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How To Create Nagios Plugins With Ruby On CentOS 6

Step 1 - Install RPMForge Repository and NRPE on client


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

Step 2 - Create your Ruby Script


It would be a good idea to keep your plugins in same directory as other Nagios plugins ( /usr/lib64/nagios/plugins/ for example). For our example, we will create a script that checks current disk usage by calling "df" from shell, and throw an alert if it is over 85% used:
#!/usr/bin/ruby
used_space=`df -h / | grep -v "Filesystem" | awk '{print $5}'`
 case used_space.chomp <=> "85%"
 when -1
   puts "OK - #{used_space.chomp!} of disk space used."
   exit 0
 when 0
   puts "WARNING - #{used_space.chomp!} of disk space used."
   exit 1
 when 1
   puts "CRITICAL - #{used_space.chomp!} of disk space used."
   exit 2
 else
   puts "UNKNOWN - #{used_space.chomp!} of disk space used."
   exit 3
end



We will save this script in /usr/lib64/nagios/plugins/usedspace.rb and make it executable:
chmod +x /usr/lib64/nagios/plugins/usedspace.rb

The entire Nagios NRPE plugin boils down to using exit codes to trigger alerts.
You introduce your level of logic to the script, and if you want to trigger an alert (whether it is OK, WARNING, CRITICAL, or UNKNOWN) - you specify an exit code.
Refer to the following Nagios Exit Codes:

Nagios Exit Codes


Exit Code Status
0 OK
1 WARNING
2 CRITICAL
3 UNKNOWN

Step 3 - Add Your Script to NRPE configuration on client host


Delete original /etc/nagios/nrpe.cfg and add the following lines to it:
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[usedspace_ruby]=/usr/lib64/nagios/plugins/usedspace.rb

Where 198.211.117.251 is our monitoring server from previous articles. Change these to your own values.
Make sure to restart Nagios NRPE service:
service nrpe restart

Step 4 - Add Your New Command to Nagios Checks on Nagios Monitoring Server


Define new command in /etc/nagios/objects/commands.cfg
define command{
        command_name    usedspace_ruby
        command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c usedspace_ruby
        }

As you can see, it uses NRPE to make TCP connections to port 5666 and run command 'usedspace_ruby', which we defined in /etc/nagios/nrpe.cfg on that remote host.
Add this check to your Nagios configuration file for client.
For our example, we will monitor a server called MyCentOS and edit /etc/nagios/servers/MyCentOS.cfg
define service {
        use                             generic-service
        host_name                       MyCentOS
        service_description             Custom Disk Checker In Ruby
        check_command                   usedspace_ruby
        }
Restart Nagios:
service nagios restart

Verify that the new check is working. And you are all done!
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