Adaptive Linux Firewalls

Source: Linux Gazette

Automatic firewall hardening is a technique used by many commercial firewalls to prevent invalid packets from reaching protected networks. The objective of this document is to demonstrate how to harden iptables in real-time.
By default, iptables can log messages via the Linux syslogd daemon. Logs by themselves are fine for basic security but do not address advanced security issues. For advanced security a system needs to run custom scripts as soon as illegal operations are logged by the firewall. Depending on the severity of the violations, you can program these scripts to perform various actions such as blocking offending IP addresses. The techniques outlined in this document are not limited to iptables, and can also be modified to process output from any application which logs via syslog e.g. intrusion detection scanners such as SNORT.