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# Probe: connection_mon

> Monitor TCP connection between a process and a service endpoint (bidirectional)

## Description

The `connection_mon` probe checks whether a specific process currently holds an ESTABLISHED TCP connection to a service endpoint. Designed for "trace between two servers" — verifying connectivity between systems.

**Bidirectional matching**: The same config can be deployed on **both ends**:

* **Forward**: this host is the source (local = source, remote = dest service)
* **Reverse**: this host is the dest (local = dest service, remote = source)

Requires both `ProcessAdapter` and `ConnAdapter`.

## Config fields

| Field               | Type     | Required | Default | Description                                                 |
| ------------------- | -------- | -------- | ------- | ----------------------------------------------------------- |
| `type`              | `string` | Yes      | —       | Must be `"connection_mon"`                                  |
| `name`              | `string` | Yes      | —       | Connection link identifier                                  |
| `processName`       | `string` | Yes      | —       | Process name glob (case-insensitive, same as `process_mon`) |
| `executablePath`    | `string` | No       | —       | Executable path to narrow match (ANDed)                     |
| `destServiceIp`     | `string` | Yes      | —       | Destination service IP (well-known end)                     |
| `destServicePort`   | `int`    | Yes      | —       | Destination service port (1–65535)                          |
| `sourceServiceIp`   | `string` | No       | —       | Source IP (optional filter)                                 |
| `sourceServicePort` | `int`    | No       | —       | Source port (usually ephemeral, left unset)                 |
| `protocol`          | `string` | No       | `"tcp"` | Only `"tcp"` supported                                      |

## Metrics

| Metric                         | Type   | Labels                                                                 | Description                                    |
| ------------------------------ | ------ | ---------------------------------------------------------------------- | ---------------------------------------------- |
| `vms.connection_mon.status`    | status | `name`, `dest_ip`, `dest_port`, `proto`, `process_name`*, `direction`* | `up` or `down`                                 |
| `vms.connection_mon.up`        | gauge  | (same)                                                                 | `1` = ESTABLISHED connection found, `0` = none |
| `vms.agent.permission_missing` | gauge  | `capability=CAP_SYS_PTRACE`                                            | When agent cannot attribute sockets to PIDs    |

## Example config

```json theme={null}
{
  "type": "connection_mon",
  "name": "app-to-postgres",
  "processName": "java",
  "destServiceIp": "10.0.1.50",
  "destServicePort": 5432
}
```

## Notes

* Only TCP `ESTABLISHED` state is considered "normal/healthy".
* Same config deployed on both ends: host A sees `direction=forward`, host B sees `direction=reverse`.
* Without `CAP_SYS_PTRACE`, a process owned by another user shows 0 connections (false negative) with `vms.agent.permission_missing` emitted.