The following instructions assume that Nagios was installed from source. With an RPM install the locations of some files may be different. However the overall principal is the same.
In Nagios you can monitor a remote server by either ding a check on the remote server by not invoking any internal command with in it or by monitoring a remote server by actually having the Nagios server invoke a command on the remote server. An example would be a simple PING check versus if a service is running on the remote server. The Ping check can be done from the Nagios server with out having to call any commands on the remote server. However, in order to check if a process is still up and running you would have to setup the remote server to do that specif check.
In order for the second type of check to be completed NRPE will used. NRPE is a utility which allows you to execute remote commands on the remote server you are monitoring. NRPE service will need to be installed on the Nagios server and on the remote server we are monitoring. When we want to do a remote check on a remote server we will have to call upon NRPE from the Nagios server.
An example of setting up the CHECK_DISK plugin. This plugin checks the disk space on the remote server we are monitoring.