

This table should as we've already noted mainly be used for mangling packets. In other words, you may freely use the mangle targets within this table, to change TOS (Type Of Service) fields and the like.

You are strongly advised not to use this table for any filtering; nor will any DNAT, SNAT or Masquerading work in this table.

The following targets are only valid in the mangle table. They can not be used outside the mangle table.

• TOS

• TTL

• MARK

The TOS target is used to set and/or change the Type of Service field in the packet. This could be used for setting up policies on the network regarding how a packet should be routed and so on. Note that this has not been perfected and is not really implemented on the Internet and most of the routers don't care about the value in this field, and sometimes, they act faulty on what they get. Don't set this in other words for packets going to the Internet unless you want to make routing decisions on it, with iproute2.

The TTL target is used to change the TTL (Time To Live) field of the packet. We could tell packets to only have a specific TTL and so on. One good reason for this could be that we don't want to give ourself away to nosy Internet Service Providers. Some Internet Service Providers do not like users running multiple computers on one single connection, and there are some Internet Service Providers known to look for a single host generating different TTL values, and take this as one of many signs of multiple computers connected to a single connection.

The MARK target is used to set special mark values to the packet. These marks could then be recognized by the iproute2 programs to do different routing on the packet depending on what mark they have, or if they don't have any. We could also do bandwidth limiting and Class Based Queuing based on these marks.

6.3. nat table

This table should only be used for NAT (Network Address Translation) on different packets. In other words, it should only be used to translate the packet's source field or destination field. Note that, as we have said before, only the first packet in a stream will hit this table. After this, the rest of the packets will automatically have the same action taken on them as the first packet. The actual targets that do these kind of things are:

• DNAT

• SNAT

• MASQUERADE

• REDIRECT

The DNAT target is mainly used in cases where you have a public IP and want to redirect accesses to the firewall to some other host (on a DMZ for example). In other words, we change the destination address of the packet and reroute it to the host.

SNAT is mainly used for changing the source address of packets. For the most part you'll hide your local networks or DMZ, etc. A very good example would be that of a firewall of which we know outside IP address, but need to substitute our local network's IP numbers with that of our firewall. With this target the firewall will automatically SNAT and De-SNAT the packets, hence making it possible to make connections from the LAN to the Internet. If your network uses 192.168.0.0/netmask for example, the packets would never get back from the Internet, because IANA has regulated these networks (among others) as private and only for use in isolated LANs.

The MASQUERADE target is used in exactly the same way as SNAT, but the MASQUERADE target takes a little bit more overhead to compute. The reason for this, is that each time that the MASQUERADE target gets hit by a packet, it automatically checks for the IP address to use, instead of doing as the SNAT target does - just using the single configured IP address. The MASQUERADE target makes it possible to work properly with Dynamic DHCP IP addresses that your ISP might provide for your PPP, PPPoE or SLIP connections to the Internet.

6.4. Filter table

The filter table is mainly used for filtering packets. We can match packets and filter them in whatever way we want. This is the place that we actually take action against packets and look at what they contain and DROP or /ACCEPT them, depending on their content. Of course we may also do prior filtering; however, this particular table is the place for which filtering was designed. Almost all targets are usable in this table. We will be more prolific about the filter table here; however you now know that this table is the right place to do your main filtering.

7.1. Introduction

The state machine is a special part within iptables that should really not be called the state machine at all, since it is really a connection tracking machine. However, most people recognize it under the first name. Throughout this chapter I will use these names more or less as if they were synonymous. This should not be overly confusing. Connection tracking is done to let the Netfilter framework know the state of a specific connection. Firewalls that implement this are generally called stateful firewalls. A stateful firewall is generally much more secure than non-stateful firewalls since it allows us to write much tighter rule-sets.

Within iptables, packets can be related to tracked connections in four different so called states. These are known as NEW, ESTABLISHED, RELATED and INVALID. We will discuss each of these in more depth later. With the --state match we can easily control who or what is allowed to initiate new sessions.

All of the connection tracking is done by special framework within the kernel called conntrack. conntrack may be loaded either as a module, or as an internal part of the kernel itself. Most of the time, we need and want more specific connection tracking than the default conntrack engine can maintain. Because of this, there are also more specific parts of conntrack that handles the TCP, UDP or ICMP protocols among others. These modules grab specific, unique, information from the packets, so that they may keep track of each stream of data. The information that conntrack gathers is then used to tell conntrack in which state the stream is currently in. For example, UDP streams are, generally, uniquely identified by their destination IP address, source IP address, destination port and source port.

In previous kernels, we had the possibility to turn on and off defragmentation. However, since iptables and Netfilter were introduced and connection tracking in particular, this option was gotten rid of. The reason for this is that connection tracking can not work properly without defragmenting packets, and hence defragmenting has been incorporated into conntrack and is carried out automatically. It can not be turned off, except by turning off connection tracking. Defragmentation is always carried out if connection tracking is turned on.

All connection tracking is handled in the PREROUTING chain, except locally generated packets which are handled in the OUTPUT chain. What this means is that iptables will do all recalculation of states and so on within the PREROUTING chain. If we send the initial packet in a stream, the state gets set to NEW within the OUTPUT chain, and when we receive a return packet, the state gets changed in the PREROUTING chain to ESTABLISHED, and so on. If the first packet is not originated by ourself, the NEW state is set within the PREROUTING chain of course. So, all state changes and calculations are done within the PREROUTING and OUTPUT chains of the nat table.

7.2. The conntrack entries

Let's take a brief look at a conntrack entry and how to read them in /proc/net/ip_conntrack. This gives a list of all the current entries in your conntrack database. If you have the ip_conntrack module loaded, a cat of /proc/net/ip_conntrack might look like:

tcp      6 117 SYN_SENT src=192.168.1.6 dst=192.168.1.9 sport=32775 \
     dport=22 [UNREPLIED] src=192.168.1.9 dst=192.168.1.6 sport=22 \
     dport=32775 [ASSURED] use=2
     

This example contains all the information that the conntrack module maintains to know which state a specific connection is in. First of all, we have a protocol, which in this case is tcp. Next, the same value in normal decimal coding. After this, we see how long this conntrack entry has to live. This value is set to 117 seconds right now and is decremented regularly until we see more traffic. This value is then reset to the default value for the specific state that it is in at that relevant point of time. Next comes the actual state that this entry is in at the present point of time. In the above mentioned case we are looking at a packet that is in the SYN_SENT state. The internal value of a connection is slightly different from the ones used externally with iptables. The value SYN_SENT tells us that we are looking at a connection that has only seen a TCP SYN packet in one direction. Next, we see the source IP address, destination IP address, source port and destination port. At this point we see a specific keyword that tells us that we have seen no return traffic for this connection. Lastly, we see what we expect of return packets. The information details the source IP address and destination IP address (which are both inverted, since the packet is to be directed back to us). The same thing goes for the source port and destination port of the connection. These are the values that should be of any interest to us.

The connection tracking entries may take on a series of different values, all specified in the conntrack headers available in linux/include/netfilter-ipv4/ip_conntrack*.h files. These values are dependent on which sub-protocol of IP we use. TCP, UDP or ICMP protocols take specific default values as specified in linux/include/netfilter-ipv4/ip_conntrack.h. We will look closer at this when we look at each of the protocols; however, we will not use them extensively through this chapter, since they are not used outside of the conntrack internals. Also, depending on how this state changes, the default value of the time until the connection is destroyed will also change.

Recently there was a new patch made available in iptables patch-o-matic, called tcp-window-tracking. This patch adds, among other things, all of the above timeouts to special sysctl variables, which means that they can be changed on the fly, while the system is still running. Hence, this makes it unnecessary to recompile the kernel every time you want to change the timeouts. These can be altered via using specific system calls available in the /proc/sys/net/ipv4/netfilter directory. You should in particular look at the /proc/sys/net/ipv4/netfilter/ip_ct_* variables.

When a connection has seen traffic in both directions, the conntrack entry will erase the [UNREPLIED] flag, and then reset it. The entry that tells us that the connection has not seen any traffic in both directions, will be replaced by the [ASSURED] flag, to be found close to the end of the entry. The [ASSURED] flag tells us that this connection is assured and that it will not be erased if we reach the maximum possible tracked connections. Thus, connections marked as [ASSURED] will not be erased, contrary to the non-assured connections (those not marked as [ASSURED]). How many connections that the connection tracking table can hold depends upon a variable that can be set through the ip-sysctl functions in recent kernels. The default value held by this entry varies heavily depending on how much memory you have. On 128 MB of RAM you will get 8192 possible entries, and at 256 MB of RAM, you will get 16376 entries. You can read and set your settings through the /proc/sys/net/ipv4/ip_conntrack_max setting.

A different way of doing this, that is more efficient, is to set the hashsize option to the ip_conntrack module once this is loaded. Under normal circumstances ip_conntrack_max equals 8 * hashsize. In other words, setting the hashsize to 4096 will result in ip_conntrack_max being set to 32768 conntrack entries. An example of this would be:

work3:/home/blueflux# modprobe ip_conntrack hashsize=4096
work3:/home/blueflux# cat /proc/sys/net/ipv4/ip_conntrack_max
32768
work3:/home/blueflux#
      

7.3. User-land states

As you have seen, packets may take on several different states within the kernel itself, depending on what protocol we are talking about. However, outside the kernel, we only have the 4 states as described previously. These states can mainly be used in conjunction with the state match which will then be able to match packets based on their current connection tracking state. The valid states are NEW, ESTABLISHED, RELATED and INVALID. The following table will briefly explain each possible state.

These states can be used together with the --state match to match packets based on their connection tracking state. This is what makes the state machine so incredibly strong and efficient for our firewall. Previously, we often had to open up all ports above 1024 to let all traffic back into our local networks again. With the state machine in place this is not necessary any longer, since we can now just open up the firewall for return traffic and not for all kinds of other traffic.

7.4. TCP connections

In this section and the upcoming ones, we will take a closer look at the states and how they are handled for each of the three basic protocols TCP, UDP and ICMP. Also, we will take a closer look at how connections are handled per default, if they can not be classified as either of these three protocols. We have chosen to start out with the TCP protocol since it is a stateful protocol in itself, and has a lot of interesting details with regard to the state machine in iptables.

A TCP connection is always initiated with the 3-way handshake, which establishes and negotiates the actual connection over which data will be sent. The whole session is begun with a SYN packet, then a SYN/ACK packet and finally an ACK packet to acknowledge the whole session establishment. At this point the connection is established and able to start sending data. The big problem is, how does connection tracking hook up into this? Quite simply really.

As far as the user is concerned, connection tracking works basically the same for all connection types. Have a look at the picture below to see exactly what state the stream enters during the different stages of the connection. As you can see, the connection tracking code does not really follow the flow of the TCP connection, from the users viewpoint. Once it has seen one packet(the SYN), it considers the connection as NEW. Once it sees the return packet(SYN/ACK), it considers the connection as ESTABLISHED. If you think about this a second, you will understand why. With this particular implementation, you can allow NEW and ESTABLISHED packets to leave your local network, only allow ESTABLISHED connections back, and that will work perfectly. Conversely, if the connection tracking machine were to consider the whole connection establishment as NEW, we would never really be able to stop outside connections to our local network, since we would have to allow NEW packets back in again. To make things more complicated, there are a number of other internal states that are used for TCP connections inside the kernel, but which are not available for us in User-land. Roughly, they follow the state standards specified within RFC 793 - Transmission Control Protocol on pages 21-23. We will consider these in more detail further along in this section.

As you can see, it is really quite simple, seen from the user's point of view. However, looking at the whole construction from the kernel's point of view, it's a little more difficult. Let's look at an example. Consider exactly how the connection states change in the /proc/net/ip_conntrack table. The first state is reported upon receipt of the first SYN packet in a connection.

tcp      6 117 SYN_SENT src=192.168.1.5 dst=192.168.1.35 sport=1031 \
     dport=23 [UNREPLIED] src=192.168.1.35 dst=192.168.1.5 sport=23 \
     dport=1031 use=1
   

As you can see from the above entry, we have a precise state in which a SYN packet has been sent, (the SYN_SENT flag is set), and to which as yet no reply has been sent (witness the [UNREPLIED] flag). The next internal state will be reached when we see another packet in the other direction.

tcp      6 57 SYN_RECV src=192.168.1.5 dst=192.168.1.35 sport=1031 \
     dport=23 src=192.168.1.35 dst=192.168.1.5 sport=23 dport=1031 \
     use=1
   

Now we have received a corresponding SYN/ACK in return. As soon as this packet has been received, the state changes once again, this time to SYN_RECV. SYN_RECV tells us that the original SYN was delivered correctly and that the SYN/ACK return packet also got through the firewall properly. Moreover, this connection tracking entry has now seen traffic in both directions and is hence considered as having been replied to. This is not explicit, but rather assumed, as was the [UNREPLIED] flag above. The final step will be reached once we have seen the final ACK in the 3-way handshake.

tcp      6 431999 ESTABLISHED src=192.168.1.5 dst=192.168.1.35 \
     sport=1031 dport=23 src=192.168.1.35 dst=192.168.1.5 \
     sport=23 dport=1031 [ASSURED] use=1
   

In the last example, we have gotten the final ACK in the 3-way handshake and the connection has entered the ESTABLISHED state, as far as the internal mechanisms of iptables are aware. Normally, the stream will be ASSURED by now.

A connection may also enter the ESTABLISHED state, but not be [ASSURED]. This happens if we have connection pickup turned on (Requires the tcp-window-tracking patch, and the ip_conntrack_tcp_loose to be set to 1 or higher). The default, without the tcp-window-tracking patch, is to have this behaviour, and is not changeable.

When a TCP connection is closed down, it is done in the following way and takes the following states.

As you can see, the connection is never really closed until the last ACK is sent. Do note that this picture only describes how it is closed down under normal circumstances. A connection may also, for example, be closed by sending a RST(reset), if the connection were to be refused. In this case, the connection would be closed down immediately.

When the TCP connection has been closed down, the connection enters the TIME_WAIT state, which is per default set to 2 minutes. This is used so that all packets that have gotten out of order can still get through our rule-set, even after the connection has already closed. This is used as a kind of buffer time so that packets that have gotten stuck in one or another congested router can still get to the firewall, or to the other end of the connection.

If the connection is reset by a RST packet, the state is changed to CLOSE. This means that the connection per default has 10 seconds before the whole connection is definitely closed down. RST packets are not acknowledged in any sense, and will break the connection directly. There are also other states than the ones we have told you about so far. Here is the complete list of possible states that a TCP stream may take, and their timeout values.

These values are most definitely not absolute. They may change with kernel revisions, and they may also be changed via the proc file-system in the /proc/sys/net/ipv4/netfilter/ip_ct_tcp_* variables. The default values should, however, be fairly well established in practice. These values are set in seconds. Early versions of the patch used jiffies (which was a bug).

Also note that the User-land side of the state machine does not look at TCP flags (i.e., RST, ACK, and SYN are flags) set in the TCP packets. This is generally bad, since you may want to allow packets in the NEW state to get through the firewall, but when you specify the NEW flag, you will in most cases mean SYN packets. This is not what happens with the current state implementation; instead, even a packet with no bit set or an ACK flag, will count as NEW. This can be used for redundant firewalling and so on, but it is generally extremely bad on your home network, where you only have a single firewall. To get around this behavior, you could use the command explained in the State NEW packets but no SYN bit set section of the Common problems and questions appendix. Another way is to install the tcp-window-tracking extension from patch-o-matic, and set the /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_loose to zero, which will make the firewall drop all NEW packets with anything but the SYN flag set.

7.5. UDP connections

UDP connections are in themselves not stateful connections, but rather stateless. There are several reasons why, mainly because they don't contain any connection establishment or connection closing; most of all they lack sequencing. Receiving two UDP datagrams in a specific order does not say anything about the order in which they were sent. It is, however, still possible to set states on the connections within the kernel. Let's have a look at how a connection can be tracked and how it might look in conntrack.

