What Is Port Monitoring and When Do You Need It?

HTTP uptime monitoring checks whether a website responds correctly to HTTP/HTTPS requests. But not every service communicates over HTTP. Databases, mail servers, FTP servers, and custom applications listen on non-HTTP ports. Port monitoring checks whether a specific TCP port is open and accepting connections — independent of what protocol runs on it.

How Port Monitoring Works

Port monitoring works by attempting a TCP connection to a specific host and port combination. If the connection succeeds (the port accepts the connection), the check passes. If it fails (connection refused, timed out, or unreachable), the check fails and triggers an alert.

This is equivalent to running:

telnet yourdomain.com 3306
# or
nc -z yourdomain.com 3306

A successful connection confirms the port is open and the service is listening. It doesn't validate the service's actual behaviour — just that it's accepting connections.

Common Services and Their Ports

When to Use Port Monitoring

Databases

Database monitoring via port is useful when:

• Your database server is externally accessible (direct connection, not via application)
• You need to verify the database daemon is running
• You're monitoring a managed database service (RDS, Cloud SQL) for availability

Important: Most production databases should NOT be publicly accessible. If your MySQL is behind a VPC, port monitoring from an external service won't work. In that case, monitor the application that connects to the database, and use application-level health checks that test database connectivity.

Mail Servers

If you run your own mail server (Postfix, Exim), monitoring SMTP ports (25, 587) confirms email delivery is functioning. A closed SMTP port means emails are not being sent or received.

SSH Availability

Port 22 monitoring confirms SSH access to a server is available. Useful for:

• Verifying jump host / bastion availability
• Alerting if SSH is accidentally blocked (security group, firewall rule change)

Custom Services

Applications that expose custom TCP ports benefit from port monitoring when:

• The service doesn't speak HTTP
• You need to confirm the service process is running

Port Monitoring vs. HTTP Monitoring

HTTP monitoring is more thorough for web applications — it verifies the application is responding correctly, not just that a port is open. A web server with port 80 open might still return 500 errors; HTTP monitoring catches this while port monitoring doesn't.

Use port monitoring for non-HTTP services (databases, mail, SSH), and HTTP monitoring for everything web-based.

Security Considerations

Port monitoring from an external service requires the monitored port to be accessible from the internet. This is fine for web ports (80, 443) but should be carefully considered for services like databases.

Never expose database ports to the public internet just to enable port monitoring. Instead:

• Monitor your application's health endpoint that tests the database connection
• Use your cloud provider's internal monitoring for database availability

If you need to monitor a port that shouldn't be publicly accessible, use an agent installed inside your network that can reach the service.

Setting Up Port Monitoring

In Domain Monitor, port monitoring is configured alongside HTTP monitors:

Monitor type: Port
Host: yourdomain.com
Port: 3306
Check interval: 5 minutes
Alert channels: Email + Slack

Choose a check interval that makes sense for the service — critical services (mail server) might warrant 1-minute checks; less critical services can use 5 minutes.

Combining Port and HTTP Monitoring

For comprehensive coverage of a web server like Nginx or Apache:

• HTTP monitor on port 443 — checks HTTPS response, content, and SSL
• Port monitor on port 22 — confirms SSH access for emergency remediation

The HTTP monitor is your primary check; the port 22 monitor ensures you can get into the server to fix problems if the HTTP check fails.

Set up port monitoring alongside HTTP and SSL checks at Domain Monitor.

Related Articles

• Website Monitoring for SaaS Companies - What to Track and Why
• "Website Monitoring vs Application Monitoring: What's the Difference?"
• "What Is DNS TTL and How Does It Affect Website Monitoring?"