A monitoring agent — the Nagios server connects here to run checks on this host.
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.
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_nrpe -H TARGETSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.