Here is a list of links to resources and where I have gotten information from, etc :
ip-sysctl.txt - from the 2.4.14 kernel. A little bit short but a good reference for the IP networking controls and what they do to the kernel.
The Internet Control Message Protocol - A good, brief document describing the ICMP protocol in detail. Written by Ralph Walden.
RFC 792 - Internet Control Message Protocol - The definitive resource for all information about ICMP packets. Whatever technical information you need about the ICMP protocol, this is where you should turn first. Written by J. Postel.
RFC 793 - Transmission Control Protocol - This is the original resource on how TCP should behave on all hosts. This document has been the standard on how TCP should work since 1981 and forward. Extremely technical, but a must read for anyone who wants to learn TCP in every detail. This was originally a Department of Defense standard written by J. Postel.
ip_dynaddr.txt - from the 2.4.14 kernel. A really short reference to the ip_dynaddr settings available via sysctl and the proc file system.
iptables.8 - The iptables 1.2.4 man page. This is an HTMLized version of the man page which is an excellent reference when reading/writing iptables rule-sets. Always have it at hand.
Firewall rules table - A small PDF document gracefully given to this project by Stuart Clark, which gives a reference form where you can write all of the information needed for your firewall, in a simple manner.
http://www.netfilter.org/ - The official Netfilter and iptables site. It is a must for everyone wanting to set up iptables and Netfilter in linux.
http://www.netfilter.org/documentation/index.html#FAQ - The official Netfilter Frequently Asked Questions. Also a good place to start at when wondering what iptables and Netfilter is about.
http://www.netfilter.org/unreliable-guides/packet-filtering-HOWTO/index.html - Rusty Russells Unreliable Guide to packet filtering. Excellent documentation about basic packet filtering with iptables written by one of the core developers of iptables and Netfilter.
http://www.netfilter.org/unreliable-guides/NAT-HOWTO/index.html - Rusty Russells Unreliable Guide to Network Address Translation. Excellent documentation about Network Address Translation in iptables and Netfilter written by one of the core developers, Rusty Russell.
http://www.netfilter.org/unreliable-guides/netfilter-hacking-HOWTO/index.html - Rusty Russells Unreliable Netfilter Hacking HOW-TO. One of the few documentations on how to write code in the Netfilter and iptables user-space and kernel space code-base. This was also written by Rusty Russell.
http://www.linuxguruz.org/iptables/ - Excellent link-page with links to most of the pages on the Internet about iptables and Netfilter. Also maintains a list of iptables scripts for different purposes.
http://www.islandsoft.net/veerapen.html - Excellent discussion on automatic hardening of iptables and how to make small changes that will make your computer automatically add hostile sites to a special ban list in iptables.
/etc/protocols - An example protocols file taken from the Slackware distribution. This can be used to find out what protocol number different protocols have, such as the IP, ICMP or TCP protocols have.
/etc/services - An example services file taken from the Slackware distribution. This is extremely good to get used to reading once in a while, specifically if you want to get a basic look at what protocols runs on different ports.
Internet Engineering Task Force - This is one of the biggest groups when it comes to setting and maintaining Internet standards. They are the ones maintaining the RFC repository, and consist of a large group of companies and individuals that work together to ensure the interoperability of the Internet.
Linux Advanced Routing and Traffic Control HOW-TO - This site hosts the Linux Advanced Routing and Traffic Control HOWTO. It is one of the biggest and best documents regarding Linux advanced routing. Maintained by Bert Hubert.
Paksecured Linux Kernel patches - A site containing all of the kernel patches written by Matthew G. Marsh. Among others, the FTOS patch is available here.
ULOGD project page - The homepage of the ULOGD site.
The Linux Documentation Project is a great site for documentation. Most big documents for Linux is available here, and if not in the TLDP, you will have to search the net very carefully. If there is anything you want to know more about, check this site out.
http://kalamazoolinux.org/presentations/20010417/conntrack.html - This presentation contains an excellent explanation of the conntrack modules and their work in Netfilter. If you are interested in more documentation on conntrack, this is a "must read".
http://www.docum.org - Excellent information about the CBQ, tc and the ip commands in Linux. One of the few sites that has any information at all about these programs. Maintained by Stef Coene.
http://lists.samba.org/mailman/listinfo/netfilter - The official Netfilter mailing-list. Extremely useful in case you have questions about something not covered in this document or any of the other links here.
And of course the iptables source, documentation and individuals who helped me.