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Sponsored by: Search | Newsletter | Conference | Tech Jobs O’Reilly’s Emerging Technology Conference: May 13-16, 2002 Articles Linux Apache MySQL Perl PHP Python BSD Essentials What is LAMP? The Best of ONLamp.com aboutSQL Big Scary Daemons FreeBSD Basics HTTP Wrangler Linux in the Enterprise Linux Network Administration The Linux Professional Perl P5P Digest Archive PHP Admin Basics PHP Phanatics Python_News Security Alerts Alphabetical Directory of Linux Commands This directory of Linux commands is from Linux in a Nutshell, 3rd Edition. Click on any of the 379 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Tech Books Online. Buy it now Read it online iptables command [options] System administration command. Configure netfilter filtering rules. In the 2.4 kernel, the ipchains firewall capabilities are replaced with the netfilter kernel module. netfilter can be configured to work just like ipchains, but it also comes with the module iptables, which is similar to ipchains but extensible. iptables rules consist of some matching criteria and a target, 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, masquerade your local area network, or just reject certain kinds of network connections. There are three built-in tables for iptables, one for network filtering (filter), one for Network Address Translation (nat), and the last for specialized packet alterations (mangle). Firewall rules are organized into chains, ordered check lists of rules that the kernel works through looking for matches. The filter table has three built-in chains: INPUT, OUTPUT, and FORWARD. The INPUT and OUTPUT chains handle packets originating from or destined for the host system. The FORWARD chain handles mail just passing through the host system. The nat table also has three built-in chains: PREROUTING, POSTROUTING, and OUTPUT. mangle has only two chains: PREROUTING and OUTPUT. netfilter checks packets entering the system. After applying any PREROUTING rules it passes them to the INPUT chain or to the FORWARD chain if the packet is just passing through. Upon leaving, the system packets are passed to the OUTPUT chain and then on to any POSTROUTING rules. Each of these chains has a default target, a policy, in case no match is found. User-defined chains can also be created and used as targets for packets but do not have 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. iptables only changes the rules in the running kernel. When the system is powered off, all changes are lost. You can use the iptables-save command to make a script you can run with iptables-restore to restore your firewall settings. Such a script is often called at bootup. Many distributions will have an iptables initialization script that uses the output from iptables-save. Commands Sponsored by: