Want to learn how to “roll your own” firewall with Netfilter? The Linux Journal details exactly how right here!

An excerpt:

“Every self-respecting Linux guru should be familiar with firewalls and how to install and configure them. With this in mind, Linux gurus also should be curious about how firewalls function and how to build a firewall of his or her own. Explaining exactly these two things is the goal of this article. Here, we attempt to write a firewall in less than 60 lines of C code. As impossible as this may sound, it actually is quite simple to do using the power of Linux kernel modules and Netfilter.

Netfilter is a packet filtering subsystem in the Linux kernel stack and has been there since kernel 2.4.x. Netfilter’s core consists of five hook functions declared in linux/netfilter_ipv4.h. Although these functions are for IPv4, they aren’t much different from those used in the IPv6 counterpart. The hooks are used to analyze packets in various locations on the network stack.” (Linux Journal)