A NATural Progression
Netfilter and iptables make an extremely powerful firewall. But to take advantage of it, you need to master the basic syntax as explained in my first article (“Taming the Wild Netfilter”, published in the September 2001 issue of LJ), have an understanding of the modules and matches available to you and have an understanding of what a particular system can know about a packet. Armed with these three things, you can build highly complex, tailored firewall solutions for whatever problem you might have.
Know how to take advantage of new and experimental matches and targets (and always test them). You learned this in last month's article with the iptables build targets of pending-patches, most-of-pom and patch-o-matic. Testing by creating and sending specific packets to a firewall interface is beyond the scope of this particular article, but a number of utilities exist to assist you here (sendip or ipmagic come to mind).
Build the missing modules for the kernel (make sure they're selected).
Build your rule chains, more specific rules first followed by more general rules. If it helps you organize things, go ahead and build custom user chains that can be called from another chain. While this was not covered specifically in this article, it was addressed in the September 2001 article. Use everything at your disposal, including the LOG target to help you see if particular rules were applied and to which packets.
With just some basic knowledge, iptables are not difficult to use. Read some iptables scripts on the Internet. I don't recommend using them as they are; they almost certainly won't work for you without a lot of tweaking, but they will show you syntax, rules (from which you can grab fragments), thought processes, etc.
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