The "service is down" troubleshooting flow

When a Linux service stops responding:

1. Is the process running? systemctl status <service> or ps aux | grep <service>
2. Is it listening? ss -tlnp — is the port open?
3. Is it reachable locally? curl localhost:<port>
4. Is it reachable from outside? curl <host>:<port> from another box
5. What do the logs say? journalctl -u <service> -n 100
6. Any system-level issues? dmesg | tail, df -h (disk full?), free -m (out of memory?)
7. Network in the way? Firewall (iptables -L, ufw status), security group, NACL
8. DNS resolving? dig <name>
9. Did anything change? last, journalctl --since filtering, check deployment history