Introduction

Nagios is very handy tool for every network/system administrator. It is an open source monitoring software application that helps on monitoring your organization infrastructure and allows us to automate any host alert. It can be used with Linux as well as other Unix variants, however we are going to create nagios plugins with Python on CentOS 6.

Initial Set Up

Before getting into the creation of Nagios plugins, you have to first install RPMForge repository and NRPE on client VPS. NRPE stands for Nagios Remote Plugin Executor.

To install RPM repo:

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

Creation of Python Script

Python 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 is given below:

#!/usr/bin/python
import os, sys
used_space=os.popen("df -h / | grep -v Filesystem | awk '{print $5}'").readline().strip() 

if used_space < "85%":
        print "OK - %s of disk space used." % used_space
        sys.exit(0)
elif used_space == "85%":
        print "WARNING - %s of disk space used." % used_space
        sys.exit(1)
elif used_space > "85%":
        print "CRITICAL - %s of disk space used." % used_space
        sys.exit(2)  
else:
        print "UKNOWN - %s of disk space used." % used_space
        sys.exit(3)

Ensure to save the script  in the same directory as other Nagios plugins, /usr/lib64/nagios/plugins/ checkdiskspace.py

To execute:

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

if you are really good with python programming then you can customize the script to apply different logic to trigger alerts and exit codes or return codes on your needs.

The list of Nagios Return Codes are as follows:

Configure NRPE

After done with scripting, we need to add the script to nrpe configuration /etc/nagios/nrpe.cfg on client host.

First of all  we will  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=198.211.117.251
dont_blame_nrpe=1
debug=0
command_timeout=60
connection_timeout=300
include_dir=/etc/nrpe.d/ 

command[checkdiskspace_python]=/usr/lib64/nagios/plugins/ checkdiskspace.py

NOTE: Make sure 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

In this step, we will be adding a new host to check Nagios on monitoring server.

edit the configuration file with your favorite editor as given below:

vi /etc/nagios/objects/commands.cfg.

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

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

In this tutorial, we will be adding ‘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 Python
        check_command                   checkdiskspace_python
        }

once the file has been updated restart the nagios service.

service nagios restart

You can verify whether the new check is working or not as given below in the picture: