Practice triggering and tracing PAM authentication decisions to find failures fast. 20.11.2025 | reading time: 2 min Want to know why a login fails even when the password is correct? This short guide shows how to trigger PAM decisions, observe module behaviour, and collect the logs needed to fix the problem quickly. When a login hides the reason PAM composes multiple modules that grant or deny access; a single failing module, a missing account condition, or a restrictive control flag can stop a login and produce little feedback, so active testing is the fastest path to the root cause. Hands-on: pamtester example Install and run a focused test to see the flow and outcome exactly: ```bash sudo apt-get install -y pamtester sudo pamtester login alice authenticate Password: Authentication succeeded ``` If pamtester prompts and returns success, PAM accepted the credentials for the service named login; a failure here isolates the problem to authentication or account modules rather than the application. Deeper checks and flags If the simple test fails, add debug to the relevant file in /etc/pam.d (for example append the word debug to a pam_unix.so line), then reproduce the attempt while watching system logs with `journalctl -f` or `tail -F /var/log/auth.log`; also inspect account state with `pam_tally2` or `faillog` and validate control flags like required, requisite, and sufficient to understand decision short-circuits. Other diagnostic techniques When PAM behaviour still puzzles, use `strace` on the service to observe system calls, enable verbose module options where supported, or create a minimal test service in /etc/pam.d to step through one module at a time and confirm which module changes the return code. Next steps for mastery PAM testing is a practical, repeatable skill: practice on throwaway users, build a checklist for common failures, and use the tools and logs to form a hypothesis and then prove it; for broader Linux competency, consider formal study toward CompTIA Linux+ or LPIC-1, and try intensive exam preparation at bitsandbytes.academy. Join Bits & Bytes Academy First class LINUX exam preparation. security troubleshooting processes utilities setup
