Refer to the following sources for additional information on packet filtering with iptables.
man iptables — Contains a description of iptables as well as a comprehensive list of targets, options, and match extensions.
http://www.netfilter.org/ — The home of the netfilter/iptables project. Contains assorted information about iptables, including a FAQ addressing specific problems and various helpful guides by Rusty Russell, the Linux IP firewall maintainer. The HOWTO documents on the site cover subjects such as basic networking concepts, 2.4 kernel packet filtering, and NAT configurations.
http://www.linuxnewbie.org/nhf/Security/IPtables_Basics.html — A basic and general look at the way packets move through the Linux kernel, plus an introduction to constructing basic iptables commands.
http://www.redhat.com/support/resources/networking/firewall.html — This webpage links to a variety of update-to-date packet filter resources.
Red Hat Enterprise Linux Security Guide; Red Hat, Inc. — Contains a chapter about the role of firewalls within an overall security strategy as well as strategies for constructing firewall rules.
Red Hat Enterprise Linux System Administration Guide; Red Hat, Inc. — Contains a chapter about configuring firewalls using Security Level Configuration Tool.