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.
InGate - InGate is a commercial firewall producer that uses Linux as the base for their firewall products. Their productrange goes from basic firewalls to SIP gateways and QoS machines.
RFC 768 - User Datagram Protocol - This is the official RFC describing how the UDP protocol should be used, in detail, and all of it's headers.
RFC 791 - Internet Protocol - The IP specification as still used on the Internet, with additions and updates. The basic is still the same for IPv4.
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.
RFC 1122 - Requirements for Internet Hosts - Communication Layers - This RFC defines the requirements of the software running on a Internet host, specifically the communication layers.
RFC 1349 - Type of Service in the Internet Protocol Suite - RFC describing some changes and clarifications of the TOS field in the IP header.
RFC 2401 - Security Architecture for the Internet Protocol - This is an RFC talking about the IPSEC implementation and standardisation. Well worth reading if you are working with IPSEC.
RFC 2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers - In this document you will find out how the DiffServ works, and you will find much needed information about the TCP/IP protocol additions/changes needed for the DiffServ protocol to work.
RFC 2638 - A Two-bit Differentiated Services Architecture for the Internet - This RFC describes a method of implementing two different differentiated service architecture into one. Both where described originally by D. Clark and van Jacobsen at the Munich IETH meeting 1997.
RFC 3168 - The Addition of Explicit Congestion Notification (ECN) to IP - This RFC defines how ECN is to be used on a technical level and how it should be implemented in the TCP and IP protocols. Written by K. Ramakrishnan, S. Floyd and D. Black.
RFC 3260 - New Terminology and Clarifications for Diffserv - This memo captures Diffserv working group agreements concerning new and improved terminology, and provides minor technical clarifications.
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 1.3.1 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.
Ipsysctl tutorial - Another tutorial I have written about the IP System Control in Linux. A try to make a complete listing of all the IP variables that can be set on the fly in Linux.
Policy Routing Using Linux - This is an excellent book that has now been opened up on the Internet regarding Policy routing in Linux. It is well written and most definitely worth buying. Written by Matthew G. Marsh.
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.insecure.org/nmap/ - Nmap is one of the best, and most known, port scanners available. It is very useful when debugging your firewall scripts. Take a closer look at it.
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.
Implementing Quality of Service Policies with DSCP - A link about the cisco implementation of DSCP. This shows some classes used in DSCP, and so on.
IETF SIP Working Group - SIP is one of the "next big things" it seems. Basically it is the defacto standards for Internet telephony today. It is horribly complex as you can see from the amount of documentation on the working groups homepage, and should hopefully be able to cope with pretty much any needs of session initiation in the future. It is used mainly to setup peer to peer connections between known users, for example to connect to firstname.lastname@example.org and setup a phone connection to that user. This is the IETF Working group handling all SIP work.
IETF TLS Working Group - TLS is a transport layer security model that is one of the most common host to server based security mechanisms. The current version is running is 1.1 and work is ongoing to get 1.2 out the door with support for newer and better cryptos as of this writing. This is a standardized way of sending and receiving public keys for servers and handling trusted certificate agents etc. For more information, read the RFC's on this page.
IPSEC Howto - This is the official IPSEC howto for Linux 2.6 kernels. It describes how IPSEC works in the 2.6 kernels and up, however, it is not the place to find out exactly how the Linux 2.2 and 2.4 kernels worked when it comes to IPSEC. Go to the FreeS/WAN site for that information.
FreeS/WAN - This is the official site for FreeS/WAN, an IPSEC implementation for the Linux 2.2 and 2.4 kernel series. This site contains documentation and all necessary downloads for the IPSEC implementation. This effort has been discontinued due to several reasons discussed on the page, but efforts will still be put into bugfixes, documentation and the forums. For an IPSEC implementation for Linux 2.6 kernels, please look at the IPSEC Howto site and the information there.
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 Assigned Numbers Authority - The IANA is the organisation that is responsible for fixing all numbers in the different protocols in an orderly fashion. If anyone has a specific addition to make to a protocol (for example, adding a new TCP option), they need to contact the IANA, which will assign the numbers requested. In other words, extremely important site to keep an eye on.
RFC-editor.org - This is an excellent site for finding RFC documents in a fast and orderly way. Functions for searching RFC documents, and general information about the RFC community (I.e., errata, news, et cetera).
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.
Snort - this 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 break-in it can do different actions that can be defined (notifying the administrator, or take action, or simply logging it).
Tripwire - tripwire is an excellent security tool which can be used to find out about host intrusions. It makes checksums of all the files specified in a configuration file, and then it tells the administrator about any files that has been tampered with in an illegit way every time it is run.
Squid - This is one of the most known webproxies available on the market. It is open source, and free. It can do several of the filtering tasks that should be done before the traffic actually hits your webserver, as well as doing the standard webcaching functions for your networks.
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/m ailman/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.