System administrators need to stay ahead of new security vulnerabilities that leave their networks exposed every day. A firewall and an intrusion detecting system (IDS) are two important weapons in that fight, enabling you to proactively deny access and monitor network traffic for signs of an attack. Linux Firewalls discusses the technical details of the iptables firewall and the Netfilter framework that are built into the Linux kernl, and it exaplins how they provide strong filtering, Network Address Translation (NAT), state tracking and application layer inspection capabilities that rival may commercial tools. You will learn how to deploy iptables as an IDS with psad and fwsnort and how to build a strong, passive authentication layer around iptables with fwknop. Concrete examples illustrate concepts such as firewall log analysis and policies, passive network authentication and authorization, exploit packet traces, Snort ruleset emulation, and more with coverage of: Application layer attack detection with the iptables string match extension and fwsnort; Building an iptables ruleset that emulates a Snort ruleset; Port knocking vs. Single Packet Authorization (SPA); Tools for visualizing iptables logs; Passive OS fingerprinting with iptables. Perl and C code snippets offer practical examples that will help you to maximize your deployment of Linux firewalls. If you are responsible for keeping a network secure, you will find Linux Firewalls invaluable in your attempt to understand attacks and use iptables - along with psad and fwsnort - to detect and even prevent compromises.