Nagios NRPE Plugin on CentOS

Nagios is is a useful tool for monitoring many servers and their services. At a glance (or via email) you can see which services/hosts are experiencing problems.
Monitoring public services such as HTTP, FTP, LDAP, SSH are relatively easy but to go a little further and check disk or swap usage on a remote machine use NRPE.
The assumption below is that the nagios service is debian/ubuntu and the server to be monitored is centos/red hat, possibly a bit weird but …

On the server to be monitored
On CentOS/Red Hat you must have the RPMforge repositories installed:

wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.i386.rpm
rpm -Uhv rpmforge*

and then:

yum install nagios-nrpe nagios-plugins-nrpe

To ensure the service is running and listening use:

chkconfig nrpe on

To view the installed plugins.

ls /usr/lib/nagios/plugins
OR
ls /usr/lib64/nagios/plugins

Next add the nagios machine to the allowed hosts:

vi /etc/nagios/nrpe.cfg

allowed_hosts=127.0.0.1,192.168.0.100

Then start the NRPE service:

service nrpe start

On the server doing the monitoring
Install the NRPE plugin on the monitoring server:

apt-get install nagios-nrpe-plugin

Edit the commands file:
vi /usr/local/nagios/etc/objects/commands.cfg

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

Edit the services file:
vi /usr/local/nagios/etc/objects/services.cfg

define service{
use generic-service
host_name clam1
service_description LOAD
check_command check_nrpe_load
}

Restart nagios:

/etc/init.d/nagios restart

Check the web application and you should be monitoring your new service.
There is a security issue with passing command arguments to the NRPE and in the logs I saw:

nrpe[12196]: Error: Request contained command arguments, but argument option is not enabled!

so rather than:

command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_disk -w 5% c 2%

we actually have to create a script on the host with the arguments hardwired i.e.

vi /usr/lib64/nagios/plugins/check_my_disk
/usr/lib64/nagios/plugins/check_disk -w 5% -c 2%
chmod a+x /usr/lib64/nagios/plugins/check_my_disk

Restart nrpe:

/etc/init.d/nrpe restart

and then from the nagios server call:

command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_my_disk

May also need to define the command on the NRPE machine in /etc/nagios – I will have to get back to this!!