This article covers a version of CentOS that is no longer supported. If you are currently operating a server running CentOS 6, we highly recommend upgrading or migrating to a supported version of CentOS.
Reason: CentOS 6 reached end of life (EOL) on November 30th, 2020 and no longer receives security patches or updates. For this reason, this guide is no longer maintained.
See Instead:
This guide might still be useful as a reference, but may not work on other CentOS releases. If available, we strongly recommend using a guide written for the version of CentOS you are using.
Ruby is a popular programming language that allows you to quickly create scripts and install additional libraries (Gems).
We have previously covered how to install Nagios monitoring server on CentOS 6.
This time, we will expand on this idea and create Nagios plugins using Ruby. These plugins will be running on client VPS, and be executed via NRPE.
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
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:
Exit Code | Status |
0 | OK |
1 | WARNING |
2 | CRITICAL |
3 | UNKNOWN |
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
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 VPS.
For our example, we will monitor a server called CentOSDroplet and edit /etc/nagios/servers/CentOSDroplet.cfg
define service { use generic-service host_name CentOSDroplet 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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