Linux Fundamentals, 2nd Edition

Network Security Using Firewalls

Firewall software helps protect your Linux server from unwanted network access. They use access control lists (ACLs) to define what network connections can be allowed and which ones should be blocked.

The core system of creating and maintaining firewall ACLs in Linux is iptables. With iptables you define chains of rules that the kernel follows to determine if an inbound or outbound packet should be allowed or denied. However, defining rules in iptables can get a bit complicated, as it uses a somewhat arcane method of defining hosts and ports. To make life easier for administrators, a few different Linux firewall programs use easy-to-read commands to create the rules needed for the kernel to allow or deny packets. ...