NetFilter

netfilter, sometimes referred to as iptables, is the filtering layer integrated with the Linux kernel. It was created by Rusty Russell in 1998.

It is an extremely flexible system that integrates with the kernel's routing and QoS functionalities, and includes its own NAT functions. It has many filtering criteria (time, data volume, etc.) and connection tracking modules for complex protocols (like FTP, SIP and H323). However, it is complicated to configure, so a third party tool is often used to generate its configuration (Shorewall, ferm, etc.).

netfilter is distributed under the GPL v2.