As you can see, the connection is brought up almost exactly in the same way as a TCP connection. That is, from the user-land point of view. Internally, conntrack information looks quite a bit different, but intrinsically the details are the same. First of all, let's have a look at the entry after the initial UDP packet has been sent.

udp      17 20 src=192.168.1.2 dst=192.168.1.5 sport=137 dport=1025 \
     [UNREPLIED] src=192.168.1.5 dst=192.168.1.2 sport=1025 \
     dport=137 use=1
   

As you can see from the first and second values, this is an UDP packet. The first is the protocol name, and the second is protocol number. This is just the same as for TCP connections. The third value marks how many seconds this state entry has to live. After this, we get the values of the packet that we have seen and the future expectations of packets over this connection reaching us from the initiating packet sender. These are the source, destination, source port and destination port. At this point, the [UNREPLIED] flag tells us that there's so far been no response to the packet. Finally, we get a brief list of the expectations for returning packets. Do note that the latter entries are in reverse order to the first values. The timeout at this point is set to 30 seconds, as per default.

udp      17 170 src=192.168.1.2 dst=192.168.1.5 sport=137 \
     dport=1025 src=192.168.1.5 dst=192.168.1.2 sport=1025 \
     dport=137 [ASSURED] use=1
   

At this point the server has seen a reply to the first packet sent out and the connection is now considered as ESTABLISHED. This is not shown in the connection tracking, as you can see. The main difference is that the [UNREPLIED] flag has now gone. Moreover, the default timeout has changed to 180 seconds - but in this example that's by now been decremented to 170 seconds - in 10 seconds' time, it will be 160 seconds. There's one thing that's missing, though, and can change a bit, and that is the [ASSURED] flag described above. For the [ASSURED] flag to be set on a tracked connection, there must have been a legitimate reply packet to the NEW packet.

udp      17 175 src=192.168.1.5 dst=195.22.79.2 sport=1025 \
     dport=53 src=195.22.79.2 dst=192.168.1.5 sport=53 \
     dport=1025 [ASSURED] use=1
   

At this point, the connection has become assured. The connection looks exactly the same as the previous example. If this connection is not used for 180 seconds, it times out. 180 Seconds is a comparatively low value, but should be sufficient for most use. This value is reset to its full value for each packet that matches the same entry and passes through the firewall, just the same as for all of the internal states.

7.6. ICMP connections

ICMP packets are far from a stateful stream, since they are only used for controlling and should never establish any connections. There are four ICMP types that will generate return packets however, and these have 2 different states. These ICMP messages can take the NEW and ESTABLISHED states. The ICMP types we are talking about are Echo request and reply, Timestamp request and reply, Information request and reply and finally Address mask request and reply. Out of these, the timestamp request and information request are obsolete and could most probably just be dropped. However, the Echo messages are used in several setups such as pinging hosts. Address mask requests are not used often, but could be useful at times and worth allowing. To get an idea of how this could look, have a look at the following image.

As you can see in the above picture, the host sends an echo request to the target, which is considered as NEW by the firewall. The target then responds with a echo reply which the firewall considers as state ESTABLISHED. When the first echo request has been seen, the following state entry goes into the ip_conntrack.

icmp     1 25 src=192.168.1.6 dst=192.168.1.10 type=8 code=0 \
     id=33029 [UNREPLIED] src=192.168.1.10 dst=192.168.1.6 \
     type=0 code=0 id=33029 use=1
   

This entry looks a little bit different from the standard states for TCP and UDP as you can see. The protocol is there, and the timeout, as well as source and destination addresses. The problem comes after that however. We now have 3 new fields called type, code and id. They are not special in any way, the type field contains the ICMP type and the code field contains the ICMP code. These are all available in ICMP types appendix. The final id field, contains the ICMP ID. Each ICMP packet gets an ID set to it when it is sent, and when the receiver gets the ICMP message, it sets the same ID within the new ICMP message so that the sender will recognize the reply and will be able to connect it with the correct ICMP request.

The next field, we once again recognize as the [UNREPLIED] flag, which we have seen before. Just as before, this flag tells us that we are currently looking at a connection tracking entry that has seen only traffic in one direction. Finally, we see the reply expectation for the reply ICMP packet, which is the inversion of the original source and destination IP addresses. As for the type and code, these are changed to the correct values for the return packet, so an echo request is changed to echo reply and so on. The ICMP ID is preserved from the request packet.

The reply packet is considered as being ESTABLISHED, as we have already explained. However, we can know for sure that after the ICMP reply, there will be absolutely no more legal traffic in the same connection. For this reason, the connection tracking entry is destroyed once the reply has traveled all the way through the Netfilter structure.

In each of the above cases, the request is considered as NEW, while the reply is considered as ESTABLISHED. Let's consider this more closely. When the firewall sees a request packet, it considers it as NEW. When the host sends a reply packet to the request it is considered ESTABLISHED.

Note that this means that the reply packet must match the criterion given by the connection tracking entry to be considered as established, just as with all other traffic types.

ICMP requests has a default timeout of 30 seconds, which you can change in the /proc/sys/net/ipv4/netfilter/ip_ct_icmp_timeout entry. This should in general be a good timeout value, since it will be able to catch most packets in transit.

Another hugely important part of ICMP is the fact that it is used to tell the hosts what happened to specific UDP and TCP connections or connection attempts. For this simple reason, ICMP replies will very often be recognized as RELATED to original connections or connection attempts. A simple example would be the ICMP Host unreachable or ICMP Network unreachable. These should always be spawned back to our host if it attempts an unsuccessful connection to some other host, but the network or host in question could be down, and hence the last router trying to reach the site in question will reply with an ICMP message telling us about it. In this case, the ICMP reply is considered as a RELATED packet. The following picture should explain how it would look.

In the above example, we send out a SYN packet to a specific address. This is considered as a NEW connection by the firewall. However, the network the packet is trying to reach is unreachable, so a router returns a network unreachable ICMP error to us. The connection tracking code can recognize this packet as RELATED. thanks to the already added tracking entry, so the ICMP reply is correctly sent to the client which will then hopefully abort. Meanwhile, the firewall has destroyed the connection tracking entry since it knows this was an error message.

The same behavior as above is experienced with UDP connections if they run into any problem like the above. All ICMP messages sent in reply to UDP connections are considered as RELATED. Consider the following image.

This time an UDP packet is sent to the host. This UDP connection is considered as NEW. However, the network is administratively prohibited by some firewall or router on the way over. Hence, our firewall receives a ICMP Network Prohibited in return. The firewall knows that this ICMP error message is related to the already opened UDP connection and sends it as a RELATED packet to the client. At this point, the firewall destroys the connection tracking entry, and the client receives the ICMP message and should hopefully abort.

7.7. Default connections

In certain cases, the conntrack machine does not know how to handle a specific protocol. This happens if it does not know about that protocol in particular, or doesn't know how it works. In these cases, it goes back to a default behavior. The default behavior is used on, for example, NETBLT, MUX and EGP. This behavior looks pretty much the same as the UDP connection tracking. The first packet is considered NEW, and reply traffic and so forth is considered ESTABLISHED.

When the default behavior is used, all of these packets will attain the same default timeout value. This can be set via the /proc/sys/net/ipv4/netfilter/ip_ct_generic_timeout variable. The default value here is 600 seconds, or 10 minutes. Depending on what traffic you are trying to send over a link that uses the default connection tracking behavior, this might need changing. Especially if you are bouncing traffic through satellites and such, which can take a long time.

7.8. Complex protocols and connection tracking

Certain protocols are more complex than others. What this means when it comes to connection tracking, is that such protocols may be harder to track correctly. Good examples of these are the ICQ, IRC and FTP protocols. Each and every one of these protocols carries information within the actual data payload of the packets, and hence requires special connection tracking helpers to enable it to function correctly.

This is a list of the complex protocols that has support inside the linux kernel, and which kernel version it was introduced in.

• FTP

• IRC

• TFTP

Let's take the FTP protocol as the first example. The FTP protocol first opens up a single connection that is called the FTP control session. When we issue commands through this session, other ports are opened to carry the rest of the data related to that specific command. These connections can be done in two ways, either actively or passively. When a connection is done actively, the FTP client sends the server a port and IP address to connect to. After this, the FTP client opens up the port and the server connects to that specified port from a random unprivileged port (>1024) and sends the data over it.

The problem here is that the firewall will not know about these extra connections, since they were negotiated within the actual payload of the protocol data. Because of this, the firewall will be unable to know that it should let the server connect to the client over these specific ports.

The solution to this problem is to add a special helper to the connection tracking module which will scan through the data in the control connection for specific syntaxes and information. When it runs into the correct information, it will add that specific information as RELATED and the server will be able to track the connection, thanks to that RELATED entry. Consider the following picture to understand the states when the FTP server has made the connection back to the client.

Passive FTP works the opposite way. The FTP client tells the server that it wants some specific data, upon which the server replies with an IP address to connect to and at what port. The client will, upon receipt of this data, connect to that specific port, from its own port 20(the FTP-data port), and get the data in question. If you have an FTP server behind your firewall, you will in other words require this module in addition to your standard iptables modules to let clients on the Internet connect to the FTP server properly. The same goes if you are extremely restrictive to your users, and only want to let them reach HTTP and FTP servers on the Internet and block all other ports. Consider the following image and its bearing on Passive FTP.

Some conntrack helpers are already available within the kernel itself. More specifically, the FTP and IRC protocols have conntrack helpers as of writing this. If you can not find the conntrack helpers that you need within the kernel itself, you should have a look at the patch-o-matic tree within user-land iptables. The patch-o-matic tree may contain more conntrack helpers, such as for the ntalk or H.323 protocols. If they are not available in the patch-o-matic tree, you have a number of options. Either you can look at the CVS source of iptables, if it has recently gone into that tree, or you can contact the Netfilter-devel mailing list and ask if it is available. If it is not, and there are no plans for adding it, you are left to your own devices and would most probably want to read the Rusty Russell's Unreliable Netfilter Hacking HOW-TO which is linked from the Other resources and links appendix.

Conntrack helpers may either be statically compiled into the kernel, or as modules. If they are compiled as modules, you can load them with the following command

modprobe ip_conntrack_ftp
modprobe ip_conntrack_irc
modprobe ip_conntrack_tftp
modprobe ip_conntrack_amanda
   

Do note that connection tracking has nothing to do with NAT, and hence you may require more modules if you are NAT'ing connections as well. For example, if you were to want to NAT and track FTP connections, you would need the NAT module as well. All NAT helpers starts with ip_nat_ and follow that naming convention; so for example the FTP NAT helper would be named ip_nat_ftp and the IRC module would be named ip_nat_irc. The conntrack helpers follow the same naming convention, and hence the IRC conntrack helper would be named ip_conntrack_irc, while the FTP conntrack helper would be named ip_conntrack_ftp.

8.1. Speed considerations

One of the largest reasons for using the iptables-save and iptables-restore commands is that they will speed up the loading and saving of larger rule-sets considerably. The main problem with running a shell script that contains iptables rules is that each invocation of iptables within the script will first extract the whole rule-set from the Netfilter kernel space, and after this, it will insert or append rules, or do whatever change to the rule-set that is needed by this specific command. Finally, it will insert the new rule-set from its own memory into kernel space. Using a shell script, this is done for each and every rule that we want to insert, and for each time we do this, it takes more time to extract and insert the rule-set.

To solve this problem, there is the iptables-save and restore commands. The iptables-save command is used to save the rule-set into a specially formatted text-file, and the iptables-restore command is used to load this text-file into kernel again. The best parts of these commands is that they will load and save the rule-set in one single request. iptables-save will grab the whole rule-set from kernel and save it to a file in one single movement. iptables-restore will upload that specific rule-set to kernel in a single movement for each table. In other words, instead of dropping the rule-set out of kernel some 30,000 times, for really large rule-sets, and then upload it to kernel again that many times, we can now save the whole thing into a file in one movement and then upload the whole thing in as little as three movements depending on how many tables you use.

As you can understand, these tools are definitely something for you if you are working on a huge set of rules that needs to be inserted. However, they do have drawbacks that we will discuss more in the next section.

8.2. Drawbacks with restore

As you may have already wondered, can iptables-restore handle any kind of scripting? So far, no, it cannot and it will most probably never be able to. This is the main flaw in using iptables-restore since you will not be able to do a huge set of things with these files. For example, what if you have a connection that has a dynamically assigned IP address and you want to grab this dynamic IP every-time the computer boots up and then use that value within your scripts? With iptables-restore, this is more or less impossible.

One possibility to get around this is to make a small script which grabs the values you would like to use in the script, then sed the iptables-restore file for specific keywords and replace them with the values collected via the small script. At this point, you could save it to a temporary file, and then use iptables-restore to load the new values. This causes a lot of problems however, and you will be unable to use iptables-save properly since it would probably erase your manually added keywords in the restore script. It is, in other words, a clumsy solution.

Another solution is to load the iptables-restore scripts first, and then load a specific shell script that inserts more dynamic rules in their proper places. Of course, as you can understand, this is just as clumsy as the first solution. iptables-restore is simply not very well suited for configurations where IP addresses are dynamically assigned to your firewall or where you want different behaviors depending on configuration options and so on.

Another drawback with iptables-restore and iptables-save is that it is not fully functional as of writing this. The problem is simply that not a lot of people use it as of today and hence there are not a lot of people finding bugs, and in turn some matches and targets will simply be inserted badly, which may lead to some strange behaviors that you did not expect. Even though these problems exist, I would highly recommend using these tools which should work extremely well for most rule-sets as long as they do not contain some of the new targets or matches that it does not know how to handle properly.

8.3. iptables-save

The iptables-save command is, as we have already explained, a tool to save the current rule-set into a file that iptables-restore can use. This command is quite simple really, and takes only two arguments. Take a look at the following example to understand the syntax of the command.

iptables-save [-c] [-t table]

The -c argument tells iptables-save to keep the values specified in the byte and packet counters. This could for example be useful if we would like to reboot our main firewall, but not lose byte and packet counters which we may use for statistical purposes. Issuing a iptables-save command with the -c argument would then make it possible for us to reboot without breaking our statistical and accounting routines. The default value is, of course, to not keep the counters intact when issuing this command.

The -t argument tells the iptables-save command which tables to save. Without this argument the command will automatically save all tables available into the file. The following is an example on what output you can expect from the iptables-save command if you do not have any rule-set loaded.

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002
*filter
:INPUT ACCEPT [404:19766]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [530:43376]
COMMIT
# Completed on Wed Apr 24 10:19:17 2002
# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002
*mangle
:PREROUTING ACCEPT [451:22060]
:INPUT ACCEPT [451:22060]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [594:47151]
:POSTROUTING ACCEPT [594:47151]
COMMIT
# Completed on Wed Apr 24 10:19:17 2002
# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002
*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [3:450]
:OUTPUT ACCEPT [3:450]
COMMIT
# Completed on Wed Apr 24 10:19:17 2002

This contains a few comments starting with a # sign. Each table is marked like *<table-name>, for example *mangle. Then within each table we have the chain specifications and rules. A chain specification looks like :<chain-name> <chain-policy> [<packet-counter>:<byte-counter>]. The chain-name may be for example PREROUTING, the policy is described previously and can, for example, be ACCEPT. Finally the packet-counter and byte-counters are the same counters as in the output from iptables -L -v. Finally, each table declaration ends in a COMMIT keyword. The COMMIT keyword tells us that at this point we should commit all rules currently in the pipeline to kernel.

The above example is pretty basic, and hence I believe it is nothing more than proper to show a brief example which contains a very small Iptables-save ruleset. If we would run iptables-save on this, it would look something like this in the output:

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002
*filter
:INPUT DROP [1:229]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A FORWARD -i eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A FORWARD -i eth1 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT 
-A OUTPUT -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT 
COMMIT
# Completed on Wed Apr 24 10:19:55 2002
# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002
*mangle
:PREROUTING ACCEPT [658:32445]
:INPUT ACCEPT [658:32445]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [891:68234]
:POSTROUTING ACCEPT [891:68234]
COMMIT
# Completed on Wed Apr 24 10:19:55 2002
# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002
*nat
:PREROUTING ACCEPT [1:229]
:POSTROUTING ACCEPT [3:450]
:OUTPUT ACCEPT [3:450]
-A POSTROUTING -o eth0 -j SNAT --to-source 195.233.192.1 
COMMIT
# Completed on Wed Apr 24 10:19:55 2002

As you can see, each command has now been prefixed with the byte and packet counters since we used the -c argument. Except for this, the command-line is quite intact from the script. The only problem now, is how to save the output to a file. Quite simple, and you should already know how to do this if you have used linux at all before. It is only a matter of piping the command output on to the file that you would like to save it as. This could look like the following:

iptables-save -c > /etc/iptables-save

The above command will in other words save the whole rule-set to a file called /etc/iptables-save with byte and packet counters still intact.

8.4. iptables-restore

The iptables-restore command is used to restore the iptables rule-set that was saved with the iptables-save command. It takes all the input from standard input and can't load from files as of writing this, unfortunately. This is the command syntax for iptables-restore:

iptables-restore [-c] [-n]

The -c argument restores the byte and packet counters and must be used if you want to restore counters that were previously saved with iptables-save. This argument may also be written in its long form --counters.

The -n argument tells iptables-restore to not overwrite the previously written rules in the table, or tables, that it is writing to. The default behavior of iptables-restore is to flush and destroy all previously inserted rules. The short -n argument may also be replaced with the longer format --noflush.

