Linux as an ADSL router

• adsl4linux.de (German, but they have a large link list as well)
• DSL-HOWTO (linuxdoc.org)

Kernel 2.4 HOWTOs

• The Netfilter Project Homepage
• Netfilter Documentation
• Rusty's Remarkably Unreliable Guides (iptables)

Kernel 2.2 HOWTO

• ipchains HOWTO

IP Masquerading HOWTO (Kernel 2.2.x and 2.4.x)

• Masquerading HOWTO

Frontends for the generation of firewall scripts:

• Firewall builder - Very good, very comfortable. Build your firewall per drag and drop. - Supports ipchains (Kernel 2.2.x) and iptables (Kernel 2.4.x); installation is rather annoying due to dependencies with numerous packages that need to be installed, with each of them having to bear exactly the needed version number.
• Easytables - iptables frontend, console based - Kernel 2.4.x
• KNetfilter - KDE frontend for iptables - Kernel 2.4.x