Linux Network Administrator's Guide, 3rd Edition

Chapter 7. TCP/IP Firewall

Security is increasingly important for companies and individuals alike. The Internet provides them with a powerful tool to distribute information about themselves and obtain information from others, but it also exposes them to dangers from which they were previously exempt. Computer crime, information theft, and malicious damage are all potential dangers.

This chapter covers the Linux features for setting up a firewall, known both by its command interface (iptables) and its kernel subsystem name (netfilter). This firewall implementation was new in the 2.4 kernel and works substantially the same way in 2.6.

A malicious person who gains access to a computer system may guess system passwords or exploit the bugs and idiosyncratic behavior of certain programs to obtain a working account on that host. Once they are able to log in to the host, they may have access to sensitive information. In a commercial setting, stealing, deleting, or modifying information such as marketing plans, new project details, or customer information databases can cause significant damage to the company.

The safest way to avoid such widespread damage is to prevent unauthorized people from gaining network access to the host. This is where firewalls come in.

Warning

Constructing secure firewalls is an art. It involves a good understanding of technology, but equally important, it requires an understanding of the philosophy behind firewall designs. We won’t cover everything you need to know ...