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: https://www.metalab.unc.edu/LDP/Firewall-HOWTO.html
DHCP mini-HOWTO: https://www.metalab.unc.edu/LDP/mini/DHCP.html
IPChains web site: https://www.rustcorp.com/linux/ipchains/
Linux Documentation Project: https://www.metalab.unc.edu/LDP/
Linux Kernel Archives: https://www.kernel.org/
“Linux Kernel Installation” by Daniel Lazenby, Linux Journal, page 54, November 1997
Linux NET-3-HOWTO, Linux Networking: https://www.metalab.unc.edu/LDP/NET-3-HOWTO.html
Linux Router Project (LRP): https://www.linuxrouter.org/
Linux Router Project Module Maker: https://www.linuxrouter.org/modmaker/
Security notices and information: https://www.cert.org/
Various newsgroups: