IPChains

One of those great new features in the 2.2.x kernel in the current generation of linux distributions. A firewall box is a must if your are fortunate enough to posess a cable modem. Until I find spare time for migration, ipfwadm on the 2.0.36 kernel is keeping my home network safe from script kiddies.

First appeared somewhere in the middle of the 2.1.x series, replacing ipfwadm. Deprecated in Linux 2.4 in favor of NAT, however there is an ipchains compatibility module.

Is the IP forwarding module present in the 2.2.* kernels.

An excerpt from IPchains' man page:

"Ipchains is used to set up, maintain, and inspect the IP firewall rules in the Linux kernel. (...)"

Efectively, what it does is analize each packet that passes by and determine what should be done with it.

It is usually used to turn a Linux station into a gateway between an intranet (or set of computers connected to each other, normaly through a hub) and the internet.

It's mainly usefull to set up a private firewall, since the gateway can control what goes in and out of the inner network.