Learn to read, test and reason about the files under /etc/pam.d so you can diagnose authentication behavior quickly. 20.11.2025 | reading time: 3 min On a Linux host the Pluggable Authentication Modules live as configuration files under /etc/pam.d; reading them tells you which modules a service loads and in what order, and that is often all you need to diagnose login issues. Quick hands-on example Look at a service file and read it: ```bash $ ls -l /etc/pam.d/sshd -rw-r--r-- 1 root root 123 Apr 10 10:00 /etc/pam.d/sshd $ cat /etc/pam.d/sshd auth required pam_sepermit.so auth include common-auth account required pam_nologin.so password include common-password session required pam_loginuid.so session include common-session ``` — you just inspected the order and includes that control SSH authentication. Control flags and why order matters PAM lines use control flags such as required, requisite, sufficient and optional; order is decisive because a "required" failure still allows later modules to run but ultimately denies access, whereas "sufficient" can grant access early; when troubleshooting, follow the file top-to-bottom and note any "include" directives that pull in other files. Useful checks and small experiments Try verbose tests and dry runs with helper tools or by enabling debug in the service; for example use `pamtester` to simulate an authentication or inspect systemd unit files to see which PAM service name they reference; never edit files on a remote system without a recovery shell because a syntax mistake can lock out console logins. Where this fits in your toolkit PAM configuration is one piece of the authentication stack: complement file inspection with log checks in /var/log/auth.log or journalctl, review NSS/PAM modules like pam_unix or pam_sss, and consider how SSSD, LDAP or Kerberos interact with the PAM modules you see. Wrap-up and next steps Inspecting /etc/pam.d files will give you clear insight into authentication flow and is a quick first step when users cannot log in; deepen this skill by testing modules safely and study certification objectives such as CompTIA Linux+ or LPIC-1, and consider intensive exam preparation at bitsandbytes.academy. Join Bits & Bytes Academy First class LINUX exam preparation. security setup utilities troubleshooting processes
