How to monitor a TCP port with Centoculus

Updated November 6, 2026

Short answer

A TCP port monitor in Centoculus opens a connection to a host and port from several check locations and reports the service as down when the connection fails. Use it for services without a web page, such as SSH on port 22, MySQL on 3306 or SMTP on 25.

The check type TCP port answers: is a service reachable on its port? It connects, and if the connection is accepted the service is up. It is the right check for services that have no web page: SSH, databases, mail servers, game servers, VPN endpoints.

Step by step

1. Choose the check type TCP port

Open Monitors, click New monitor and choose TCP port.

2. Enter host and port

Enter the host name or IP address, for example db.example.com, and the port, for example 3306. The port is required. The host must be publicly reachable.

Form of a new TCP port monitor for db.example.com on port 3306

3. Choose interval and locations, then create

Pick the check interval and the check locations, then click Create monitor. The interval and the number of locations depend on your plan.

4. Read the results

The monitor page shows per location whether the port is reachable and how long the connection took.

Detail page of a TCP port monitor with connection time per check location

Typical ports

Service	Port
SSH	22
SMTP (server to server)	25
SMTP submission with TLS	465
SMTP submission	587
IMAP with TLS	993
POP3 with TLS	995
MySQL / MariaDB	3306
PostgreSQL	5432
Redis	6379
RDP	3389

Fields and defaults

Field	What it does	Default
Host name or IP address	Server that is connected to	–
Port	TCP port, 1 to 65535	required
Check interval	How often the check runs	1 minute, depending on your plan

Frequently asked questions

My database should not be open to the internet. How do I monitor it?

Do not open the database port for this. Install the server agent instead: it connects outbound only and monitors the database service from the inside.

The port monitor reports down, but the service runs. Why?

A firewall drops connections from outside, or the service only listens on 127.0.0.1. Check with ss -tlnp on the server which address the service listens on. The guide on connection refused covers the common causes.

Does the check log in or send data?

No. It only opens the TCP connection and closes it again. It shows that the service accepts connections, not that it answers correctly.

Can I check UDP ports?

No. The check type uses TCP connections.

Guide

ERR_CONNECTION_REFUSED: causes and how to fix it

Free tool

Website down checker

Plans

Which check types, intervals and locations your plan includes.

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