The first thing to do when testing connections is to go to the target server and run a command to verify that it works. Here is an example of a command that is found in the /etc/nagios/nagios.cfg file.
# /usr/lib/nagios/plugins/./check_users -w 5 -c 10
USERS WARNING – 5 users currently logged in |users=5;5;10;0
You can see that the plugin works locally.
Now run the command from the Nagios Server. Be sure to run the command that you used on the target server to verify that it is working. You should see the information below returned.
/usr/lib/nagios/plugins/./check_nrpe -H 192.168.5.178 -c check_users
USERS WARNING – 7 users currently logged in |users=7;5;10;0
If you do not receive the correct output then you will need to check logs and check your firewall has port 5666/TCP open on the target server.