---
title: TCP & UDP port monitors
description: Check that a specific port on a server is open and answering.
category: monitors
updated: 2026-10-12
tags: [tcp, udp, port, smtp]
---

Port monitors check a single port on a host — perfect for services that aren't
websites, like databases, mail servers, SSH, or game servers.

![A TCP monitor's detail panel for a mail relay on port 25, showing the connection time and recent check history](/img/docs/app-monitor-tcp.png)
*A TCP monitor on a mail relay. The target is written as `host:port`.*

- **TCP** monitors (all plans) confirm a TCP port accepts a connection.
- **UDP** monitors (Pro, Business and MSP) confirm a UDP service is reachable.

## Setting it up

- **Target** — `host:port`, e.g. `db.example.com:5432` or `8.8.8.8:53`.

## What counts as up or down

- A **TCP** monitor is **up** when a connection to the port completes, and **down**
  only if every attempt is refused or times out. We retry briefly before calling it
  down, so a single dropped packet won't cause a false alarm.
- A **UDP** monitor is **up** when the service replies or silently accepts the probe,
  and **down** only when the port is confirmed closed or unreachable. Because UDP is
  connectionless, "up" often just means "nothing rejected it".

## Monitoring SMTP (port 25)

Mail servers listen on port 25, and you can absolutely watch it with a TCP monitor.
One rule applies: **port 25 checks run at most once every 2 minutes.** Connecting to
a mail port too frequently looks like spam activity to anti-spam systems, which can
get the checking servers temporarily blocked — and that would show up as false
"down" alerts. The 2-minute minimum keeps your monitor accurate and protects the
checking network. This is a fixed technical limit, not a plan restriction, and the
form enforces it automatically.

## Good to know

- Some networks block or rate-limit certain outbound ports (25 in particular). If a
  port monitor flaps, check whether something between us and your host is throttling
  connections.
- A port being open doesn't guarantee the service behind it is healthy — but it's a
  strong, lightweight signal.

## Get started

- [Create a port monitor](/docs/getting-started/quickstart)
- [Set a check interval](/docs/configuration/check-intervals)

## Related

- [Ping (ICMP) monitors](/docs/monitors/ping)
- [Firewall allowlist](/docs/advanced/firewall-allowlist)