Is Your Service Actually Reachable?
You just deployed a new API. Nginx is running, your app is listening on port 8080, and the reverse proxy should be forwarding port 443 to it. But users are reporting connection timeouts. Is the problem your app, your reverse proxy, or your firewall?
Test port 443 on your server’s hostname. If it’s open, the firewall and Nginx are fine — the problem’s deeper (maybe your upstream config is wrong). If it’s closed, either Nginx isn’t running or a firewall rule is blocking HTTPS. Test port 8080 directly to see if the app is listening. Now you’ve narrowed it down in 10 seconds.
Quick-select buttons for the most common ports make testing fast: HTTP (80), HTTPS (443), SSH (22), MySQL (3306), PostgreSQL (5432), Redis (6379), and more.
What the Results Tell You
Open means a service is actively listening and accepting TCP connections on that port. Your web server is running, your database is reachable, your SSH daemon is ready.
Closed means nothing’s listening. The server received the connection attempt and explicitly refused it. Either the service isn’t running or it’s listening on a different port.
Filtered means a firewall, security group, or network device silently dropped the connection attempt. You can’t tell whether a service is running behind the firewall or not.
Quick Reference
| Port | Service | What it does |
|---|---|---|
| 22 | SSH | Remote terminal access |
| 80 | HTTP | Unencrypted web traffic |
| 443 | HTTPS | Encrypted web traffic |
| 3306 | MySQL | MySQL database connections |
| 5432 | PostgreSQL | PostgreSQL database connections |
| 6379 | Redis | Redis cache/data store |
| 27017 | MongoDB | MongoDB database connections |
Real Troubleshooting Workflows
Post-deployment check. You deployed a new service on port 5000. Before updating the load balancer, verify port 5000 is open on the new instance.
Firewall rule validation. You added a rule allowing port 8443 for a new internal service. Test it from outside to confirm the rule actually took effect.
Database connectivity. Your app can’t connect to the database. Is the database server accepting connections on port 5432? If the port’s filtered, check your security group rules. If it’s closed, the database service isn’t running.
Security audit. Check for unexpectedly open ports on your production servers. That open port 27017 (MongoDB) with no authentication? That’s a breach waiting to happen.
The URL Availability Checker tests whether a full URL responds (HTTP-level check). The HTTP Headers Analyzer inspects response headers. The SSL Certificate Checker verifies HTTPS certs. This tool operates at the TCP level — lower and simpler.