Linux Security Errors: SELinux denies, expired certs, firewall blocks, fail2ban
Working reference for the 10 most common Linux security errors. SELinux AVC denial (ausearch + audit2allow + restorecon), AppArmor DENIED (aa-complain + aa-genprof), TLS cert expired (certbot timer), firewall blocking expected traffic with iptables -L -nv counter diagnosis, fail2ban banned-IP recovery, SSH brute force defenses (key-only / fail2ban / port move), sudo timeout in long scripts, NET::ERR_CERT_REVOKED, audit log full, GPG signature verification. Cross-linked to auth errors, file permissions, and networking errors.