To load a rule-set with the iptables-restore command, we could do this in several ways, but we will mainly look at the simplest and most common way here.

    cat /etc/iptables-save | iptables-restore -c
   

The following will also work:

     iptables-restore -c < /etc/iptables-save
   

This would cat the rule-set located within the /etc/iptables-save file and then pipe it to iptables-restore which takes the rule-set on the standard input and then restores it, including byte and packet counters. It is that simple to begin with. This command could be varied until oblivion and we could show different piping possibilities, however, this is a bit out of the scope of this chapter, and hence we will skip that part and leave it as an exercise for the reader to experiment with.

The rule-set should now be loaded properly to kernel and everything should work. If not, you may possibly have run into a bug in these commands.

9.1. Basics of the iptables command

As we have already explained, each rule is a line that the kernel looks at to find out what to do with a packet. If all the criteria - or matches - are met, we perform the target - or jump - instruction. Normally we would write our rules in a syntax that looks something like this:

iptables [-t table] command [match] [target/jump]

There is nothing that says that the target instruction has to be the last function in the line. However, you would usually adhere to this syntax to get the best readability. Anyway, most of the rules you'll see are written in this way. Hence, if you read someone else's script, you'll most likely recognize the syntax and easily understand the rule.

If you want to use a table other than the standard table, you could insert the table specification at the point at which [table] is specified. However, it is not necessary to state explicitly what table to use, since by default iptables uses the filter table on which to implement all commands. Neither do you have to specify the table at just this point in the rule. It could be set pretty much anywhere along the line. However, it is more or less standard to put the table specification at the beginning.

One thing to think about though: The command should always come first, or alternatively directly after the table specification. We use 'command' to tell the program what to do, for example to insert a rule or to add a rule to the end of the chain, or to delete a rule. We shall take a further look at this below.

The match is the part of the rule that we send to the kernel that details the specific character of the packet, what makes it different from all other packets. Here we could specify what IP address the packet comes from, from which network interface, the intended IP address, port, protocol or whatever. There is a heap of different matches that we can use that we will look closer at further on in this chapter.

Finally we have the target of the packet. If all the matches are met for a packet, we tell the kernel what to do with it. We could, for example, tell the kernel to send the packet to another chain that we've created ourselves, and which is part of this particular table. We could tell the kernel to drop the packet dead and do no further processing, or we could tell the kernel to send a specified reply to the sender. As with the rest of the content in this section, we'll look closer at it further on in the chapter.

9.2. Tables

The -t option specifies which table to use. Per default, the filter table is used. We may specify one of the following tables with the -t option. Do note that this is an extremely brief summary of some of the contents of the Traversing of tables and chains chapter.

The above details should have explained the basics about the three different tables that are available. They should be used for totally different purposes, and you should know what to use each chain for. If you do not understand their usage, you may well dig a pit for yourself in your firewall, into which you will fall as soon as someone finds it and pushes you into it. We have already discussed the requisite tables and chains in more detail within the Traversing of tables and chains chapter. If you do not understand this fully, I advise you to go back and read through it again.

9.3. Commands

In this section we will cover all the different commands and what can be done with them. The command tells iptables what to do with the rest of the rule that we send to the parser. Normally we would want either to add or delete something in some table or another. The following commands are available to iptables:

You should always enter a complete command line, unless you just want to list the built-in help for iptables or get the version of the command. To get the version, use the -v option and to get the help message, use the -h option. As usual, in other words. Next comes a few options that can be used with various different commands. Note that we tell you with which commands the options can be used and what effect they will have. Also note that we do not include any options here that affect rules or matches. Instead, we'll take a look at matches and targets in a later section of this chapter.

10.1. Generic matches

This section will deal with Generic matches. A generic match is a kind of match that is always available, whatever kind of protocol we are working on, or whatever match extensions we have loaded. No special parameters at all are needed to use these matches; in other words. I have also included the --protocol match here, even though it is more specific to protocol matches. For example, if we want to use a TCP match, we need to use the --protocol match and send TCP as an option to the match. However, --protocol is also a match in itself, since it can be used to match specific protocols. The following matches are always available.

10.2. Implicit matches

This section will describe the matches that are loaded implicitly. Implicit matches are implied, taken for granted, automatic. For example when we match on --protocol tcp without any further criteria. There are currently three types of implicit matches for three different protocols. These are TCP matches, UDP matches and ICMP matches. The TCP based matches contain a set of unique criteria that are available only for TCP packets. UDP based matches contain another set of criteria that are available only for UDP packets. And the same thing for ICMP packets. On the other hand, there can be explicit matches that are loaded explicitly. Explicit matches are not implied or automatic, you have to specify them specifically. For these you use the -m or --match option, which we will discuss in the next section.

10.3. Explicit matches

Explicit matches are those that have to be specifically loaded with the -m or --match option. State matches, for example, demand the directive -m state prior to entering the actual match that you want to use. Some of these matches may be protocol specific . Some may be unconnected with any specific protocol - for example connection states. These might be NEW (the first packet of an as yet unestablished connection), ESTABLISHED (a connection that is already registered in the kernel), RELATED (a new connection that was created by an older, established one) etc. A few may just have been evolved for testing or experimental purposes, or just to illustrate what iptables is capable of. This in turn means that not all of these matches may at first sight be of any use. Nevertheless, it may well be that you personally will find a use for specific explicit matches. And there are new ones coming along all the time, with each new iptables release. Whether you find a use for them or not depends on your imagination and your needs. The difference between implicitly loaded matches and explicitly loaded ones, is that the implicitly loaded matches will automatically be loaded when, for example, you match on the properties of TCP packets, while explicitly loaded matches will never be loaded automatically - it is up to you to discover and activate explicit matches.

---

10.3.1. AH/ESP match

These matches are used for the IPSEC AH and ESP protocols. IPSEC is used to create secure tunnels over an insecure Internet connection. The AH and ESP protocols are used by IPSEC to create these secure connections. The AH and ESP matches are really two separate matches, but are both described here since they look very much alike, and both are used in the same function.

I will not go into detail to describe IPSEC here, instead look at the following pages and documents for more information:

• RFC 2401 - Security Architecture for the Internet Protocol

• FreeS/WAN

• IPSEC Howto

• Linux Advanced Routing and Traffic Control HOW-TO

There is also a ton more documentation on the Internet on this, but you are free to look it up as needed.

To use the AH/ESP matches, you need to use -m ah to load the AH matches, and -m esp to load the ESP matches.

In 2.2 and 2.4 kernels, Linux used something called FreeS/WAN for the IPSEC implementation, but as of Linux kernel 2.5.47 and up, Linux kernels have a direct implementation of IPSEC that requires no patching of the kernel. This is a total rewrite of the IPSEC implementation on Linux.

Table 10-5. AH match options

Match	--ahspi
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p 51 -m ah --ahspi 500
Explanation	This matches the AH Security Parameter Index (SPI) number of the AH packets. Please note that you must specify the protocol as well, since AH runs on a different protocol than the standard TCP, UDP or ICMP protocols. The SPI number is used in conjunction with the source and destination address and the secret keys to create a security association (SA). The SA uniquely identifies each and every one of the IPSEC tunnels to all hosts. The SPI is used to uniquely distinguish each IPSEC tunnel connected between the same two peers. Using the --ahspi match, we can match a packet based on the SPI of the packets. This match can match a whole range of SPI values by using a : sign, such as 500:520, which will match the whole range of SPI's.

Table 10-6. ESP match options

Match	--espspi
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p 50 -m esp --espspi 500
Explanation	The ESP counterpart Security Parameter Index (SPI) is used exactly the same way as the AH variant. The match looks exactly the same, with the esp/ah difference. Of course, this match can match a whole range of SPI numbers as well as the AH variant of the SPI match, such as --espspi 200:250 which matches the whole range of SPI's.

---

10.3.3. DSCP match

This match is used to match on packets based on their DSCP (Differentiated Services Code Point) field. This is documented in the RFC 2638 - A Two-bit Differentiated Services Architecture for the Internet RFC. The match is explicitly loaded by specifying -m dscp. The match can take two mutually exclusive options, described below.

Table 10-8. DSCP match options

Match	--dscp
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m dscp --dscp 32
Explanation	This option takes a DSCP value in either decimal or in hex. If the option value is in decimal, it would be written like 32 or 16, et cetera. If written in hex, it should be prefixed with 0x, like this: 0x20. It can also be inverted by using the ! character, like this: -m dscp ! --dscp 32.
Match	--dscp-class
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m dscp --dscp-class BE
Explanation	The --dscp-class match is used to match on the DiffServ class of a packet. The values can be any of the BE, EF, AFxx or CSx classes as specified in the various RFC's. This match can be inverted just the same way as the --dscp option.

Please note that the --dscp and --dscp-class options are mutually exclusive and can not be used in conjunction with each other.

---

10.3.9. MAC match

The MAC (Ethernet Media Access Control) match can be used to match packets based on their MAC source address. As of writing this documentation, this match is a little bit limited, however, in the future this may be more evolved and may be more useful. This match can be used to match packets on the source MAC address only as previously said.

Do note that to use this module we explicitly load it with the -m mac option. The reason that I am saying this is that a lot of people wonder if it should not be -m mac-source, which it should not.

Table 10-15. MAC match options

Match	--mac-source
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -m mac --mac-source 00:00:00:00:00:01
Explanation	This match is used to match packets based on their MAC source address. The MAC address specified must be in the form XX:XX:XX:XX:XX:XX, else it will not be legal. The match may be reversed with an ! sign and would look like --mac-source ! 00:00:00:00:00:01. This would in other words reverse the meaning of the match, so that all packets except packets from this MAC address would be matched. Note that since MAC addresses are only used on Ethernet type networks, this match will only be possible to use for Ethernet interfaces. The MAC match is only valid in the PREROUTING, FORWARD and INPUT chains and nowhere else.

---

10.3.11. Multiport match

The multiport match extension can be used to specify multiple destination ports and port ranges. Without the possibility this match gives, you would have to use multiple rules of the same type, just to match different ports.

You can not use both standard port matching and multiport matching at the same time, for example you can't write: --sport 1024:63353 -m multiport --dport 21,23,80. This will simply not work. What in fact happens, if you do, is that iptables honors the first element in the rule, and ignores the multiport instruction.

Table 10-17. Multiport match options

Match	--source-port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --source-port 22,53,80,110
Explanation	This match matches multiple source ports. A maximum of 15 separate ports may be specified. The ports must be comma delimited, as in the above example. The match may only be used in conjunction with the -p tcp or -p udp matches. It is mainly an enhanced version of the normal --source-port match.
Match	--destination-port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --destination-port 22,53,80,110
Explanation	This match is used to match multiple destination ports. It works exactly the same way as the above mentioned source port match, except that it matches destination ports. It too has a limit of 15 ports and may only be used in conjunction with -p tcp and -p udp.
Match	--port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --port 22,53,80,110
Explanation	This match extension can be used to match packets based both on their destination port and their source port. It works the same way as the --source-port and --destination-port matches above. It can take a maximum of 15 ports and can only be used in conjunction with -p tcp and -p udp. Note that the --port match will only match packets coming in from and going to the same port, for example, port 80 to port 80, port 110 to port 110 and so on.

---

10.3.14. Recent match

The recent match is a rather large and complex matching system, which allows us to match packets based on recent events that we have previously matched. For example, if we would see an outgoing IRC connection, we could set the IP addresses into a list of hosts, and have another rule that allows identd requests back from the IRC server within 15 seconds of seeing the original packet.

Before we can take a closer look at this match, let's try and explain a little bit how it works. First of all, we use several different rules to accomplish the use of the recent match. The recent match uses several different lists of recent events. The default list being used is the DEFAULT list. We create a new entry in a list with the set option, so once a rule is entirely matched (the set option is always a match), we also add an entry in the recent list specified. The list entry contains a timestamp, and the source IP address used in the packet that triggered the set option. Once this has happened, we can use a series of different recent options to match on this information, as well as update the entries timestamp, et cetera.

Finally, if we would for some reason want to remove a list entry, we would do this using the remove match from the recent module. All rules using the recent match, must load the recent module (-m recent) as usual. Before we go on with an example of the recent match, let's take a look at all the options.

Table 10-20. Recent match options

Match	--name
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --name examplelist
Explanation	The name option gives the name of the list to use. Per default the DEFAULT list is used, which is probably not what we want if we are using more than one list.
Match	--set
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --set
Explanation	This creates a new list entry in the named recent list, which contains a timestamp and the source IP address of the host that triggered the rule. This match will always return success, unless it is preceded by a ! sign, in which case it will return failure.
Match	--rcheck
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --name examplelist --rcheck
Explanation	The --rcheck option will check if the source IP address of the packet is in the named list. If it is, the match will return true, otherwise it returns false. The option may be inverted by using the ! sign. In the later case, it will return true if the source IP address is not in the list, and false if it is in the list.
Match	--update
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --name examplelist --update
Explanation	This match is true if the source combination is available in the specified list and it also updates the last-seen time in the list. This match may also be reversed by setting the ! mark in front of the match. For example, ! --update.
Match	--remove
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --remove
Explanation	This match will try to find the source address of the packet in the list, and returns true if the packet is there. It will also remove the corresponding list entry from the list. The command is also possible to inverse with the ! sign.
Match	--seconds
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --check --seconds 60
Explanation	This match is only valid together with the --check and --update matches. The --seconds match is used to specify how long since the "last seen" column was updated in the recent list. If the last seen column was older than this amount in seconds, the match returns false. Other than this the recent match works as normal, so the source address must still be in the list for a true return of the match.
Match	--hitcount
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --check --hitcount 20
Explanation	The --hitcount match must be used together with the --check or --update matches and it will limit the match to only include packets that have seen at least the hitcount amount of packets. If this match is used together with the --seconds match, it will require the specified hitcount packets to be seen in the specific timeframe. This match may also be reversed by adding a ! sign in front of the match. Together with the --seconds match, this means that a maximum of this amount of packets may have been seen during the specified timeframe. If both of the matches are inversed, then a maximum of this amount of packets may have been seen during the last minumum of seconds.
Match	--rttl
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --check --rttl
Explanation	The --rttl match is used to verify that the TTL value of the current packet is the same as the original packet that was used to set the original entry in the recent list. This can be used to verify that people are not spoofing their source address to deny others access to your servers by making use of the recent match.
Match	--rsource
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --rsource
Explanation	The --rsource match is used to tell the recent match to save the source address and port in the recent list. This is the default behavior of the recent match.
Match	--rdest
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --rdest
Explanation	The --rdest match is the opposite of the --rsource match in that it tells the recent match to save the destination address and port to the recent list.

I have created a small sample script of how the recent match can be used, which you can find in the Recent-match.txt section.

Briefly, this is a poor replacement for the state engine available in netfilter. This version was created with a http server in mind, but will work with any TCP connection. First we have created two chains named http-recent and http-recent-final. The http-recent chain is used in the starting stages of the connection, and for the actual data transmission, while the http-recent-final chain is used for the last and final FIN, FIN/ACK handshake.

This is a very bad replacement for the built in state engine and can not handle all of the possibilities that the state engine can handle. However, it is a good example of what can be done with the recent match without being too specific. Do not use this example in a real world environment. It is slow, handles special cases badly, and should generally never be used more than as an example. For example, it does not handle closed ports on connection, asyncronuous FIN handshake (where one of the connected parties closes down, while the other continues to send data), etc.

Let's follow a packet through the example ruleset. First a packet enters the INPUT chain, and we send it to the http-recent chain.

1. The first packet should be a SYN packet, and should not have the ACK,FIN or RST bits set. Hence it is matched using the --tcp-flags SYN,ACK,FIN,RST SYN line. At this point we add the connection to the httplist using -m recent --name httplist --set line. Finally we accept the packet.

2. After the first packet we should receive a SYN/ACK packet to acknowledge that the SYN packet was received. This can be matched using the --tcp-flags SYN,ACK,FIN,RST SYN,ACK line. FIN and RST should be illegal at this point as well. At this point we update the entry in the httplist using -m recent --name httplist --update and finally we ACCEPT the packet.

3. By now we should get a final ACK packet, from the original creater of the connection, to acknowledge the SYN/ACK sent by the server. SYN, FIN and RST are illegal at this point of the connection, so the line should look like --tcp-flags SYN,ACK,FIN,RST ACK. We update the list in exactly the same way as in the previous step, and ACCEPT it.

4. At this point the data tranmission can start. The connection should never contain any SYN packet now, but it will contain ACK packets to acknowledge the data packets that are sent. Each time we see any packet like this, we update the list and ACCEPT the packets.

5. The transmission can be ended in two ways, the simplest is the RST packet. RST will simply reset the connection and it will die. With FIN, the other endpoint answers with a FIN,ACK, and this closes down the connection so that the original source of the FIN can no longer send any data. The receiver of the FIN, will still be able to send data, hence we send the connection to a "final" stage chain to handle the rest.

