TrinityOS is a step-by-step, example-driven HOWTO on building a very functional Linux box with strong security in mind. TrinityOS is well known for its strong packet firewall ruleset, Chrooted and Split DNS (v9 and v8), secured Sendmail (8.x), Linux PPTP, Serial consoles and Reverse TELNET, DHCPd, SSHd, UPSes, system performance tuning, the automated TrinityOS-Security implementation scripts, and much more.
SLIS is a TCP/IP gateway for schools. It provides advanced functions to students and teachers, such as a filtering proxy/cache, email addresses, access control, Web replication, Web mirroring, an intranet Web server with PHP and PostgreSQL, user management, DHCP, remote monitoring of a global set of SLIS servers, live update, etc. All of this is provided by an easy-to-use Web interface. All of the project is in French, but work is in progress to make it international.
The IP Masquerade HOWTO is the document that contains instructions on understanding, configuring, and troubleshooting NAT or Network Address Translation for Linux. It covers topics such as IPTABLES, PORTFW, IPCHAINS, IPFWADM, stronger packet firewalls, multiple network segments, and configuring many client operating systems. It also has an extensive FAQ and troubleshooting section.