Preface

A networking firewall is a logical barrier designed to prevent unauthorized or unwanted communications between sections of a computer network. Linux-based firewalls besides being highly customizable and versatile are also robust, inexpensive, and reliable.

The two things needed to build firewalls and QoS with Linux are two packages named netfilter and iproute. While netfilter is a packet-filtering framework included in the Linux kernels 2.4 and 2.6, iproute is a package containing a few utilities that allow Linux users to do advanced routing and traffic shaping.

L7-filter is a packet classifier for the Linux kernel that doesn't look up port numbers or Layer 4 protocols, but instead looks at the data in an IP packet and does a regular expression match on it to determine what kind of data it is, mainly what application protocol is being used. IP2P is an alternative to L7-filter, but has been designed for filtering only P2P applications while L7-filter takes into consideration a wider range of applications.