Jonathan Gazeley

This a short and simple guide, explaining how to set up remote monitoring of Linux hosts using NRPE in Nagios. The procedure is simple, but having searched for information on this earlier today I didn’t find a straightforward all-inclusive guide, so I’ve written my own.

These instructions were written with Nagios 3.0.6, and they assume that you already have a working Nagios monitoring server. They assume that the monitoring server was installed from RPM, not from source (some paths will vary).

Configuring the remote server

First, we install the NRPE on the remote server to be monitored. This comes as standard in the Fedora repositories, but on CentOS you’ll need to add the EPEL repository first.

yum install nrpe

We’ll need to make one or two changes to get it working. First open up /etc/nagios/nrpe.cfg and find the allowed_hosts directive. Replace it with the IP address of your Nagios monitoring server.

allowed_hosts=123.123.123.123

Edit your /etc/sysconfig/iptables and add a line to allow port 5666/TCP from the monitoring server’s IP address.

-A INPUT -m tcp -p tcp -s 123.123.123.123--dport 5666 -j ACCEPT

Finally, restart iptables and start NRPE to get it working. We also tell NRPE to start on boot.

service iptables restart
service nrpe start
chkconfig nrpe on

Configuring the Nagios server

Edit your commands.cfg (usually in /etc/nagios/objects/ if you installed from RPM) and add the following command definition:

define command{
        command_name    check_nrpe
        command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
        }

If this is your first remote Linux host to monitor, create a new host definition file in the same directory as commands.cfg, e.g. linux.cfg. Make a host definition for your new server:

define host{
        use                     linux-server
        host_name               myserver
        alias                   My Server
        address                 234.234.234.234
        }

Add the following to it as a test to show it works:

define service{
        use                         generic-service
        host_name                   myserver
        service_description         PING
        check_command               check_ping!100.0,20%!500.0,60%
        }

define service{
        use                         generic-service
        host_name                   yourserver
        service_description         Load
        check_command               check_nrpe!check_load
        }

Restart Nagios and ensure that both tests work OK. If so, we can move on to some custom test.

Custom checks

The default NRPE client comes with a handful of built-in tests. You can see these near the bottom of nrpe.cfg on your remote machine. But they’re not very exciting, and you’ll probably want to use some of the other checks. If you want to see a list of the available checks in your yum repo, try this:

yum list available nagios-plugins-*

Install any that take your fancy. You’ll need to set up a definition for them in your nrpe.cfg. Use the examples in the file, and try running the Nagios plugin yourself to see if it gives you any clues about the arguments it wants.

Please note, in the default config of NRPE, you cannot use placeholders like $ARG1$, for security reasons. Either hardcode the values in, like

command[check_hda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/hda1

or enable dont_blame_nrpe=1 further up in the file. There is a security risk associated with doing this. Your funeral!

Restart NRPE again, and let’s move on to setting up your Nagios server. There is no need to create a new command definition, since we are using NRPE again. So open up linux.cfg and let’s add a service definition for the check_hda1 that exists in nrpe.cfg.

define service{
        use                             generic-service
        host_name                       myserver
        service_description             Disk status
        check_command                   check_nrpe!check_hda1
        }

Restart Nagios again and your new checks should appear. Go ahead and install any useful plugins from your yum repository, or have a look at Monitoring Exchange, a great source of free Nagios plugins.

I wrote my own plugins for monitoring your account balance with AQL and checking for the latest installed kernel. One day I will probably get round to uploading them to Monitoring Exchange.