Security checklist/howto:
CERT UNIX security checklist
http://www.cert.org/tech_tips/usc20_full.html
Linux security checklist
http://www.wfu.edu/~rbhm/linux.html
SANS institute securing solaris paper
http://rr.sans.org/unix/sec_solaris.php
Linux security
http://www.linuxsecurity.com
Linux security HOWTO
http://www.tldp.org/HOWTO/Security-HOWTO.html

General documentation:
The Linux documentation project
http://www.tldp.org/

Routing and firewall docs:
Linux advanced routing and traffic control HOWTO
http://lartc.org/HOWTO//cvs/2.4routing/2.4routing-howto.html
Linux firewall and proxy HOWTO
http://www.tldp.org/HOWTO/Firewall-HOWTO.html
Linux rootkit checker
http://www.chkrootkit.org/
