In the eighth lesson of the Fail2ban Mastery course, Dargslan demonstrates how to write custom filters for Fail2ban, the intrusion prevention tool for Linux. The video outlines a six-step workflow that sysadmins can use to create tailored filters for any application.
The 6-Step Workflow
- Find: Locate the relevant log entries for the service you want to protect.
- Generalize: Create a regular expression that matches the failed authentication attempts.
- Multi-line failregex: For logs that span multiple lines, write a failregex that captures the entire pattern.
- Ignoreregex: Define patterns to exclude from triggering bans (e.g., known good IPs).
- Datepattern: Specify the date/time format used in the logs so Fail2ban can track timing.
- Ship + Test: Deploy the filter and verify it works using
fail2ban-regex.
By following these steps, users can create filters that automatically ban IP addresses based on custom criteria, enhancing server security without relying on pre-built filters alone.