6. In the http-recent-final chain we check if the packet is still in the httplist, and if so, we send it to the http-recent-final1 chain. In that chain we remove the connection from the httplist and add it to the http-recent-final list instead. If the connection has already been removed and moved over to the http-recent-final list, we send te packet to the http-recent-final2 chain.

7. In the final http-recent-final2 chain, we wait for the non-closed side to finish sending its data, and to close the connection from their side as well. Once this is done, the connection is completely removed.

As you can see the recent list can become quite complex, but it will give you a huge set of possibilities if need be. Still, try and remember not to reinvent the wheel. If the ability you need is already implemented, try and use it instead of trying to create your own solution.

11.1. ACCEPT target

This target needs no further options. As soon as the match specification for a packet has been fully satisfied, and we specify ACCEPT as the target, the rule is accepted and will not continue traversing the current chain or any other ones in the same table. Note however, that a packet that was accepted in one chain might still travel through chains within other tables, and could still be dropped there. There is nothing special about this target whatsoever, and it does not require, nor have the possibility of, adding options to the target. To use this target, we simply specify -j ACCEPT.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.2. CLASSIFY target

The CLASSIFY target can be used to classify packets in such a way that can be used by a couple of different qdiscs (Queue Disciplines). For example, atm, cbq, dsmark, pfifo_fast, htb and the prio qdiscs. For more information about qdiscs and traffic controlling, visit the Linux Advanced Routing and Traffic Control HOW-TO webpage.

The CLASSIFY target is only valid in the POSTROUTING chain of the mangle table.

Works under Linux kernel 2.5 and 2.6.

11.3. DNAT target

The DNAT target is used to do Destination Network Address Translation, which means that it is used to rewrite the Destination IP address of a packet. If a packet is matched, and this is the target of the rule, the packet, and all subsequent packets in the same stream will be translated, and then routed on to the correct device, host or network. This target can be extremely useful, for example,when you have a host running your web server inside a LAN, but no real IP to give it that will work on the Internet. You could then tell the firewall to forward all packets going to its own HTTP port, on to the real web server within the LAN. We may also specify a whole range of destination IP addresses, and the DNAT mechanism will choose the destination IP address at random for each stream. Hence, we will be able to deal with a kind of load balancing by doing this.

Note that the DNAT target is only available within the PREROUTING and OUTPUT chains in the nat table, and any of the chains called upon from any of those listed chains. Note that chains containing DNAT targets may not be used from any other chains, such as the POSTROUTING chain.

Since DNAT requires quite a lot of work to work properly, I have decided to add a larger explanation on how to work with it. Let's take a brief example on how things would be done normally. We want to publish our website via our Internet connection. We only have one IP address, and the HTTP server is located on our internal network. Our firewall has the external IP address $INET_IP, and our HTTP server has the internal IP address $HTTP_IP and finally the firewall has the internal IP address $LAN_IP. The first thing to do is to add the following simple rule to the PREROUTING chain in the nat table:

iptables -t nat -A PREROUTING --dst $INET_IP -p tcp --dport 80 -j DNAT \
--to-destination $HTTP_IP
    

Now, all packets from the Internet going to port 80 on our firewall are redirected (or DNAT'ed) to our internal HTTP server. If you test this from the Internet, everything should work just perfect. So, what happens if you try connecting from a host on the same local network as the HTTP server? It will simply not work. This is a problem with routing really. We start out by dissecting what happens in a normal case. The external box has IP address $EXT_BOX, to maintain readability.

1. Packet leaves the connecting host going to $INET_IP and source $EXT_BOX.

2. Packet reaches the firewall.

3. Firewall DNAT's the packet and runs the packet through all different chains etcetera.

4. Packet leaves the firewall and travels to the $HTTP_IP.

5. Packet reaches the HTTP server, and the HTTP box replies back through the firewall, if that is the box that the routing database has entered as the gateway for $EXT_BOX. Normally, this would be the default gateway of the HTTP server.

6. Firewall Un-DNAT's the packet again, so the packet looks as if it was replied to from the firewall itself.

7. Reply packet travels as usual back to the client $EXT_BOX.

Now, we will consider what happens if the packet was instead generated by a client on the same network as the HTTP server itself. The client has the IP address $LAN_BOX, while the rest of the machines maintain the same settings.

1. Packet leaves $LAN_BOX to $INET_IP.

2. The packet reaches the firewall.

3. The packet gets DNAT'ed, and all other required actions are taken, however, the packet is not SNAT'ed, so the same source IP address is used on the packet.

4. The packet leaves the firewall and reaches the HTTP server.

5. The HTTP server tries to respond to the packet, and sees in the routing databases that the packet came from a local box on the same network, and hence tries to send the packet directly to the original source IP address (which now becomes the destination IP address).

6. The packet reaches the client, and the client gets confused since the return packet does not come from the host that it sent the original request to. Hence, the client drops the reply packet, and waits for the "real" reply.

The simple solution to this problem is to SNAT all packets entering the firewall and leaving for a host or IP that we know we do DNAT to. For example, consider the above rule. We SNAT the packets entering our firewall that are destined for $HTTP_IP port 80 so that they look as if they came from $LAN_IP. This will force the HTTP server to send the packets back to our firewall, which Un-DNAT's the packets and sends them on to the client. The rule would look something like this:

iptables -t nat -A POSTROUTING -p tcp --dst $HTTP_IP --dport 80 -j SNAT \
--to-source $LAN_IP
    

Remember that the POSTROUTING chain is processed last of the chains, and hence the packet will already be DNAT'ed once it reaches that specific chain. This is the reason that we match the packets based on the internal address.

This last rule will seriously harm your logging, so it is really advisable not to use this method, but the whole example is still a valid one. What will happen is this, packet comes from the Internet, gets SNAT'ed and DNAT'ed, and finally hits the HTTP server (for example). The HTTP server now only sees the request as if it was coming from the firewall, and hence logs all requests from the internet as if they came from the firewall. This can also have even more severe implications. Take an SMTP server on the LAN, that allows requests from the internal network, and you have your firewall set up to forward SMTP traffic to it. You have now effectively created an open relay SMTP server, with horrenduously bad logging! One solution to this problem is to simply make the above rule even more specific in the match part, and to only work on packets that come in from our LAN interface. In other words, add a -i $LAN_IFACE to the whole command as well. This will make the rule only work on streams that come in from the LAN, and hence will not affect the Source IP, so the logs will look correct, except for streams coming from our LAN. You will, in other words, be better off solving these problems by either setting up a separate DNS server for your LAN, or to actually set up a separate DMZ, the latter being preferred if you have the money.

You think this should be enough by now, and it really is, unless considering one final aspect to this whole scenario. What if the firewall itself tries to access the HTTP server, where will it go? As it looks now, it will unfortunately try to get to its own HTTP server, and not the server residing on $HTTP_IP. To get around this, we need to add a DNAT rule in the OUTPUT chain as well. Following the above example, this should look something like the following:

iptables -t nat -A OUTPUT --dst $INET_IP -p tcp --dport 80 -j DNAT \
--to-destination $HTTP_IP
    

Adding this final rule should get everything up and running. All separate networks that do not sit on the same net as the HTTP server will run smoothly, all hosts on the same network as the HTTP server will be able to connect and finally, the firewall will be able to do proper connections as well. Now everything works and no problems should arise.

Everyone should realize that these rules only affect how the packet is DNAT'ed and SNAT'ed properly. In addition to these rules, you may also need extra rules in the filter table (FORWARD chain) to allow the packets to traverse through those chains as well. Don't forget that all packets have already gone through the PREROUTING chain, and should hence have their destination addresses rewritten already by DNAT.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.4. DROP target

The DROP target does just what it says, it drops packets dead and will not carry out any further processing. A packet that matches a rule perfectly and is then Dropped will be blocked. Note that this action might in certain cases have an unwanted effect, since it could leave dead sockets around on either host. A better solution in cases where this is likely would be to use the REJECT target, especially when you want to block port scanners from getting too much information, such as on filtered ports and so on. Also note that if a packet has the DROP action taken on it in a subchain, the packet will not be processed in any of the main chains either in the present or in any other table. The packet is in other words totally dead. As we've seen previously, the target will not send any kind of information in either direction, nor to intermediaries such as routers.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.5. DSCP target

This is a target that changes the DSCP (Differentiated Services Field) marks inside a packet. The DSCP target is able to set any DSCP value inside a TCP packet, which is a way of telling routers the priority of the packet in question. For more information about DSCP, look at the RFC 2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers RFC document.

Basically, DSCP is a way of differentiating different services into separate categories, and based on this, give them different priority through the routers. This way, you can give interactive TCP sessions (such as telnet, SSH, POP3) a very high fast connection, that may not be very suitable for large bulk transfers. If on the other hand the connection is one of low importance (SMTP, or whatever you classify as low priority), you could send it over a large bulky network with worse latency than the other network, that is cheaper to utilize than the faster and lower latency connections.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.6. ECN target

This target can be great, used in the correct way. Simply put, the ECN target can be used to reset the ECN bits from the IPv4 header, or to put it correctly, reset them to 0 at least. Since ECN is a relatively new thing on the net, there are problems with it. For example, it uses 2 bits that are defined in the original RFC for the TCP protocol to be 0. Some routers and other internet appliances will not forward packets that have these bits set to 1. If you want to make use of at least parts of the ECN functionality from your hosts, you could for example reset the ECN bits to 0 for specific networks that you know you are having troubles reaching because of ECN.

Please do note that it isn't possible to turn ECN on in the middle of a stream. It isn't allowed according to the RFC's, and it isn't possible anyways. Both endpoints of the stream must negotiate ECN. If we turn it on, then one of the hosts is not aware of it, and can't respond properly to the ECN notifications.

Works under Linux kernel 2.5 and 2.6.

11.7. LOG target options

The LOG target is specially designed for logging detailed information about packets. These could, for example, be considered as illegal. Or, logging can be used purely for bug hunting and error finding. The LOG target will return specific information on packets, such as most of the IP headers and other information considered interesting. It does this via the kernel logging facility, normally syslogd. This information may then be read directly with dmesg, or from the syslogd logs, or with other programs or applications. This is an excellent target to use to debug your rule-sets, so that you can see what packets go where and what rules are applied on what packets. Note as well that it could be a really great idea to use the LOG target instead of the DROP target while you are testing a rule you are not 100% sure about on a production firewall, since a syntax error in the rule-sets could otherwise cause severe connectivity problems for your users. Also note that the ULOG target may be interesting if you are using really extensive logging, since the ULOG target has support for direct logging to MySQL databases and suchlike.

Note that if you get undesired logging direct to consoles, this is not an iptables or Netfilter problem, but rather a problem caused by your syslogd configuration - most probably /etc/syslog.conf. Read more in man syslog.conf for information about this kind of problem. You may also need to tweak your dmesg settings. dmesg is the command that changes which errors from the kernel that should be shown on the console. dmesg -n 1 should prevent all messages from showing up on the console, except panic messages. The dmesg message levels matches exactly the syslogd levels, and it only works on log messages from the kernel facility. For more information, see man dmesg.

The LOG target currently takes five options that could be of interest if you have specific information needs, or want to set different options to specific values. They are all listed below.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.8. MARK target

The MARK target is used to set Netfilter mark values that are associated with specific packets. This target is only valid in the mangle table, and will not work outside there. The MARK values may be used in conjunction with the advanced routing capabilities in Linux to send different packets through different routes and to tell them to use different queue disciplines (qdisc), etc. For more information on advanced routing, check out the Linux Advanced Routing and Traffic Control HOW-TO. Note that the mark value is not set within the actual package, but is a value that is associated within the kernel with the packet. In other words, you can not set a MARK for a packet and then expect the MARK still to be there on another host. If this is what you want, you will be better off with the TOS target which will mangle the TOS value in the IP header.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.9. MASQUERADE target

The MASQUERADE target is used basically the same as the SNAT target, but it does not require any --to-source option. The reason for this is that the MASQUERADE target was made to work with, for example, dial-up connections, or DHCP connections, which gets dynamic IP addresses when connecting to the network in question. This means that you should only use the MASQUERADE target with dynamically assigned IP connections, which we don't know the actual address of at all times. If you have a static IP connection, you should instead use the SNAT target.

When you masquerade a connection, it means that we set the IP address used on a specific network interface instead of the --to-source option, and the IP address is automatically grabbed from the information about the specific interface. The MASQUERADE target also has the effect that connections are forgotten when an interface goes down, which is extremely good if we, for example, kill a specific interface. If we would have used the SNAT target, we may have been left with a lot of old connection tracking data, which would be lying around for days, swallowing up useful connection tracking memory. This is, in general, the correct behavior when dealing with dial-up lines that are probably assigned a different IP every time they are brought up. In case we are assigned a different IP, the connection is lost anyways, and it is more or less idiotic to keep the entry around.

It is still possible to use the MASQUERADE target instead of SNAT even though you do have a static IP, however, it is not favorable since it will add extra overhead, and there may be inconsistencies in the future which will thwart your existing scripts and render them "unusable".

Note that the MASQUERADE target is only valid within the POSTROUTING chain in the nat table, just as the SNAT target. The MASQUERADE target takes one option specified below, which is optional.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.10. MIRROR target

Be warned, the MIRROR is dangerous and was only developed as an example code of the new conntrack and NAT code. It can cause dangerous things to happen, and very serious DDoS/DoS will be possible if used improperly. Avoif using it at all costs! It was removed from 2.5 and 2.6 kernels due to it's bad security implications!

The MIRROR target is an experimental and demonstration target only, and you are warned against using it, since it may result in really bad loops hence, among other things, resulting in serious Denial of Service. The MIRROR target is used to invert the source and destination fields in the IP header, and then to retransmit the packet. This can cause some really funny effects, and I'll bet that, thanks to this target, not just one red faced cracker has cracked his own box by now. The effect of using this target is stark, to say the least. Let's say we set up a MIRROR target for port 80 at computer A. If host B were to come from yahoo.com, and try to access the HTTP server at host A, the MIRROR target would return the yahoo host's own web page (since this is where the request came from).

Note that the MIRROR target is only valid within the INPUT, FORWARD and PREROUTING chains, and any user-defined chains which are called from those chains. Also note that outgoing packets resulting from the MIRROR target are not seen by any of the normal chains in the filter, nat or mangle tables, which could give rise to loops and other problems. This could make the target the cause of unforeseen headaches. For example, a host might send a spoofed packet to another host that uses the MIRROR command with a TTL of 255, at the same time spoofing its own packet, so as to seem as if it comes from a third host that uses the MIRROR command. The packet will then bounce back and forth incessantly, for the number of hops there are to be completed. If there is only 1 hop, the packet will jump back and forth 240-255 times. Not bad for a cracker, in other words, to send 1500 bytes of data and eat up 380 kbyte of your connection. Note that this is a best case scenario for the cracker or script kiddie, whatever we want to call them.

Works under Linux kernel 2.3 and 2.4. It was removed from 2.5 and 2.6 kernels due to it's inherent insecurity. Do not use this target!

11.11. NETMAP target

NETMAP is a new implementation of the SNAT and DNAT targets where the host part of the IP address isn't changed. It provides a 1:1 NAT function for whole networks which isn't available in the standard SNAT and DNAT functions. For example, lets say we have a network containing 254 hosts using private IP addresses (a /24 network), and we just got a new /24 network of public IP's. Instead of walking around and changing the IP of each and every one of the hosts, we would be able to simply use the NETMAP target like -j NETMAP -to 10.5.6.0/24 and voila, all the hosts are seen as 10.5.6.x when they leave the firewall. For example, 192.168.0.26 would become 10.5.6.26.

Works under Linux kernel 2.5 and 2.6.

11.12. QUEUE target

The QUEUE target is used to queue packets to User-land programs and applications. It is used in conjunction with programs or utilities that are extraneous to iptables and may be used, for example, with network accounting, or for specific and advanced applications which proxy or filter packets. We will not discuss this target in depth, since the coding of such applications is out of the scope of this tutorial. First of all it would simply take too much time, and secondly such documentation does not have anything to do with the programming side of Netfilter and iptables. All of this should be fairly well covered in the Netfilter Hacking HOW-TO.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.13. REDIRECT target

The REDIRECT target is used to redirect packets and streams to the machine itself. This means that we could for example REDIRECT all packets destined for the HTTP ports to an HTTP proxy like squid, on our own host. Locally generated packets are mapped to the 127.0.0.1 address. In other words, this rewrites the destination address to our own host for packets that are forwarded, or something alike. The REDIRECT target is extremely good to use when we want, for example, transparent proxying, where the LAN hosts do not know about the proxy at all.

