# WHATPORT(5666)

## NAME

port 5666 — NRPE (Nagios Remote Plugin Executor) — A monitoring agent — the Nagios server connects here to run checks on this host.

Protocol: TCP  
IANA registry

## DESCRIPTION

NRPE runs on a monitored machine and executes local plugins (disk, load, process checks) on request from a Nagios, Icinga, or Naemon server, returning the status text and exit code. IANA registered 5666 to Nagios Enterprises in 2017. Access control is a plaintext allowed_hosts list plus, in NRPE 3 and later, TLS — but the default anonymous-DH mode encrypts without authenticating either side. Enabling dont_blame_nrpe so callers may pass arguments has repeatedly turned into remote command execution.

## EXPOSURE

**Do not expose.** It exists to run commands on the host for whoever asks — restrict it to the monitoring server's address, keep command arguments disabled, and never publish it.

## CHECK

```sh
check_nrpe -H TARGET
```

## SEE ALSO

- source — [NagiosEnterprises/nrpe](https://github.com/NagiosEnterprises/nrpe) (github.com)
- iana — [IANA port registry — 5666 (nrpe)](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=5666) (iana.org)

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