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Linux Journal: Taming the Wild Netfilter

Aug 27, 2001, 01:05 (1 Talkback[s])
(Other stories by David A. Bandel)
"For those of you who have taken the plunge and upgraded from kernel 2.2.X (or even 2.0.X) to 2.4.X, congratulations. If, like a number of folks, you're running some form of firewall using either ipchains or ipfwadm, your scripts may work fine. But sooner or later you're probably going to want to upgrade.

In the 2.4.X kernels, Rusty Russell, the Linux packet-filter guru, and his crew of coders have implemented Netfilter into the kernel. Netfilter is the replacement for ipchains or ipfwadm. Fortunately, Netfilter permits you to keep using ipchains or ipfwadm until you can come to grips with iptables by adding a compatibility layer via a kernel module that permits these older packet filters to run. But Netfilter has so many exciting new additions, you'll want to convert those rules as soon as possible. One word of caution, though, if you load the ipchains or ipfwadm modules, you can't load ip_tables (and vice versa). So it's all or nothing. After reading this article, however, making the change should be easy.

For those new to packet filtering, ignore the ipchains translations and use the iptables examples. While not all ipchains commands and options will be translated to iptables, this text should provide a good idea about how to construct a packet-filter firewall by translating ipchains commands into iptables commands.

The reason you'll want to upgrade to Netfilter is because it, unlike ipchains or ipfwadm, is stateful. What this means is it can track connections and permit incoming responses to outgoing requests without creating gaping holes in the firewall. The connection tracking opens a specific, temporary hole for responses and only from the contacted server. We'll see how this works later. The drawback is that with connection tracking in use, Netfilter will need to use a little more memory because the connections are tracked in RAM. So your 4MB 386-16 may no longer be up to the job, depending on your filtering requirements."

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