Adding a Custom Check/Creating Plugins for Nagios.

This post will go over how to add a custom check to a host being monitored by nagios.  In this case, nagios will check to make sure crond is running on the puppetmaster server, which is a centos machine.

Step 1:
Write the script that will check for the given condition, and verify its functionality.  This is a simple script that will check that the crond process is running. The data after the pipe is interpreted by nagios as performance data, and is being added in so that the status of the process can be graphed over a period of time in an rrd graph. Adding graphs to nagios will be covered in another post.

Note that the exit codes get interpreted by nagios as follows:
0 - OK
1 - WARNING
2 - CRITICAL
3 - UNKNOWN

#!/bin/bash

lineCount=`ps -eaf|grep -v grep|grep " crond"|wc -l`

if [ $lineCount -eq "0" ]; then
        echo "WARNING - crond is not running|proc=$lineCount"
        exit 1;
fi
if [ $lineCount -eq "1" ]; then
        echo "OK - crond is running|proc=$lineCount"
        exit 0;
fi
if [ $lineCount -gt "1" ]; then
        echo "UNKNOWN - crond process count > 1|proc=$lineCount"
        exit 3;
fi
echo "UNKNOWN - crond process count is unknown|proc=$lineCount"
exit 3;

Step 2:
Add the command to /etc/nagios/nrpe.cfg on the host.  The necessary line to add in this case is:

command[check_crond]=/usr/lib/nagios/plugins/check_crond

Step 3:
Add the service check to the nagios server.  Restart the nagios process.

define service{
        use generic-service
        host_name puppetmaster
        service_description Crond Process
        check_command check_nrpe!check_crond

}

service nagios restart

Step 4:
Verify functionality.