Note that the REDIRECT target is only valid within the PREROUTING and OUTPUT chains of the nat table. It is also valid within user-defined chains that are only called from those chains, and nowhere else. The REDIRECT target takes only one option, as described below.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.14. REJECT target

The REJECT target works basically the same as the DROP target, but it also sends back an error message to the host sending the packet that was blocked. The REJECT target is as of today only valid in the INPUT, FORWARD and OUTPUT chains or their sub chains. After all, these would be the only chains in which it would make any sense to put this target. Note that all chains that use the REJECT target may only be called by the INPUT, FORWARD, and OUTPUT chains, else they won't work. There is currently only one option which controls the nature of how this target works, though this may in turn take a huge set of variables. Most of them are fairly easy to understand, if you have a basic knowledge of TCP/IP.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.15. RETURN target

The RETURN target will cause the current packet to stop traveling through the chain where it hit the rule. If it is the subchain of another chain, the packet will continue to travel through the superior chains as if nothing had happened. If the chain is the main chain, for example the INPUT chain, the packet will have the default policy taken on it. The default policy is normally set to ACCEPT, DROP or similar.

For example, let's say a packet enters the INPUT chain and then hits a rule that it matches and that tells it to --jump EXAMPLE_CHAIN. The packet will then start traversing the EXAMPLE_CHAIN, and all of a sudden it matches a specific rule which has the --jump RETURN target set. It will then jump back to the INPUT chain. Another example would be if the packet hit a --jump RETURN rule in the INPUT chain. It would then be dropped to the default policy as previously described, and no more actions would be taken in this chain.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.16. SAME target

The SAME target works almost in the same fashion as the SNAT target, but it still differs. Basically, the SAME target will try to always use the same outgoing IP address for all connections initiated by a single host on your network. For example, say you have one /24 network (192.168.1.0) and 3 IP addresses (10.5.6.7-9). Now, if 192.168.1.20 went out through the .7 address the first time, the firewall will try to keep that machine always going out through that IP address.

Works under Linux kernel 2.5 and 2.6.

11.17. SNAT target

The SNAT target is used to do Source Network Address Translation, which means that this target will rewrite the Source IP address in the IP header of the packet. This is what we want, for example, when several hosts have to share an Internet connection. We can then turn on ip forwarding in the kernel, and write an SNAT rule which will translate all packets going out from our local network to the source IP of our own Internet connection. Without doing this, the outside world would not know where to send reply packets, since our local networks mostly use the IANA specified IP addresses which are allocated for LAN networks. If we forwarded these packets as is, no one on the Internet would know that they were actually from us. The SNAT target does all the translation needed to do this kind of work, letting all packets leaving our LAN look as if they came from a single host, which would be our firewall.

The SNAT target is only valid within the nat table, within the POSTROUTING chain. This is in other words the only chain in which you may use SNAT. Only the first packet in a connection is mangled by SNAT, and after that all future packets using the same connection will also be SNATted. Furthermore, the initial rules in the POSTROUTING chain will be applied to all the packets in the same stream.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.18. TCPMSS target

The TCPMSS target can be used to alter the MSS (Maximum Segment Size) value of TCP SYN packets that the firewall sees. The MSS value is used to control the maximum size of packets for specific connections. Under normal circumstances, this means the size of the MTU (Maximum Transfer Unit) value, minus 40 bytes. This is used to overcome some ISP's and servers that block ICMP fragmentation needed packets, which can result in really weird problems which can mainly be described such that everything works perfectly from your firewall/router, but your local hosts behind the firewall can't exchange large packets. This could mean such things as mail servers being able to send small mails, but not large ones, web browsers that connect but then hang with no data received, and ssh connecting properly, but scp hangs after the initial handshake. In other words, everything that uses any large packets will be unable to work.

The TCPMSS target is able to solve these problems, by changing the size of the packets going out through a connection. Please note that we only need to set the MSS on the SYN packet since the hosts take care of the MSS after that. The target takes two arguments.

Works under Linux kernel 2.5 and 2.6.

11.19. TOS target

The TOS target is used to set the Type of Service field within the IP header. The TOS field consists of 8 bits which are used to help in routing packets. This is one of the fields that can be used directly within iproute2 and its subsystem for routing policies. Worth noting, is that if you handle several separate firewalls and routers, this is the only way to propagate routing information within the actual packet between these routers and firewalls. As previously noted, the MARK target - which sets a MARK associated with a specific packet - is only available within the kernel, and can't be propagated with the packet. If you feel a need to propagate routing information for a specific packet or stream, you should therefore set the TOS field, which was developed for this.

There are currently a lot of routers on the Internet which do a pretty bad job at this, so as of now it may prove to be a bit useless to attempt TOS mangling before sending the packets on to the Internet. At best the routers will not pay any attention to the TOS field. At worst, they will look at the TOS field and do the wrong thing. However, as stated above, the TOS field can most definitely be put to good use if you have a large WAN or LAN with multiple routers. You then in fact have the possibility of giving packets different routes and preferences, based on their TOS value - even though this might be confined to your own network.

The TOS target is only capable of setting specific values, or named values on packets. These predefined TOS values can be found in the kernel include files, or more precisely, the Linux/ip.h file. The reasons are many, and you should actually never need to set any other values; however, there are ways around this limitation. To get around the limitation of only being able to set the named values on packets, you can use the FTOS patch available at the Paksecured Linux Kernel patches site maintained by Matthew G. Marsh. However, be cautious with this patch! You should not need to use any other than the default values, except in extreme cases.

Note that this target is only valid within the mangle table and can't be used outside it.

Also note that some old versions (1.2.2 or below) of iptables provided a broken implementation of this target which did not fix the packet checksum upon mangling, hence rendering the packets bad and in need of retransmission. That in turn would most probably lead to further mangling and the connection never working.

The TOS target only takes one option as described below.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.20. TTL target

This patch requires the TTL patch from the patch-o-matic tree available in the base directory from http://www.netfilter.org/.

The TTL target is used to modify the Time To Live field in the IP header. One useful application of this is to change all Time To Live values to the same value on all outgoing packets. One reason for doing this is if you have a bully ISP which don't allow you to have more than one machine connected to the same Internet connection, and who actively pursues this. Setting all TTL values to the same value, will effectively make it a little bit harder for them to notice that you are doing this. We may then reset the TTL value for all outgoing packets to a standardized value, such as 64 as specified in the Linux kernel.

For more information on how to set the default value used in Linux, read the ip-sysctl.txt, which you may find within the Other resources and links appendix.

The TTL target is only valid within the mangle table, and nowhere else. It takes 3 options as of writing this, all of them described below in the table.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

11.21. ULOG target

The ULOG target is used to provide user-space logging of matching packets. If a packet is matched and the ULOG target is set, the packet information is multicasted together with the whole packet through a netlink socket. One or more user-space processes may then subscribe to various multicast groups and receive the packet. This is in other words a more complete and more sophisticated logging facility that is only used by iptables and Netfilter so far, and it contains much better facilities for logging packets. This target enables us to log information to MySQL databases, and other databases, making it much simpler to search for specific packets, and to group log entries. You can find the ULOGD user-land applications at the ULOGD project page.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

12.1. Debugging, a necessity

Debugging is more or less a necessity when it comes to iptables and netfilter and most firewalls in general. The problem with 99% of all firewalls is that in the end there is a human being that decides upon the policies and how the rulesets are created, and I can promise you, it is easy to make a mistake while writing your rulesets. Sometimes, these errors are very hard to see with the naked eye, or to see the holes that they are creating through the firewall. Holes that you don't know of or didn't intend to happen in your scripts can create havoc on your networks, and create an easy entry for your attackers. Most of these holes can be found rather easily with a few good tools.

Other than this, you may write bugs into your scripts in other ways as well, which can create the problem of being unable to login to the firewall. This can also be solved by using a little bit of cleverness before running the scripts at all. Using the full power of both the scripting language as well as the system environment can prove incredibly powerful, which almost all experienced Unix administrators should already have noticed from before, and this is basically all we do when debugging our scripts as well.

12.2. Bash debugging tips

There are quite a few things that can be done with bash to help debugging your scripts containing the rulesets. One of the first problems with finding a bug is to know on which line the problem appears. This can be solved in two different ways, either using the bash -x flag, or by simply entering some echo statements to find the place where the problem happens. Ideally, you would, with the echo statement, add something like the following echo statement at regular intervals in the code:

  ...
  echo "Debugging message 1."
  ...
  echo "Debugging message 2."
  ...
      

In my case, I generally use pretty much worthless messages, as long as they have something in them that is unique so I can find the error message by a simple grep or search in the script file. Now, if the error message shows up after the "Debugging message 1." message, but before "Debugging message 2.", then we know that the erroneous line of code is somewhere in between the two debugging messages. As you can understand, bash has the not really bad, but at least peculiar idea of continuing to execute commands even if there is an error in one of the commands before. In netfilter, this can cause some very interesting problems for you. The above idea of simply using echo statements to find the errors is extremely simple, but it is at the same time very nice since you can narrow the whole problem down to a single line of code and see what the problem is directly.

The second possibility to find the above problem is to use the -x variable to bash, as we spoke of before. This can of course be a little problem, especially if your script is large, and if your console buffer isn't large enough. What the -x variable means is quite simple, it tells the script to just echo every single line of code in the script into the standard output of the shell (generally your console). What you do is to change your normal start line of the script from this:

#!/bin/bash
      

Into the line below:

#!/bin/bash -x
      

As you will see, this changes your output from perhaps a couple of lines, to copious amounts of data on the output. The code shows you every single command line that is executed, and with the values of all the variables et cetera, so that you don't have to try and figure out exactly what the code is doing. Simply put, each line that gets executed is output to your screen as well. One thing that may be nice to see, is that all of the lines that bash outputs are prefixed by a + sign. This makes it a little bit easier to discern error or warning messages from the actual script, rather than just one big mesh of output.

The -x option is also very interesting for debugging a couple of other rather common problems that you may run into with a little bit more complex rulesets. The first of them is to find out exactly what happens with what you thought was a simple loop, such as an for, if or while statement? For example, let's look at an example.

  #!/bin/bash 
  iptables="/sbin/iptables"
  $iptables -N output_int_iface
  cat /etc/configs/machines | while read host; do
    $iptables -N output-$host
    $iptables -A output_int_iface -p tcp -d $host -j output-$host

    cat /etc/configs/${host}/ports | while read row2; do
      $iptables -A output-$host -p tcp --dport $row2 -d $host -j ACCEPT
    done
  done
      

This set of rules may look simple enough, but we continue to run into a problem with it. We get the following error messages that we know come from the above code by using the simple echo debugging method.

work3:~# ./test.sh
Bad argument `output-'
Try `iptables -h' or 'iptables --help' for more information.
cat: /etc/configs//ports: No such file or directory
      

So we turn on the -x option to bash and look at the output. The output is shown below, and as you can see there is something very weird going on in it. There are a couple of commands where the $host and $row2 variables are replaced by nothing. Looking closer, we see that it is only the last iteration of code that causes the trouble. Either we have done a programmatical error, or there is something strange with the data. In this case, it is a simple error with the data, which contains a single extra linebreak at the end of the file. This causes the loop to iterate one last time, which it shouldn't. Simply remove the trailing linebreak of the file, and the problem is solved. This may not be a very elegant solution, but for private work it should be enough. Otherwise, you could add code that looks to see that there is actually some data in the $host and $row2 variables.

work3:~# ./test.sh
+ iptables=/sbin/iptables
+ /sbin/iptables -N output_int_iface
+ cat /etc/configs/machines
+ read host
+ /sbin/iptables -N output-sto-as-101
+ /sbin/iptables -A output_int_iface -p tcp -d sto-as-101 -j output-sto-as-101
+ cat /etc/configs/sto-as-101/ports
+ read row2
+ /sbin/iptables -A output-sto-as-101 -p tcp --dport 21 -d sto-as-101 -j ACCEPT
+ read row2
+ /sbin/iptables -A output-sto-as-101 -p tcp --dport 22 -d sto-as-101 -j ACCEPT
+ read row2
+ /sbin/iptables -A output-sto-as-101 -p tcp --dport 23 -d sto-as-101 -j ACCEPT
+ read row2
+ read host
+ /sbin/iptables -N output-sto-as-102
+ /sbin/iptables -A output_int_iface -p tcp -d sto-as-102 -j output-sto-as-102
+ cat /etc/configs/sto-as-102/ports
+ read row2
+ /sbin/iptables -A output-sto-as-102 -p tcp --dport 21 -d sto-as-102 -j ACCEPT
+ read row2
+ /sbin/iptables -A output-sto-as-102 -p tcp --dport 22 -d sto-as-102 -j ACCEPT
+ read row2
+ /sbin/iptables -A output-sto-as-102 -p tcp --dport 23 -d sto-as-102 -j ACCEPT
+ read row2
+ read host
+ /sbin/iptables -N output-sto-as-103
+ /sbin/iptables -A output_int_iface -p tcp -d sto-as-103 -j output-sto-as-103
+ cat /etc/configs/sto-as-103/ports
+ read row2
+ /sbin/iptables -A output-sto-as-103 -p tcp --dport 21 -d sto-as-103 -j ACCEPT
+ read row2
+ /sbin/iptables -A output-sto-as-103 -p tcp --dport 22 -d sto-as-103 -j ACCEPT
+ read row2
+ /sbin/iptables -A output-sto-as-103 -p tcp --dport 23 -d sto-as-103 -j ACCEPT
+ read row2
+ read host
+ /sbin/iptables -N output-
+ /sbin/iptables -A output_int_iface -p tcp -d -j output-
Bad argument `output-'
Try `iptables -h' or 'iptables --help' for more information.
+ cat /etc/configs//ports
cat: /etc/configs//ports: No such file or directory
+ read row2
+ read host
      

The third and final problem you run into that can be partially solved with the help of the -x option is if you are executing the firewall script via SSH, and the console hangs in the middle of executing the script, and the console simply won't come back, nor are you able to connect via SSH again. In 99.9% of the cases, this means there is some kind of problem inside the script with a couple of the rules. By turning on the -x option, you will see exactly at which line the script locks dead, hopefully at least. There are a couple of circumstances where this is not true, unfortunately. For example, what if the script sets up a rule that blocks incoming traffic, but since the ssh/telnet server sends the echo first as outgoing traffic, netfilter will remember the connection, and hence allow the incoming traffic anyways if you have a rule above that handles connection states.

As you can see, it can become quite complex to debug your ruleset to its full extent in the end. However, it is not impossible at all. You may also have noticed, if you have worked remotely on your firewalls via SSH, for example, that the firewall may hang when you load bad rulesets. There is one more thing that can be done to save the day in these circumstances. Cron is an excellent way of saving your day. For example, say you are working on a firewall 50 kilometers away, you add some rules, delete some others, and then delete and insert the new updated ruleset. The firewall locks dead, and you can't reach it. The only way of fixing this is to go to the firewall's physical location and fix the problem from there, unless you have taken precautions that is!

12.3. System tools used for debugging

One of the best precautions you may take against a locked down firewall is to simply use cron to add a script that is run every 5 minutes or so that resets the firewall, and then remove that cron line once you are sure the installation works fine. The cron line may look something like the one below and be entered with the command crontab -e.

*/5 * * * * /etc/init.d/rc.flush-iptables.sh stop
      

Make absolutely sure, that the line will actually work and do what you expect it to do before you start doing something you expect will or may freeze the server you are working on.

Another tool that is constantly used to debug your scripts is the syslog facility. This is the facility that logs all log-messages created by a ton of different programs. In fact, almost all large programs support syslog logging, including the kernel. All messages sent to syslog have two basic variables set to them that are very important to remember, the facility and the log level/priority.

The facility tells the syslog server from which facility the log entry came from, and where to log it. There are several specified facilities, but the one in question right now is the Kern facility, or kernel facility as it may also be called. All netfilter generated messages are sent using this facility.

The log level tells syslog how high priority the log messages have. There are several priorities that can be used, listed below.

1. debug

2. info

3. notice

4. warning

5. err

6. crit

7. alert

8. emerg

Depending on these priorities, we can send them to different log files using the syslog.conf. For example, to send all messages from the kern facility with warning priority to a file called /var/log/kernwarnings, we could do as shown below. The line should go into the /etc/syslog.conf.

kern.warning                       /var/log/kernwarnings
      

As you can see, it's quite simple. Now you will hopefully find your netfilter logs in the file /var/log/kernwarnings (after restarting, or HUP'ing the syslog server). Of course, this also depends on what log levels you set in your netfilter logging rules. The log level can be set there with the --log-level option.

The logs entered into this file will give you information about all the packets that you wish to log via specific log rules in the ruleset. With these, you can see if there is anything specific that goes wrong. For example, you can set logrules in the end of all the chains to see if there are any packets that are carried over the boundary of the chains. A log entry may look something like the example below, and include quite a lot of information as you can see.

  Oct 23 17:09:34 localhost kernel: IPT INPUT packet died: IN=eth1 OUT= 
MAC=08:00:09:cd:f2:27:00:20:1a:11:3d:73:08:00 SRC=200.81.8.14 DST=217.215.68.146 
LEN=78 TOS=0x00 PREC=0x00 TTL=110 ID=12818 PROTO=UDP SPT=1027 DPT=137 LEN=58
      

As you can understand, syslog can really help you out when debugging your rulesets. Looking at these logs may help you understand why some port that you wanted to open doesn't work.

12.4. Iptables debugging

Iptables can be rough to debug sometimes, since the error messages from iptables itself aren't very user friendly at all times. For this reason, it may be a good idea to take a look at the most common error messages you can get from iptables, and why you may have gotten them.

One of the first error messages to look at is the "Unknown arg" error. This may show up for several reasons. For example, look below.

work3:~# iptables -A INPUT --dport 67 -j ACCEPT
iptables v1.2.9: Unknown arg `--dport'
Try `iptables -h' or 'iptables --help' for more information.
      

This error is simpler than normal to solve, since we have only used a single argument. Normally, you may have used a long, long command and get this error message. The problem in the above scenario is that we have forgotten to use the --protocol match, and because of that, the --dport match isn't available to us. Adding the --protocol match would also solve the problem in this match. Make absolutely certain that you are not missing any special preconditions that are required to use a specific match.

Another very common error is if you miss a dash (-) somewhere in the command line, like below. The proper solution is to simply add the dash, and the command will work.

work3:~# iptables -A INPUT --protocol tcp -dport 67 -j ACCEPT
Bad argument `67'
Try `iptables -h' or 'iptables --help' for more information.
      

And finally, there is the simple misspelling, which is rather common as well. This is shown below. The error message, as you will notice, is exactly the same as when you forget to add another prerequisite match to the rule, so it needs to be carefully looked into.

work3:~# iptables -A INPUT --protocol tcp --destination-ports 67 -j ACCEPT
iptables v1.2.9: Unknown arg `--destination-ports'
Try `iptables -h' or 'iptables --help' for more information.
      

There is also one more possible cause for the "Unknown arg" error shown above. If you can see that the argument is perfectly written, and no possible errors in the prerequisites, there is a possibility that the target/match/table was simply not compiled into the kernel. For example, let's say we forgot to compile the filter table support into the kernel, this would then look something like this:

work3:~# iptables -A INPUT -j ACCEPT
iptables v1.2.9: can't initialize iptables table `filter': Table does not exist 
(do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.
      

Normally, iptables should be able to automatically modprobe a specific module that isn't already inside the kernel, so this is generally a sign of either not having done a proper depmod after restarting with the new kernel, or you may simply have forgotten about the module(s). If the problematic module would be a match instead, the error message would be a little bit more cryptic and hard to understand. For example, look at this error message.

work3:~# iptables -A INPUT -m state 
--state ESTABLISHED -j ACCEPT
iptables: No chain/target/match by that name
      

In this case, we forgot to compile the state module, and as you can see the error message isn't very nice and easy to understand. But it does give you a hint at what is wrong. Finally, we have the same error again, but this time, the target is missing. As you understand from looking at the error message, it get's rather complicated since it is the exact same error message for both errors (missing match and/or target).

work3:~# iptables -A INPUT -m state 
--state ESTABLISHED -j REJECT
iptables: No chain/target/match by that name
      

The easiest way to see if we have simply forgotten to depmod, or if the module is actually missing is to look in the directory where the modules should be. This is the /lib/modules/2.6.4/kernel/net/ipv4/netfilter directory. All ipt_* files that are written in uppercase letters are targets, while all the ones with lowercase letters are matches. For example, ipt_REJECT.ko is a target, while the ipt_state.ko is a match.

In 2.4 kernels and older, the file extension for all kernel modules was .o, which changed to .ko for files in the 2.6 kernels.

Another way of getting help from iptables itself is to simply comment out a whole chain from your script to see if that fixes the problem. This is kind of a last resort problem solver, that may be very effective if you don't even know which chain is causing the problem. By removing the whole chain and simply setting a default policy of ACCEPT, and then testing, if it works better, then this is the chain that is causing the problems. If it doesn't work better, then it is another chain, and you can go on to find the problem elsewhere.

12.5. Other debugging tools

There are of course other tools that may be extremely useful when debugging your firewall scripts. This section will briefly touch the most common tools used to find out fast how your firewall looks from all sides of it (inside, outside, etc). The tools I have chosen here are the nmap and nessus tools.

blueflux@work3:~$ nmap -p 1-1024 192.168.0.1

Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-03-18 17:19 CET
Interesting ports on firewall (192.168.0.1):
(The 1021 ports scanned but not shown below are in state: closed)
PORT    STATE SERVICE
22/tcp  open  ssh
25/tcp  open  smtp
587/tcp open  submission

Nmap run completed -- 1 IP address (1 host up) scanned in 3.877 seconds
        

work3:/home/blueflux# nmap -O -p 1-1024 192.168.0.1

Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-03-18 17:38 CET
Interesting ports on firewall (192.168.0.1):
(The 1021 ports scanned but not shown below are in state: closed)
PORT    STATE SERVICE
22/tcp  open  ssh
25/tcp  open  smtp
587/tcp open  submission
Device type: general purpose
Running: Linux 2.4.X|2.5.X
OS details: Linux Kernel 2.4.0 - 2.5.20
Uptime 6.201 days (since Fri Mar 12 12:49:18 2004)

Nmap run completed -- 1 IP address (1 host up) scanned in 14.303 seconds
        

---

12.5.2. Nessus

While nmap is more of a low level scanner, showing open ports etcetera, the nessus program is an actual security scanner. It tries to connect to different ports, and to find out at most, what kind of version the different servers are running. Nessus takes this a step further, by finding all open ports, finding out what is running on that specific port, what program and which version is running, and then testing for different security threats to that program, and finally creating a complete report of all the security threats that are available.

As you can understand, this is an extremely useful tool to find out more about your host. The program is built up in a server client way, so it should be fairly easy to find out more about your firewall from the outside by using an external nessus daemon, or internal for that matter. The client is a graphical user interface where you login to the nessus daemon, set your settings, and specify which host you would like to scan for vulnerabilities. The generated report may look something like in the example below.

Nessus should be used with some caution however, since it can crash a machine or a service that it is specified to attack. Those attacks that risk crashing a machine are per default turned off luckily.

12.6. What's next?

In this chapter we have looked in detail at different techniques you can use to debug your firewall scripts. Debugging of firewall scripts can become rather tedious and longwinded, however it is a necessity. If you use some small simple steps while doing this, it can become very easy in the end as well. We have looked at the following techniques in particular:

• Bash help in debugging

• System tools fit for debugging

• Iptables debugging

• Other tools for debugging

13.1. example rc.firewall

OK, so you have everything set up and are ready to check out an example configuration script. You should at least be if you have come this far. This example rc.firewall.txt (also included in the Example scripts code-base appendix) is fairly large but not a lot of comments in it. Instead of looking for comments, I suggest you read through the script file to get a basic hum about how it looks, and then you return here to get the nitty gritty about the whole script.

13.2. explanation of rc.firewall

---

13.2.2. Initial loading of extra modules

First, we see to it that the module dependencies files are up to date by issuing a /sbin/depmod -a command. After this we load the modules that we will require for this script. Always avoid loading modules that you do not need, and if possible try to avoid having modules lying around at all unless you will be using them. This is for security reasons, since it will take some extra effort to make additional rules this way. Now, for example, if you want to have support for the LOG, REJECT and MASQUERADE targets and don't have this compiled statically into your kernel, we load these modules as follows:

/sbin/insmod ipt_LOG
/sbin/insmod ipt_REJECT
/sbin/insmod ipt_MASQUERADE
    

In these scripts we forcedly load the modules, which could lead to failures of loading the modules. If a module fails to load, it could depend upon a lot of factors, and it will generate an error message. If some of the more basic modules fail to load, its biggest probable error is that the module, or functionality, is statically compiled into the kernel. For further information on this subject, read the Problems loading modules section in the Common problems and questions appendix.

Next is the option to load ipt_owner module, which could for example be used to only allow certain users to make certain connections, etc. I will not use that module in this example but basically, you could allow only root to do FTP and HTTP connections to redhat.com and DROP all the others. You could also disallow all users but your own user and root to connect from your box to the Internet. Might be boring for others, but you will be a bit more secure to bouncing hacker attacks and attacks where the hacker will only use your host as an intermediate host. For more information about the ipt_owner match, look at the Owner match section within the How a rule is built chapter.

We may also load extra modules for the state matching code here. All modules that extend the state matching code and connection tracking code are called ip_conntrack_* and ip_nat_*. Connection tracking helpers are special modules that tell the kernel how to properly track the specific connections. Without these so called helpers, the kernel would not know what to look for when it tries to track specific connections. The NAT helpers on the other hand, are extensions of the connection tracking helpers that tell the kernel what to look for in specific packets and how to translate these so the connections will actually work. For example, FTP is a complex protocol by definition, and it sends connection information within the actual payload of the packet. So, if one of your NATed boxes connect to a FTP server on the Internet, it will send its own local network IP address within the payload of the packet, and tell the FTP server to connect to that IP address. Since this local network address is not valid outside your own network, the FTP server will not know what to do with it and hence the connection will break down. The FTP NAT helpers do all of the translations within these connections so the FTP server will actually know where to connect. The same thing applies for DCC file transfers (sends) and chats. Creating these kind of connections requires the IP address and ports to be sent within the IRC protocol, which in turn requires some translation to be done. Without these helpers, some FTP and IRC stuff will work no doubt, however, some other things will not work. For example, you may be able to receive files over DCC, but not be able to send files. This is due to how the DCC starts a connection. First off, you tell the receiver that you want to send a file and where he should connect to. Without the helpers, the DCC connection will look as if it wants the receiver to connect to some host on the receiver's own local network. In other words, the whole connection will be broken. However, the other way around, it will work flawlessly since the sender will (most probably) give you the correct address to connect to.

If you are experiencing problems with mIRC DCCs over your firewall and everything works properly with other IRC clients, read the mIRC DCC problems section in the Common problems and questions appendix.

As of this writing, there is only the option to load modules which add support for the FTP and IRC protocols. For a long explanation of these conntrack and nat modules, read the Common problems and questions appendix. There are also H.323 conntrack helpers within the patch-o-matic, as well as some other conntrack as well as NAT helpers. To be able to use these helpers, you need to use the patch-o-matic and compile your own kernel. For a better explanation on how this is done, read the Preparations chapter.

Note that you need to load the ip_nat_irc and ip_nat_ftp if you want Network Address Translation to work properly on any of the FTP and IRC protocols. You will also need to load the ip_conntrack_irc and ip_conntrack_ftp modules before actually loading the NAT modules. They are used the same way as the conntrack modules, but it will make it possible for the computer to do NAT on these two protocols.

---

13.2.3. proc set up

At this point we start the IP forwarding by echoing a 1 to /proc/sys/net/ipv4/ip_forward in this fashion:

echo "1" > /proc/sys/net/ipv4/ip_forward

It may be worth a thought where and when we turn on the IP forwarding. In this script and all others within the tutorial, we turn it on before actually creating any kind of IP filters (i.e., iptables rule-sets). This will lead to a brief period of time where the firewall will accept forwarding of any kind of traffic for everything between a millisecond to a minute depending on what script we are running and on what box. This may give malicious people a small time-frame to actually get through our firewall. In other words, this option should really be turned on after creating all firewall rules, however, I have chosen to turn it on before loading any rules to maintain consistency with the script breakdown currently used in all scripts.

In case you need dynamic IP support, for example if you use SLIP, PPP or DHCP you may enable the next option, ip_dynaddr by doing the following :

echo "1" > /proc/sys/net/ipv4/ip_dynaddr

If there is any other options you might need to turn on you should follow that style. There's other documentation on how to do these things and this is out of the scope of this documentation. There is a good but rather brief document about the proc system available within the kernel, which is also available within the Other resources and links appendix. The Other resources and links appendix is generally a good place to start looking when you have specific areas that you are looking for information on, that you do not find here.

The rc.firewall.txt script, and all other scripts contained within this tutorial, do contain a small section of non-required proc settings. These may be a good primer to look at when something is not working exactly as you want it to, however, do not change these values before actually knowing what they mean.

---

13.2.5. Setting up default policies

Quite early on in the process of creating our rule-set, we set up the default policies. We set up the default policies on the different chains with a fairly simple command, as described below.

iptables [-P {chain} {policy}]

The default policy is used every time the packets do not match a rule in the chain. For example, let's say we get a packet that matches no single rule in our whole rule-set. If this happens, we must decide what should happen to the packet in question, and this is where the default policy comes into the picture. The default policy is used on all packets that does not match with any other rule in our rule-set.

Do be cautious with what default policy you set on chains in other tables since they are simply not made for filtering, and it may lead to very strange behaviors.

---

13.2.6. Setting up user specified chains in the filter table

Now you have a good picture of what we want to accomplish with this firewall, so let us get on to the actual implementation of the rule-set. It is now high time that we take care of setting up all the rules and chains that we wish to create and to use, as well as all of the rule-sets within the chains.

After this, we create the different special chains that we want to use with the -N command. The new chains are created and set up with no rules inside of them. The chains we will use are, as previously described, icmp_packets, tcp_packets, udp_packets and the allowed chain, which is used by the tcp_packets chain. Incoming packets on $INET_IFACE, of ICMP type, will be redirected to the chain icmp_packets. Packets of TCP type, will be redirected to the tcp_packets chain and incoming packets of UDP type from $INET_IFACE go to udp_packets chain. All of this will be explained more in detail in the INPUT chain section below. To create a chain is quite simple and only consists of a short declaration of the chain as this:

iptables [-N chain]

In the upcoming sections we will have a closer look at each of the user defined chains that we have by now created. Let us have a closer look at how they look and what rules they contain and what we will accomplish within them.

---

13.2.6.1. The bad_tcp_packets chain

The bad_tcp_packets chain is devoted to contain rules that inspect incoming packets for malformed headers or other problems. As it is, we have only chosen to include a packet filter which blocks all incoming TCP packets that are considered as NEW but do not have the SYN bit set, as well as a rule that blocks SYN/ACK packets that are considered NEW. This chain could be used to check for all possible inconsistencies, such as above or XMAS port-scans etc. We could also add rules that looks for state INVALID.

If you want to fully understand the NEW not SYN, you need to look at the State NEW packets but no SYN bit set section in the Common problems and questions appendix regarding state NEW and non-SYN packets getting through other rules. These packets could be allowed under certain circumstances but in 99% of the cases we wouldn't want these packets to get through. Hence, we log them to our logs and then we DROP them.

The reason that we REJECT SYN/ACK packets that are considered NEW is also very simple. It is described in more depth in the SYN/ACK and NEW packets section in the Common problems and questions appendix. Basically, we do this out of courtesy to other hosts, since we will prevent them from being attacked in a sequence number prediction attack.

---

13.2.6.2. The allowed chain

If a packet comes in on $INET_IFACE and is of TCP type, it travels through the tcp_packets chain and if the connection is against a port that we want to allow traffic on, we want to do some final checks on it to see if we actually do want to allow it or not. All of these final checks are done within the allowed chain.

First of all, we check if the packet is a SYN packet. If it is a SYN packet, it is most likely to be the first packet in a new connection so, of course, we allow this. Then we check if the packet comes from an ESTABLISHED or RELATED connection, if it does, then we, again of course, allow it. An ESTABLISHED connection is a connection that has seen traffic in both directions, and since we have seen a SYN packet, the connection then must be in state ESTABLISHED, according to the state machine. The last rule in this chain will DROP everything else. In this case this pretty much means everything that has not seen traffic in both directions, i.e., we didn't reply to the SYN packet, or they are trying to start the connection with a non SYN packet. There is no practical use of not starting a connection with a SYN packet, except to port scan people pretty much. There is no currently available TCP/IP implementation that supports opening a TCP connection with something else than a SYN packet to my knowledge, hence, DROP it since it is 99% sure to be a port scan.

---

13.2.6.3. The TCP chain

The tcp_packets chain specifies what ports are allowed to use on the firewall from the Internet. There is, however, even more checks to do, hence we send each and every one of the packets on to the allowed chain, which we described previously.

