The external service is unavailable, but everything is running on the server. How do you check?

If the external service is unavailable but it is running on the server, I check step by step:

1. Process and logs check: Make sure the process is actually running (ps, systemctl status), and look at logs for errors.

2. Local access check: Try to connect to the service from the server itself (for example, curl localhost:port or telnet localhost port). If accessible locally — the service is working.

3. Network rules check: Check firewall settings (iptables, firewalld, ufw) and security groups (if in cloud), the port might be blocked.

4. Network interface check: Ensure the service listens on the correct interface (for example, 0.0.0.0, not just localhost). netstat -tulnp or ss -tulnp will show on which addresses the service is listening.

5. Routing and DNS check: Verify that IP and DNS are correctly configured, and requests reach the server.

6. Proxy and load balancer check: If there is nginx, haproxy, or other proxies, check their configuration and logs.

Thus, a sequential check from the service to the network will help identify the problem.