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Last updated: 5 Apr 2022

Debug Nagios NRPE connection failures

Nagios uses a protocol called NRPE (Nagios Remote Plugin Executor) to perform checks on remote machines. Monitored machines run an ‘NRPE agent’ which listens for requests to execute monitoring checks.

Occasionally the Nagios server is unable to connect to an NRPE agent on one of the monitored machines. In this case you will see an alert with a message such as ‘connection to NRPE could not be established’.

In some cases this may be a false alarm due to load on the monitored machine. In this case the NRPE errors usually disappear of their own accord when Nagios tries the check again (within a minute or two). If the errors persist for longer than a few minutes there may be a genuine issue and you should investigate.


In this example we use the placeholders broken-machine and monitoring. To find the proper hostnames and learn how to ssh there please consult:

First verify that NRPE is running on the monitored machine:

$ ssh <broken-machine>
$ nc -v localhost 5666
Connection to localhost 5666 port [tcp/nrpe] succeeded!

If the connection succeeds then the NRPE agent is running on that machine.

A failure indicates that the agent is not running, and you should investigate. Try running govuk_puppet --test on the machine, which should restart the service. If it fails check the output for errors.

If the agent is running ok, next check that you can connect from the monitoring server:

$ ssh <monitoring>
$ nc -v <broken-machine> 5666
Connection to localhost 5666 port [tcp/nrpe] succeeded!

A failure indicates a networking issue between the monitoring server and the monitored machine.

Note that the NRPE port is firewalled and only accessible from the monitoring machine and the box itself.