-A tcp_packets tells iptables in which chain to add the new rule, the rule will be added to the end of the chain. -p TCP tells it to match TCP packets and -s 0/0 matches all source addresses from 0.0.0.0 with netmask 0.0.0.0, in other words all source addresses. This is actually the default behavior but I am using it just to make everything as clear as possible. --dport 21 means destination port 21, in other words if the packet is destined for port 21 they also match. If all the criteria are matched, then the packet will be targeted for the allowed chain. If it doesn't match any of the rules, they will be passed back to the original chain that sent the packet to the tcp_packets chain.

As it is now, I allow TCP port 21, or FTP control port, which is used to control FTP connections and later on I also allow all RELATED connections, and that way we allow PASSIVE and ACTIVE connections since the ip_conntrack_ftp module is, hopefully, loaded. If we do not want to allow FTP at all, we can unload the ip_conntrack_ftp module and delete the $IPTABLES -A tcp_packets -p TCP -s 0/0 --dport 21 -j allowed line from the rc.firewall.txt file.

Port 22 is SSH, which is much better than allowing telnet on port 23 if you want to allow anyone from the outside to use a shell on your box at all. Note that you are dealing with a firewall. It is always a bad idea to give others than yourself any kind of access to a firewall box. Firewalls should always be kept to a bare minimum and no more.

Port 80 is HTTP, in other words your web server, delete it if you do not want to run a web server directly on your firewall.

And finally we allow port 113, which is IDENTD and might be necessary for some protocols like IRC, etc to work properly. Do note that it may be worth it to use the oidentd package if you NAT several hosts on your local network. oidentd has support for relaying IDENTD requests on to the correct boxes within your local network.

If you feel like adding more open ports with this script, well, it should be quite obvious how to do that by now. Just cut and paste one of the other lines in the tcp_packets chain and change it to the port you want to open.

---

13.2.6.4. The UDP chain

If we do get a UDP packet on the INPUT chain, we send them on to udp_packets where we once again do a match for the UDP protocol with -p UDP and then match everything with a source address of 0.0.0.0 and netmask 0.0.0.0, in other words everything again. Except this time, we only accept specific UDP ports that we want to be open for hosts on the Internet. Do note that we do not need to open up holes depending on the sending hosts source port, since this should be taken care of by the state machine. We only need to open up ports on our host if we are to run a server on any UDP port, such as DNS etc. Packets that are entering the firewall and that are part of an already established connection (by our local network) will automatically be accepted back in by the --state ESTABLISHED,RELATED rules at the top of the INPUT chain.

As it is, we do not ACCEPT incoming UDP packets from port 53, which is what we use to do DNS lookups. The rule is there, but it is per default commented out. If you want your firewall to act as a DNS server, uncomment this line.

I personally also allow port 123, which is NTP or network time protocol. This protocol is used to set your computer clock to the same time as certain other time servers which have very accurate clocks. Most of you probably do not use this protocol and hence I am not allowing it per default. The same thing applies here, however, the rule is there and it is simple to uncomment to get it working.

We do not currently allow port 2074, which is used for certain real-time multimedia applications like speak freely which you can use to talk to other people in real-time by using speakers and a microphone, or even better, a headset. If you would like to use this, you could turn it on quite simply by removing the comment.

Port 4000 is the ICQ protocol. This should be an extremely well known protocol that is used by the Mirabilis application named ICQ. There are at least 2-3 different ICQ clones for Linux and it is one of the most widely used chat programs in the world. I doubt there is any further need to explain what it is.

At this point, two extra rules are available if you are experiencing a lot of log entries due to different circumstances. The first rule will block broadcast packets to destination ports 135 through 139. These are used by NetBIOS, or SMB for most Microsoft users. This will block all log entries we may get from iptables logging Microsoft network activity on the outside of our firewall. The second rule was also created to take care of excessive logging problems, but instead takes care of DHCP queries from the outside. This is specifically true if your outside network consists of a non-switched Ethernet type of network, where the clients receive their IP addresses by DHCP. During these circumstances, you could wind up with a lot of logs from just that.

Do note that the last two rules are specifically opted out since some people may be interested in these kind of logs. If you are experiencing problems with excessive legit logging, try to drop these types of packages at this point. There are also more rules of this type just before the log rules in the INPUT chain.

---

13.2.6.5. The ICMP chain

This is where we decide what ICMP types to allow. If a packet of ICMP type comes in on eth0 on the INPUT chain, we then redirect it to the icmp_packets chain as explained before. Here we check what kind of ICMP types to allow. For now, I only allow incoming ICMP Echo requests, TTL equals 0 during transit and TTL equals 0 during reassembly. The reason that we do not allow any other ICMP types per default here, is that almost all other ICMP types should be covered by the RELATED state rules.

If an ICMP packet is sent as a reply to an already existing packet or packet stream it is considered RELATED to the original stream. For more information on the states, read the The state machine chapter.

The reason that I allow these ICMP packets is as follows, Echo Requests are used to request an echo reply, which in turn is used to mainly ping other hosts to see if they are available on any of the networks. Without this rule, other hosts will not be able to ping us to see if we are available on any network connection. Do note that some people would tend to erase this rule, since they simply do not want to be seen on the Internet. Deleting this rule will effectively render any pings to our firewall totally useless from the Internet since the firewall will simply not respond to them.

Time Exceeded (i.e., TTL equals 0 during transit and TTL equals 0 during reassembly), is allowed in the case we want to trace-route some host or if a packet gets its Time To Live set to 0, we will get a reply about this. For example, when you trace-route someone, you start out with TTL = 1, and it gets down to 0 at the first hop on the way out, and a Time Exceeded is sent back from the first gateway en route to the host we are trying to trace-route, then TTL = 2 and the second gateway sends Time Exceeded, and so on until we get an actual reply from the host we finally want to get to. This way, we will get a reply from each host on our way to the actual host we want to reach, and we can see every host in between and find out what host is broken.

For a complete listing of all ICMP types, see the ICMP types appendix . For more information on ICMP types and their usage, i suggest reading the following documents and reports:

• RFC 792 - Internet Control Message Protocol by J. Postel.

As a side-note, I might be wrong in blocking some of these ICMP types for you, but in my case, everything works perfectly while blocking all the ICMP types that I do not allow.

---

13.2.10. PREROUTING chain of the nat table

The PREROUTING chain is pretty much what it says, it does network address translation on packets before they actually hit the routing decision that sends them onward to the INPUT or FORWARD chains in the filter table. The only reason that we talk about this chain in this script is that we once again feel obliged to point out that you should not do any filtering in it. The PREROUTING chain is only traversed by the first packet in a stream, which means that all subsequent packets will go totally unchecked in this chain. As it is with this script, we do not use the PREROUTING chain at all, however, this is the place we would be working in right now if we wanted to do DNAT on any specific packets, for example if you want to host your web server within your local network. For more information about the PREROUTING chain, read the Traversing of tables and chains chapter.

The PREROUTING chain should not be used for any filtering since, among other things, this chain is only traversed by the first packet in a stream. The PREROUTING chain should be used for network address translation only, unless you really know what you are doing.

14.1. rc.firewall.txt script structure

All scripts written for this tutorial have been written after a specific structure. The reason for this is that they should be fairly similar to each other and to make it easier to find the differences between the scripts. This structure should be fairly well documented in this brief chapter. This chapter should hopefully give a short understanding to why all the scripts have been written as they have, and why I have chosen to maintain this structure.

Even though this is the structure I have chosen, do note that this may not be the best structure for your scripts. It is only a structure that I have chosen to use since it fits the need of being easy to read and follow the best according to my logic.

---

14.1.1. The structure

This is the structure that all scripts in this tutorial should follow. If they differ in some way it is probably an error on my part, unless it is specifically explained why I have broken this structure.

1. Configuration - First of all we have the configuration options which the rest of the script should use. Configuration options should pretty much always be the first thing in any shell-script.

   1. Internet - This is the configuration section which pertains to the Internet connection. This could be skipped if we do not have any Internet connection. Note that there may be more subsections than those listed here, but only such that pertain to our Internet connection.

      1. DHCP - If there are possibly any special DHCP requirements with this specific script, we will add the DHCP specific configuration options here.

      2. PPPoE - If there is a possibility that the user that wants to use this specific script, and if there are any special circumstances that raises the chances that he is using a PPPoE connection, we will add specific options for those here.

   2. LAN - If there is any LAN available behind the firewall, we will add options pertaining to that in this section. This is most likely, hence this section will almost always be available.

   3. DMZ - If there is any reason to it, we will add a DMZ zone configuration at this point. Most scripts lacks this section, mainly because any normal home network, or small corporate network, will not have one.

   4. Localhost - These options pertain to our local-host. These variables are highly unlikely to change, but we have put most of it into variables anyway. Hopefully, there should be no reason to change these variables.

   5. iptables - This section contains iptables specific configuration. In most scripts and situations this should only require one variable which tells us where the iptables binary is located.

   6. Other - If there are any other specific options and variables, they should first of all be fitted into the correct subsection (If it pertains to the Internet connection, it should be sub-sectioned there, etc). If it does not fit in anywhere, it should be sub-sectioned directly to the configuration options somewhere.

2. Module loading - This section of the scripts should maintain a list of modules. The first part should contain the required modules, while the second part should contain the non-required modules.

   Note that some modules that may raise security, or add certain services or possibilities, may have been added even though they are not required. This should normally be noted in such cases within the example scripts.

   1. Required modules - This section should contain the required modules, and possibly special modules that add to the security or add special services to the administrator or clients.

   2. Non-required modules - This section contains modules that are not required for normal operations. All of these modules should be commented out per default, and if you want to add the service it provides, it is up to you.

3. proc configuration - This section should take care of any special configuration needed in the proc file system. If some of these options are required, they will be listed as such, if not, they should be commented out per default, and listed under the non-required proc configurations. Most of the useful proc configurations will be listed here, but far from all of them.

   1. Required proc configuration - This section should contain all of the required proc configurations for the script in question to work. It could possibly also contain configurations that raise security, and possibly which add special services or possibilities for the administrator or clients.

   2. Non-required proc configuration - This section should contain non-required proc configurations that may prove useful. All of them should be commented out, since they are not actually necessary to get the script to work. This list will contain far from all of the proc configurations or nodes.

4. rules set up - By now the scripts should most probably be ready to insert the rule-set. I have chosen to split all the rules down after table and then chain names in the rule-sets, to make them easier to follow and read. All user specified chains are created before we do anything to the system built in chains. I have also chosen to set the chains and their rule specifications in the same order as they are output by the iptables -L command.

   1. Filter table - First of all we go through the filter table and its content. First of all we should set up all the policies in the table.

      1. Set policies - Set up all the default policies for the system chains. Normally I will set DROP policies on the chains in the filter table, and specifically ACCEPT services and streams that I want to allow inside. This way we will get rid of all ports that we do not want to let people use.

      2. Create user specified chains - At this point we create all the user specified chains that we want to use later on within this table. We will not be able to use these chains in the system chains anyway if they are not already created so we might as well get to it as soon as possible.

      3. Create content in user specified chains - After creating the user specified chains we may as well enter all the rules within these chains. The only reason I have to enter this data at this point already is that you may as well put it close to the creation of the user specified chains. You may as well put this later on in your script, it is totally up to you.

      4. INPUT chain - When we have come this far, we do not have a lot of things left to do within the filter table so we get onto the INPUT chain. At this point we should add all rules within the INPUT chain.

         At this point we start following the output from the iptables -L command as you may see. There is no reason for you to stay with this structure, however, do try to avoid mixing up data from different tables and chains since it will become much harder to read such rule-sets and to fix possible problems.

      5. FORWARD chain - At this point we go on to add the rules within the FORWARD chain. Nothing special about this decision.

      6. OUTPUT chain - Last of all in the filter table, we add the rules dealing with the OUTPUT chain. There should, hopefully, not be too much to do at this point.

   2. nat table - After the filter table we take care of the nat table. This is done after the filter table because of a number of reasons within these scripts. First of all we do not want to turn the whole forwarding mechanism and NAT function on at too early a stage, which could possibly lead to packets getting through the firewall at just the wrong time point (i.e., when the NAT has been turned on, but none of the filter rules has been run). Also, I look upon the nat table as a sort of layer that lies just outside the filter table and kind of surrounds it. The filter table would hence be the core, while the nat table acts as a layer lying around the filter table, and finally the mangle table lies around the nat table as a second layer. This may be wrong in some perspectives, but not too far from reality.

      1. Set policies - First of all we set up all the default policies within the nat table. Normally, I will be satisfied with the default policy set from the beginning, namely the ACCEPT policy. This table should not be used for filtering anyways, and we should not let packets be dropped here since there are some really nasty things that may happen in such cases due to our own presumptions. I let these chains be set to ACCEPT since there is no reason not to do so.

      2. Create user specified chains - At this point we create any user specified chains that we want within the nat table. Normally I do not have any of these, but I have added this section anyways, just in case. Note that the user specified chains must be created before they can actually be used within the system chains.

      3. Create content in user specified chains - By now it should be time to add all the rules to the user specified chains in the nat table. The same thing goes here as for the user specified chains in the filter table. We add this material here since I do not see any reason not to.

      4. PREROUTING chain - The PREROUTING chain is used to do DNAT on packets in case we have a need for it. In most scripts this feature is not used, or at the very least commented out. The reason being that we do not want to open up big holes to our local network without knowing about it. Within some scripts we have this turned on by default since the sole purpose of those scripts is to provide such services.

      5. POSTROUTING chain - The POSTROUTING chain should be fairly well used by the scripts I have written since most of them depend upon the fact that you have one or more local networks that we want to firewall against the Internet. Mainly we will try to use the SNAT target, but in certain cases we are forced to use the MASQUERADE target instead.

      6. OUTPUT chain - The OUTPUT chain is barely used at all in any of the scripts. As it looks now, it is not broken, but I have been unable to find any good reasons to use this chain so far. If anyone has a reason to use this chain, send me a line and I will add it to the tutorial.

   3. mangle table - The last table to do anything about is the mangle table. Normally I will not use this table at all, since it should normally not be used for anyone, unless they have specific needs, such as masking all boxes to use the exact same TTL or to change TOS fields etc. I have in other words chosen to leave these parts of the scripts more or less blank, with a few exceptions where I have added a few examples of what it may be used for.

      1. Set policies - Set the default policies within the chain. The same thing goes here as for the nat table, pretty much. The table was not made for filtering, and hence you should avoid it alltogether. I have not set any policies in any of the scripts in the mangle table one way or the other, and you are encouraged not to do so either.

      2. Create user specified chains - Create all the user specified chains. Since I have barely used the mangle table at all in the scripts, I have neither created any chains here since it is fairly unusable without any data to use within it. However, this section was added just in case someone, or I, would have the need for it in the future.

      3. Create content in user specified chains - If you have any user specified chains within this table, you may at this point add the rules that you want within them here.

      4. PREROUTING - At this point there is barely any information in any of the scripts in this tutorial that contains any rules here.

      5. INPUT chain - At this point there is barely any information in any of the scripts in this tutorial that contains any rules here.

      6. FORWARD chain - At this point there is barely any information in any of the scripts in this tutorial that contains any rules here.

      7. OUTPUT chain - At this point there is barely any information in any of the scripts in this tutorial that contains any rules here.

      8. POSTROUTING chain - At this point there is barely any information in any of the scripts in this tutorial that contains any rules here.

Hopefully this should explain more in detail how each script is structured and why they are structured in such a way.

Do note that these descriptions are extremely brief, and should mainly just be seen as a brief explanation to what and why the scripts have been split down as they have. There is nothing that says that this is the only and best way to go.

14.2. rc.firewall.txt

The rc.firewall.txt script is the main core on which the rest of the scripts are based upon. The rc.firewall file chapter should explain every detail in the script most thoroughly. Mainly it was written for a dual homed network. For example, where you have one LAN and one Internet Connection. This script also makes the assumption that you have a static IP to the Internet, and hence don't use DHCP, PPP, SLIP or some other protocol that assigns you an IP automatically. If you are looking for a script that will work with those setups, please take a closer look at the rc.DHCP.firewall.txt script.

The rc.firewall.txt script requires the following options to be compiled statically to the kernel, or as modules. Without one or more of these, the script will become more or less flawed since parts of the script's required functionalities will be unusable. As you change the script you use, you could possibly need more options to be compiled into your kernel depending on what you want to use.

