Linux in a Nutshell, Fourth Edition

Name

ipchains

Synopsis

                  ipchains 
                  command [options]

System administration command. Edit IP firewall rules in the 2.2 Linux kernel. A 2.2 Linux kernel compiled with firewall support will examine the headers of all network packets and compare them to matching rules to see what it should do with the packet. A firewall rule consists of some matching criteria and a target, which is a result to be applied if the packet matches the criteria. The rules are organized into chains. You can use these rules to build a firewall or just reject certain kinds of network connections.

Firewall rules are organized into chains, ordered checklists that the kernel works through looking for matches. There are three built-in chains: input, output, and forward. Packets entering the system are tested against the input chain; those exiting the system are checked against the output chain. If an incoming packet is destined for some other system, it is checked against the forward chain. Each of these chains has a default target (a policy) in case no match is found. User-defined chains can be created and used as targets for packets, but they have no default policies. If no match can be found in a user-defined chain, the packet is returned to the chain from which it was called and tested against the next rule in that chain.

ipchains changes only the rules in the running kernel. When the system is powered off, all those changes are lost. You can use the ipchains-save command to make a script you can later run with ...