There is a known error when your Nagios Monitoring Server is running Ubuntu 18.04 and the host you are monitoring using NRPE is running an earlier version eg 16.04.
In 18.04 NRPE uses a 2048 bit DH key, in earlier versions NRPE uses a 512 but DH key.
This is a bit of a hack, but does work:
– Go to a machine running a version below Ubuntu 18.04
– apt install nagios-nrpe-plugin
– Copy /usr/lib/nagios/plugins/check_nrpe over to your Nagios server
– On the Nagios server:
– mv /usr/lib/nagios/plugins/check_nrpe /usr/lib/nagios/plugins/check_nrpe.v3
You can now use the earlier version which should now work.