[ Thanks to Harry for
this link. ]
“Ipchains is a packet-filtering firewall package. You can findan
RPM of Ipchains in RedHat/RPMS in the latest PC Quest RedHat CD.
First you have to check whether the Linux kernel supports Ipchains.
For this, look for a file named ip_fwchains in /proc/net. If it’s
not there, then you will have to recompile your kernel. The
KERNEL-HOWTO and IPCHAINS-HOWTO will help you. Ifthe file exists,
then you can check whether Ipchains is already installed….”
“Ipchains is so called because it deals with IP packets at
the Network Layer, and the rules defined in it are based on three
builtin chains called input chain, output chain and forward
chain. A rule can be something like “if the sourceof the
packet is Sachin’s machine, then deny access”. Packets arriving
atthe machine running Ipchains are compared against rules defined
in the input chain. If these packets are destined for another
machine, they are redirectedafter being compared against rules in
the forward chain. The output chain processes packets going out of
the firewall. Apart from these three, you can also have your own
user-defined chains. The rules for each chain define access control
based on source, destination, port, protocol or other information
contained in IP headers.”