> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vms.verolabs.co/llms.txt
> Use this file to discover all available pages before exploring further.

# Probe: check_port

> Check TCP port reachability

## Description

The `check_port` probe performs a TCP connect to verify whether a port on a host is reachable. Supports both expecting the port to be open (default) and expecting it to be closed.

This is an active check — emits both metrics and a check result with latency.

## Config fields

| Field        | Type     | Required | Default | Description                                                                                                  |
| ------------ | -------- | -------- | ------- | ------------------------------------------------------------------------------------------------------------ |
| `type`       | `string` | Yes      | —       | Must be `"check_port"`                                                                                       |
| `host`       | `string` | Yes      | —       | Host or IP to check                                                                                          |
| `port`       | `int`    | Yes      | —       | TCP port (1–65535)                                                                                           |
| `expectOpen` | `bool`   | No       | `true`  | `true`: expect port open → `up` on successful connect. `false`: expect port closed → `up` on connect failure |

## Metrics

| Metric                      | Type   | Unit | Description      |
| --------------------------- | ------ | ---- | ---------------- |
| `vms.check_port.status`     | status | —    | `up` or `down`   |
| `vms.check_port.latency_ms` | gauge  | ms   | TCP connect time |

## Example config

```json theme={null}
{
  "type": "check_port",
  "host": "10.0.0.10",
  "port": 443,
  "expectOpen": true
}
```

**Check that a port is closed:**

```json theme={null}
{
  "type": "check_port",
  "host": "10.0.0.10",
  "port": 3389,
  "expectOpen": false
}
```

## Notes

* When the result doesn't match the expectation, severity is `critical`.
* A connect failure due to network is a normal result (`status=down`), not an agent error.