Linux Authentication & User Errors: 10 Common Login Problems
Working reference for the 10 most common Linux authentication errors. PAM authentication failure (auth.log diagnosis), user not in sudoers (visudo + /etc/sudoers.d/ drop-ins), UNPROTECTED PRIVATE KEY (SSH 700/600 perms), pam_faillock account lockout (faillock --reset), sudo no-tty for cron (NOPASSWD scoped), KEX algorithm mismatch with old servers, /sbin/nologin shell trap, stale group memberships requiring re-login or newgrp, sudo 3-strikes, SSH wrong-key-offered diagnosis. Cross-links to User & Service Management, File Permissions, and Security Errors.