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.
In this tutorial, you will learn how to create Nagios plugins with Perl. Here, I will be using Ubuntu 12.10.
Initial Set Up
Before getting into the creation of Nagios plugins, you have to first install NRPE on client VPS. NRPE stands for Nagios Remote Plugin Executor.
NRPE can be installed using the following command:
apt-get install -y perl nagios-nrpe-server useradd nrpe && update-rc.d nagios-nrpe-server defaults
Creation of Perl Script
Perl 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 be calling ‘df’ from shell in order to check.
The script will look like this:
#!/usr/bin/perl use strict; use warnings; use feature qw(switch say); my $used_space = `df -h / \|awk 'FNR == 2 {print \$5}'`; given ($used_space) { chomp($used_space); when ($used_space lt '85%') { print "OK - $used_space of disk space used."; exit(0); } when ($used_space eq '85%') { print "WARNING - $used_space of disk space used."; exit(1); } when ($used_space gt '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/lib/nagios/plugins/ checkdiskspace.pl.
Make it executable:
chmod +x /usr/lib/nagios/plugins/checkdiskspace.perl
You can customize the script to apply different logic to trigger alerts and exit codes or return codes.
Here is the list of Nagios Return Codes:
Exit Codes / Return Codes | Status |
0 | OK |
1 | WARNING |
2 | CRITICAL |
3 | UNKNOWN |
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/nagios/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/nagios/nrpe.d/ command[checkdiskspace_perl]=/usr/lib/nagios/plugins/checkdiskspace.perl
You have to specify the correct values for allowed_hosts, which should be the IP of monitoring server.
Restart the Nagios nrpe service:
service nagios-nrpe-server restart
Set Up on Nagios Monitoring Server
In this step, 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_perl command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c checkdiskspace _perl }
This will use nrpe to make TCP connections to port 5666 and will run the command ‘checkdiskspace_perl’ 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 ‘UbuntuDroplet’ as server for monitoring and will update /etc/nagios/servers/UbuntuDroplet.cfg with following lines of code:
define service { use generic-service host_name UbuntuDroplet service_description Custom Disk Checker In Perl check_command checkdiskspace_perl }
Once the changes are done, you should restart the Nagios:
service nagios restart
Verify whether the new check is working or not:
That’s it!