The following is a list of resources that may assist you in setting up your firewall. There are many more--just look around on the Internet.
Firewall HOWTO: http://www.metalab.unc.edu/LDP/Firewall-HOWTO.html
DHCP mini-HOWTO: http://www.metalab.unc.edu/LDP/mini/DHCP.html
IPChains web site: http://www.rustcorp.com/linux/ipchains/
Linux Documentation Project: http://www.metalab.unc.edu/LDP/
Linux Kernel Archives: http://www.kernel.org/
``Linux Kernel Installation'' by Daniel Lazenby, Linux Journal, page 54, November 1997
Linux NET-3-HOWTO, Linux Networking: http://www.metalab.unc.edu/LDP/NET-3-HOWTO.html
Linux Router Project (LRP): http://www.linuxrouter.org/
Linux Router Project Module Maker: http://www.linuxrouter.org/modmaker/
Security notices and information: http://www.cert.org/
Various newsgroups: