Introduction

Nagios is an open source monitoring software application that helps organizations to prevent IT infrastructure problems. It can be used with Linux as well as other Unix variants.

you will learn  to create Nagios plugins using PHP on CentOS 6.

Initial Set Up

You have to first install RPMForge repository and NRPE(Nagios Remote Plugin Executor) on client VPS.

The commands are given below:

rpm -ivh http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el6.rf.x86_64.rpm
yum -y install php nagios-nrpe
useradd nrpe && chkconfig nrpe on

Creation of PHP Script

PHP programming language allows us to create scripts and install additional libraries. Here, we will be creating a script which will check disk usage and throw alert if it is over 85%.

The script will look like this:

#!/usr/bin/php
<?

php$used_space=chop(shell_exec("df -h / | grep -v Filesystem | awk '{print $5}'")); 

switch ($used_space) {
        case "$used_space" < "85%":
        print "OK - $used_space of disk space used.";
        exit(0);
       
        case "$used_space" == "85%":
        print "WARNING - $used_space of disk space used.";
        exit(1);

        case $used_space > "85%":
        print "CRITICAL - $used_space of disk space used.";
        exit(2);

        default:
        print "UNKNOWN - $used_space of disk space used.";
        exit(3);
}
?>

This script has to be saved in the same directory as other Nagios plugins, /usr/lib64/nagios/plugins/ checkdiskspace.php

To make it executable:

chmod +x /usr/lib64/nagios/plugins/checkdiskspace.php

You can customize the script to apply different logic to trigger alerts and exit codes or return codes.

the list of Nagios Return Codes:

Configure NRPE

Now that we are done creating the script, we have to add the script to nrpe configuration /etc/nagios/nrpe.cfg on client host. For that, we will first delete the original config file (/etc/nagios/nrpe.cfg) and update it with the following lines of code:

log_facility=daemon
pid_file=/var/run/nrpe/nrpe.pid
server_port=5666
nrpe_user=nrpe
nrpe_group=nrpe
allowed_hosts=x.x.x.x
dont_blame_nrpe=1
debug=0
command_timeout=60
connection_timeout=300
include_dir=/etc/nrpe.d/

 command[usedspace_php]=/usr/lib64/nagios/plugins/ checkdiskspace.php

You have to specify the correct values for allowed_hosts, which should be the IP of monitoring server.
Restart the Nagios nrpe service:

service nrpe restart

Set Up on Nagios Monitoring Server

We will add a new command to check Nagios on monitoring server. For that, we have to  update the commands file /etc/nagios/objects/commands.cfg.

define command{
        command_name    checkdiskspace_php
        command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c checkdiskspace_php
        }

This will use nrpe to make TCP connections to port 5666 and will run the command ‘checkdiskspace_php’ which we already defined in /etc/nagios/nrpe.cfg on remote host.

Now we will add this check to Nagios configuration file for client VPS. In this tutorial, we will be using ‘CentOS6Droplet’ as server for monitoring and will update /etc/nagios/servers/CentOS6Droplet.cfg with following lines of code:

define service {
        use                             generic-service
        host_name                       CentOSDroplet
        service_description             Custom Disk Checker In PHP
        check_command                   checkdiskspace_php
        }

Once the changes are done, you should restart the Nagios:

service nagios restart

You can verify whether the new check is working or not: