This is the Linux IPCHAINS-HOWTO; see Where? for the master site, which contains the latest copy. You should read the Linux NET-3-HOWTO as well. The IP-Masquerading HOWTO, the PPP-HOWTO, the Ethernet-HOWTO and the Firewall HOWTO might make interesting reading. (Then again, so might the alt.fan.bigfoot FAQ).
If packet filtering is passe to you, read Section Why?, Section How?, and scan through the titles in Section IP Firewalling Chains.
If you are converting from ipfwadm, read Section Introduction, Section How?, and Appendices in section Differences between ipchains and ipfwadm and section Using the `ipfwadm-wrapper' script.
Linux ipchains is a rewrite of the Linux IPv4 firewalling code (which was mainly stolen from BSD) and a rewrite of ipfwadm, which was a rewrite of BSD's ipfw, I believe. It is required to administer the IP packet filters in Linux kernel versions 2.1.102 and above.
The older Linux firewalling code doesn't deal with fragments, has 32-bit counters (on Intel at least), doesn't allow specification of protocols other than TCP, UDP or ICMP, can't make large changes atomically, can't specify inverse rules, has some quirks, and can be tough to manage (making it prone to user error).
Currently the code is in the mainstream kernel from 2.1.102. For the 2.0 kernel series, you will need to download a kernel patch from the web page. If your 2.0 kernel is more recent than the supplied patch, the older patch should be OK; this part of the 2.0 kernels is fairly stable (eg. the 2.0.34 kernel patch works just fine on the 2.0.35 kernel). Since the 2.0 patch is incompatible with the ipportfw and ipautofw patches, I don't recommend applying it unless you really need some functionality that ipchains offers.
The official page is in three places: Thanks to Penguin Computing Thanks to the SAMBA Team Thanks to Jim Pick
There is a mailing list for bug reports, discussion, development and usage. Join the mailing list by sending a message containing the word ``subscribe ipchains-list'' to subscribe at east.balius.com. To mail to everyone on the list use ipchains-list at east.balius.com.