From this page you can browse several online resources for Firewall designers.
The TIS Firewall Toolkit Documentation.
The Linux Patches for the fwtk.
The TIS Firewall Toolkit Users Archive.
S/Key version 2.2 for Linux.
Tripwire is a file integrity checker.
The Official Internet Firewall FAQ.
Commercial Firewall Vendors.
Firewall Mailing List Archive.
Linux Multiple Ethernet mini-HOWTO