Here is a list of handy pages & sites, as originally seen on http://www.linuxfirewall.com/ :
Linux IP Firewall Information
rc.firewall – a powerful firewall based on netfilter/iptables http://projectfiles.com/firewall/
ZDNet – Linux Firewall On A 486: http://www.zdnet.com/zdhelp/stories/main/0,5594,2503199,00.html
Firewall HOWTO: http://www.linuxfaq.com/LDP/HOWTO/Firewall-HOWTO.html
IPCHAINS HOWTO: http://www.linuxfaq.com/LDP/HOWTO/IPCHAINS-HOWTO.html
The rcf Linux Firewall: http://rcf.mvlan.net/
Security HOWTO: http://www.linuxfaq.com/LDP/HOWTO/Security-HOWTO.html
VPN-Masquerade HOWTO: http://www.linuxfaq.com/LDP/HOWTO/VPN-Masquerade-HOWTO.html
IPCHAINS script “rc.firewall”: http://www.jsmoriss.dyndns.org/linux/
SmoothWall (Linux-based GPLed Firewall) http://smoothwall.org/
Fireparse (ipchains log file analyzer): http://freecode.com/projects/fireparse
Misc. links on keeping your Linux box secured:
http://www.linuxworld.com/linuxworld/lw-1999-05/lw-05-ramparts_p.html
http://www.securityfocus.com/focus/linux/articles/linux-securing.html
http://www.isr.umd.edu/~danielf/Linux/securinglinux.html
http://www.rootprompt.org/article.php3?article=903
http://www.rootprompt.org/article.php3?article=922
http://securityportal.com/closet/closet20001101.html
http://www.gl.umbc.edu/~jjasen1/unix/linux.html
Stormix Firewall: http://www.stormix.com/products/firewall/
Intel Open Source Security Software CDSA: http://www.intel.com/pressroom/archive/releases/cn041100.htm
Linux Intrusion Detection System & hardening (LIDS): http://www.lids.org/
Libsafe (cool stuff from Bell Labs to prevent some buffer overflow exploits): http://www.bell-labs.com/org/11356/libsafe.html
Linux 2.4 NetFilter: http://www.netfilter.org/
Internet Firewalls Frequently Asked Questions: http://www.interhack.net/pubs/fwfaq/
You may also want to consider OpenBSD as your Firewall: http://www.openbsd.org/
FireStarter – Firewalls made easy: http://firestarter.sourceforge.net/
Internet Firewalls: http://www.cerias.purdue.edu/coast/firewalls/
SSH (get it!!): http://www.ssh.fi/ download: ftp://ftp.cs.hut.fi/pub/ssh/
PHP Firewall Generator: http://phpfwgen.sourceforge.net/
Protectix ProWall Security Solution: http://protectix.com/
Phoenix Adaptive Firewall Personal Edition: http://www.progressive-systems.com/downloads/phoenix/personal/
National Security Agency Selects Secure Computing to Provide Type Enforcement(TM) on Linux OS: http://biz.yahoo.com/prnews/000113/ca_secure__1.html
Darren Reed’s IP Filter: http://coombs.anu.edu.au/~avalon/
Trinux: http://www.trinux.org/
PortSentry: http://www.psionic.com/abacus/portsentry/
Tripwire: http://www.tripwiresecurity.com
AIDE (Tripwire replacement): http://www.cs.tut.fi/~rammer/aide.html
SOCKS: http://www.socks.nec.com/
TIS Firewall Toolkit: ftp://ftp.tis.com/pub/firewalls/toolkit/
Juniper Firewall Toolkit: http://www.obtuse.com/juniper/
NMAP (get it!!): http://www.insecure.org/nmap/
Dante: http://www.inet.no/dante/
Linux Router Project: http://www.linuxrouter.org/
COAST Project & tools: http://www.cerias.purdue.edu/coast/coast-tools.html
PMFirewall: http://www.pointman.org/PMFirewall/
Langistix ipchains-firewall: http://firewall.langistix.com/
Bastille Linux: http://bastille-linux.sourceforge.net/
Security-Script: http://halliday.wl.vg/scripts/howto.html
Linux™ is a registered trademark of Linus Torvalds.