"This article will cover the basics of ipchains, including
how to compile the kernel, and the basics of configuring it. The
assumption is you have a basic knowledge of TCP/IP and firewalling.
ipchains is the firewall software available with Linux 2.2. It
is a rewrite of the older ipfwadm, used for configuring packet
filtering, ip masquerading and other parts of a firewall, under
"To use ipchains, you need support in the kernel. Specifically,
you need at least support for the following options:
ipchains allows you to manage rules in the kernel packet filtering
area, by adding or deleting rules from various chains. A chain is
simply a set of rules that decide what to do with each packet as it
traverses the chain, hence the name. By default, there are 3
chains: the input chain, the forward chain, and the output chain.
When a chain goes though the firewall, it first goes through the
input chain to see if it is allowed it. After this, we have the
routing decision, and if the packet isn't local, i.e. destined for
a different machine, it goes though the forward chain. Then, just
before the packet leaves the machine, it goes through the output
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