• CONFIG_NETFILTER

• CONFIG_IP_NF_CONNTRACK

• CONFIG_IP_NF_IPTABLES

• CONFIG_IP_NF_MATCH_LIMIT

• CONFIG_IP_NF_MATCH_STATE

• CONFIG_IP_NF_FILTER

• CONFIG_IP_NF_NAT

• CONFIG_IP_NF_TARGET_LOG

14.3. rc.DMZ.firewall.txt

The rc.DMZ.firewall.txt script was written for those people out there that have one Trusted Internal Network, one De-Militarized Zone and one Internet Connection. The De-Militarized Zone is in this case 1-to-1 NATed and requires you to do some IP aliasing on your firewall, i.e., you must make the box recognize packets for more than one IP. There are several ways to get this to work, one is to set 1-to-1 NAT, another one if you have a whole subnet is to create a subnetwork, giving the firewall one IP both internally and externally. You could then set the IP's to the DMZed boxes as you wish. Do note that this will "steal" two IP's for you, one for the broadcast address and one for the network address. This is pretty much up to you to decide and to implement. This tutorial will give you the tools to actually accomplish the firewalling and NATing part, but it will not tell you exactly what you need to do since it is out of the scope of the tutorial.

The rc.DMZ.firewall.txt script requires these options to be compiled into your kernel, either statically or as modules. Without these options, at the very least, available in your kernel, you will not be able to use this scripts functionality. You may in other words get a lot of errors complaining about modules and targets/jumps or matches missing. If you are planning to do traffic control or any other things like that, you should see to it that you have all the required options compiled into your kernel there as well.

• CONFIG_NETFILTER

• CONFIG_IP_NF_CONNTRACK

• CONFIG_IP_NF_IPTABLES

• CONFIG_IP_NF_MATCH_LIMIT

• CONFIG_IP_NF_MATCH_STATE

• CONFIG_IP_NF_FILTER

• CONFIG_IP_NF_NAT

• CONFIG_IP_NF_TARGET_LOG

You need to have two internal networks with this script as you can see from the picture. One uses IP range 192.168.0.0/24 and consists of a Trusted Internal Network. The other one uses IP range 192.168.1.0/24 and consists of the De-Militarized Zone which we will do 1-to-1 NAT to. For example, if someone from the Internet sends a packet to our DNS_IP, then we use DNAT to send the packet on to our DNS on the DMZ network. When the DNS sees our packet, the packet will be destined for the actual DNS internal network IP, and not to our external DNS IP. If the packet would not have been translated, the DNS wouldn't have answered the packet. We will show a short example of how the DNAT code looks:

$IPTABLES -t nat -A PREROUTING -p TCP -i $INET_IFACE -d $DNS_IP \
--dport 53 -j DNAT --to-destination $DMZ_DNS_IP
    

First of all, DNAT can only be performed in the PREROUTING chain of the nat table. Then we look for TCP protocol on our $INET_IFACE with destination IP that matches our $DNS_IP, and is directed to port 53, which is the TCP port for zone transfers between name servers. If we actually get such a packet we give a target of DNAT. After that we specify where we want the packet to go with the --to-destination option and give it the value of $DMZ_DNS_IP, in other words the IP of the DNS on our DMZ network. This is how basic DNAT works. When the reply to the DNATed packet is sent through the firewall, it automatically gets un-DNATed.

By now you should have enough understanding of how everything works to be able to understand this script pretty well without any huge complications. If there is something you don't understand that hasn't been gone through in the rest of the tutorial, mail me since it is probably a fault on my side.

14.4. rc.DHCP.firewall.txt

The rc.DHCP.firewall.txt script is pretty much identical to the original rc.firewall.txt. However, this script no longer uses the STATIC_IP variable, which is the main change to the original rc.firewall.txt script. The reason is that this won't work together with a dynamic IP connection. The actual changes needed to be done to the original script are minimal, however, I've had some people mail me and ask about the problem so this script will be a good solution for you. This script will allow people who uses DHCP, PPP and SLIP connections to connect to the Internet.

The rc.DHCP.firewall.txt script requires the following options to be compiled statically to the kernel, or as modules, as a bare minimum to run properly.

• CONFIG_NETFILTER

• CONFIG_IP_NF_CONNTRACK

• CONFIG_IP_NF_IPTABLES

• CONFIG_IP_NF_MATCH_LIMIT

• CONFIG_IP_NF_MATCH_STATE

• CONFIG_IP_NF_FILTER

• CONFIG_IP_NF_NAT

• CONFIG_IP_NF_TARGET_MASQUERADE

• CONFIG_IP_NF_TARGET_LOG

The main changes done to the script consist of erasing the STATIC_IP variable as I already said and deleting all references to this variable. Instead of using this variable the script now does its main filtering on the variable INET_IFACE. In other words -d $STATIC_IP has been changed to -i $INET_IFACE. This is pretty much the only change made and that's all that's needed really.

There are some more things to think about though. We can no longer filter in the INPUT chain depending on, for example, --in-interface $LAN_IFACE --dst $INET_IP. This in turn forces us to filter only based on interfaces in such cases where the internal machines must access the Internet addressable IP. One great example is if we are running an HTTP on our firewall. If we go to the main page (i.e., http://192.168.0.1/), which contains static links back to the same host (i.e., http://foobar.dyndns.net/fuubar.html), which could be some dyndns solution, we would get a minor problem. The NATed box would ask the DNS for the IP of the HTTP server, then try to access that IP. In case we filter based on interface and IP, the NATed box would be unable to get to the HTTP because the INPUT chain would DROP the packets flat to the ground. This also applies in a sense to the case where we got a static IP, but in such cases it could be gotten around by adding rules which check the LAN interface packets for our INET_IP, and if so ACCEPT them.

As you may read from above, it may be a good idea to get a script, or write one, that handles dynamic IP in a better sense. We could for example make a script that grabs the IP from ifconfig and adds it to a variable, upon boot-up of the Internet connection. A good way to do this, would be to use, for example, the ip-up scripts provided with pppd and some other programs. For a good site, check out the linuxguruz.org iptables site which has a huge collection of scripts available to download. You will find a link to the linuxguruz.org site from the Other resources and links appendix.

This script might be a bit less secure than the rc.firewall.txt script. I would definitely advise you to use that script if at all possible since this script is more open to attacks from the outside.

Also, there is the possibility to add something like this to your scripts:

INET_IP=`ifconfig $INET_IFACE | grep inet | cut -d : -f 2 | \
cut -d ' ' -f 1`
    

The above would automatically grab the IP address of the $INET_IFACE variable, grep the correct line which contains the IP address and then cuts it down to a manageable IP address. For a more elaborate way of doing this, you could apply the snippets of code available within the retreiveip.txt script, which will automatically grab your Internet IP address when you run the script. Do note that this may in turn lead to a little bit of "weird" behavior, such as stalling connections to and from the firewall on the internal side. The most common strange behaviors are described in the following list.

1. If the script is run from within a script which in turn is executed by, for example, the PPP daemon, it will hang all currently active connections due to the NEW not SYN rules (see the State NEW packets but no SYN bit set section). It is possible to get by, if you get rid of the NEW not SYN rules for example, but it is questionable.

2. If you got rules that are static and always want to be around, it is rather harsh to add and erase rules all the time, without hurting the already existing ones. For example, if you want to block hosts on your LAN to connect to the firewall, but at the same time operate a script from the PPP daemon, how would you do it without erasing your already active rules blocking the LAN?

3. It may get unnecessarily complicated, as seen above which, in turn, could lead to security compromises. If the script is kept simple, it is easier to spot problems, and to keep order in it.

14.5. rc.UTIN.firewall.txt

The rc.UTIN.firewall.txt script will in contrast to the other scripts block the LAN that is sitting behind us. In other words, we don't trust anyone on any networks we are connected to. We also disallow people on our LAN to do anything but specific tasks on the Internet. The only things we actually allow are POP3, HTTP and FTP access to the Internet. We also don't trust the internal users to access the firewall more than we trust users on the Internet.

The rc.UTIN.firewall.txt script requires the following options to be compiled statically to the kernel, or as modules. Without one or more of these, the script will become more or less flawed since parts of the script's required functionalities will be unusable. As you change the script you use, you could possibly need more options to be compiled into your kernel depending on what you want to use.

• CONFIG_NETFILTER

• CONFIG_IP_NF_CONNTRACK

• CONFIG_IP_NF_IPTABLES

• CONFIG_IP_NF_MATCH_LIMIT

• CONFIG_IP_NF_MATCH_STATE

• CONFIG_IP_NF_FILTER

• CONFIG_IP_NF_NAT

• CONFIG_IP_NF_TARGET_LOG

This script follows the golden rule to not trust anyone, not even our own employees. This is a sad fact, but a large part of the hacks and cracks that a company gets hit by are a matter of people from their own staff perpetrating the hit. This script will hopefully give you some clues as to what you can do with your firewall to strengthen it. It's not very different from the original rc.firewall.txt script, but it does give a few hints at what we would normally let through etc.

14.6. rc.test-iptables.txt

The rc.test-iptables.txt script can be used to test all the different chains, but it might need some tweaking depending on your configuration, such as turning on ip_forwarding, and setting up masquerading etc. It will work for most everyone who has all the basic set up and all the basic tables loaded into kernel. All it really does is set some LOG targets which will log ping replies and ping requests. This way, you will get information on which chain was traversed and in which order. For example, run this script and then do:

ping -c 1 host.on.the.internet
    

And tail -n 0 -f /var/log/messages while doing the first command. This should show you all the different chains used, and in which order, unless the log entries are swapped around for some reason.

This script was written for testing purposes only. In other words, it's not a good idea to have rules like this that log everything of one sort since your log partitions might get filled up quickly and it would be an effective Denial of Service attack against you and might lead to real attacks on you that would be unlogged after the initial Denial of Service attack.

14.7. rc.flush-iptables.txt

The rc.flush-iptables.txt script should not really be called a script in itself. The rc.flush-iptables.txt script will reset and flush all your tables and chains. The script starts by setting the default policies to ACCEPT on the INPUT, OUTPUT and FORWARD chains of the filter table. After this we reset the default policies of the PREROUTING, POSTROUTING and OUTPUT chains of the nat table. We do this first so we won't have to bother about closed connections and packets not getting through. This script is intended for actually setting up and troubleshooting your firewall, and hence we only care about opening the whole thing up and resetting it to default values.

After this we flush all chains first in the filter table and then in the NAT table. This way we know there are no redundant rules lying around anywhere. When all of this is done, we jump down to the next section where we erase all the user specified chains in the NAT and filter tables. When this step is done, we consider the script done. You may consider adding rules to flush your mangle table if you use it.

One final word on this issue. Certain people have mailed me asking me to put this script into the original rc.firewall script using Red Hat Linux syntax where you type something like rc.firewall start and the script starts. However, I will not do that since this is a tutorial and should be used as a place to fetch ideas mainly and it shouldn't be filled up with shell scripts and strange syntax. Adding shell script syntax and other things makes the script harder to read as far as I am concerned and the tutorial was written with readability in mind and will continue being so.

14.8. Limit-match.txt

The limit-match.txt script is a minor test script which will let you test the limit match and see how it works. Load the script up, and then send ping packets at different intervals to see which gets through, and how often they get through. All echo replies will be blocked until the threshold for the burst limit has again been reached.

14.9. Pid-owner.txt

The pid-owner.txt is a small example script that shows how we could use the PID owner match. It does nothing real, but you should be able to run the script, and then from the output of iptables -L -v be able to tell that the rule actually matches.

14.10. Recent-match.txt

The recent-match.txt script is a small example of how the recent match can be used. For a complete explanation of this script take a look at the Recent match section in the Iptables matches chapter.

14.11. Sid-owner.txt

The sid-owner.txt is a small example script that shows how we could use the SID owner match. It does nothing real, but you should be able to run the script, and then from the output of iptables -L -v be able to tell that the rule actually matches.

14.12. Ttl-inc.txt

A small example ttl-inc.txt script. This script shows how we could make the firewall/router invisible to traceroutes, which would otherwise reveal much information to possible attackers.

14.13. Iptables-save ruleset

A small example script used in the Saving and restoring large rule-sets chapter to illustrate how iptables-save may be used. This script is non-working, and should hence not be used for anything else than a reference.

15.1. fwbuilder

Firewall Builder, or simply fwbuilder, is an extremely versatile and powerful tool that can be used to build your own firewalls, or to maintain several firewalls for that matter. It can be used to create policies for several different types of firewalls, including iptables (Linux 2.4 and 2.6), ipfilter (freebsd, netbsd, etc), openbsd pf, and, with a module that must be bought, Cisco PIX.

Fwbuilder has, as you can see, a very big audience and is well taken care of and continues to be developed. It is run on a separate host system, where you create the policy files, and then copy them over and run them on the target system. It is able to handle everything from very simple rulesets to large and rather complicated ones. It has extensive abilities to handle different versions and installations of iptables, by configuration of which targets/matches are available on each host system, etcetera. The end result may be saved in an xml file, or a system parsable configuration file (e.g., the real firewall scripts).

You can see the configuration of the "firewall" in the above example, and the main menus of the whole fwbuilder system. fwbuilder can be found at http://www.fwbuilder.org.

15.2. Turtle Firewall Project

Turtle Firewall is an excellent, yet simpler kind of user interface to iptables. It is integrated in something called webmin (a web administration interface). It is fairly basic, and neither as complex nor able to handle as complex changes as the fwbuilder package, but it is more than able to handle most simpler firewalls, as well as some more advanced ones as well.

One big advantage with Turtle Firewall is the fact that it is web-based, and hence can be remotely controlled in a totally different manner than with fwbuilder and most other tools. Of course, it also adds more of a security risk since webmin is a separate extra service running on the firewall itself.

The above screenshot shows the items page of the Turtle Firewall, where you can configure network interfaces and networks, and other items.

This final screenshot shows the turtlefirewalls main screen, and with the whole ruleset expanded at the bottom. The whole ruleset isn't showing, as you can see, but you get a good general idea of what it looks like in Turtle Firewall.

You can find the Turtle Firewall Project and more information over at http://www.turtlefirewall.com/.

15.3. Integrated Secure Communications System

The Integrated Secure Communications System, or shortly ISCS, is still undergoing development, and no public version has been released. However, this looks like it will become an extremely helpful tool once it is finished. The developer has very high standards, and this is the main reason that it has not been released yet. ISCS integrates several functionalities into a single suite of administration and management user interface. Basically this means that once this project is released, you will be able to fully configure all your firewalls from a centralized point using a single GUI, including VPN's, VLAN's, Tunnels, sysctl's, etcetera.

The main attack angle that the developer(s) of ISCS has, is to simplify management and administration and to remove tedious work for the administrators, so to save as much work hours as possible for the administrators. This is done by putting together policies, and then the programs creates the rulesets and "pushes" them out to the "enforcements points" (e.g., firewalls, proxies, etcetera). The administrator doesn't actually "write" or "click" together the rulesets, just simply put together policies that are then enforced by ISCS.

This tool isn't finished yet, as of writing this. However, I have been in touch with the main developer of this project before, and this is indeed a very large project. When it is finished, I believe this will be one of the best tools on the market. Of course, time can only tell, but it is well worth mentioning here. You can find the ISCS project over at http://iscs.sourceforge.net/.

The main developer, John Sullivan, of ISCS has specifically asked me to ask people to join his development efforts. The project is very big, and he would definitely like as much help with the project as possible. If you are able to help, you are, in other words, more than welcome.

15.4. IPMenu

IPMenu is a very able program, yet simple to operate and not too demanding on resources nor bandwidth. It is a console based program, so it works perfect over an SSH connection for example. It works perfectly on machines running over a simple and old modem as well.

As you can see from the screenshot, it is able to handle all iptables functionality, including filtering, mangling and nating. It is also able to handle routing tables and bandwidth shaping and to save and restore rulesets. You can add new rules directly into the currently running iptables script easily, and handle all of the different tables. Including adding and removing custom chains.

As you can see from the screenshot above, the program is rather basic, but still able to handle most situations rather well. And first of all, it is very simple, and can be used for remote administration simply enough, and since it runs on top of ssh via a standard console, it should also be fairly secure. You can find the homepage of IPMenu at http://users.pandora.be/stes/ipmenu.html.

15.5. Easy Firewall Generator

Easy Firewall Generator is another interesting development when it comes to iptables and netfilter. Basically, Easy Firewall Generator is a PHP webpage where you specify options and specifics of your firewall, and once all of the configurations are done, you click a button, and the webpage spits out an iptables ruleset that you can utilize.

The script contains all the basic rules, and more specific ones to contain strange patterns in packets. It also contains specific IP sysctl changes that may be needed, loads necessary modules, et cetera. The whole ruleset is also written in a redhat init.d format.

This screenshot shows one of the final stages of configuring the firewall script that is about to be created by the script. You can find more information, and a working version of the Easy Firewall Generator at http://easyfwgen.morizot.net/.

15.6. What's next?

In this chapter we have looked closer at what can be done with some different graphical user interfaces, and other user interfaces as well. Note that there are several more user interfaces around on the market. This chapter has mainly given you an idea of the different types of firewall administration interfaces around on the market. Most of them are open source and free to use, while some will cost a bit of money to get full support or functionality from.