How to install and configure nagios nrpe client in Ubuntu with apt-get command

A few days back I wrote on how to configure nagios server in Ubuntu 12.04.
In this tutorial we will learn how to monitor the ubuntu machines means we will install nrpe in ubuntu machines so that nagios server can monitor it.

NRPE works on port no. 5666.

Follow the steps to configure nrpe in Ubuntu systems :
note: here # prompt is for root and $ prompt is for non-root user.

Step 1 : Login in to the ubuntu client machine and become a superuser & do “apt-get update”(I prefer to work as root)

$ sudo su -
# apt-get update

Step 2 : Now install nrpe,open-ssl and nagios plugins in ubuntu system

# apt-get install openssl nagios-nrpe-server nagios-plugins nagios-plugins-basic nagios-plugins-standard

Step 3: Take the backup of original nrpe.cfg file

# cp -p  /etc/nagios/nrpe.cfg /etc/nagios/nrpe.cfg.orig

Step 4 : Now edit the /etc/nagios/nrpe.cfg file and in allowed_hosts give the ip address of Nagios Server from which monitoring will be done.

Below is the reference of my Nagios system’s nrpe.cfg file :

root@webserver:/etc/nagios# egrep -v '^#|^$' nrpe.cfg
log_facility=daemon
pid_file=/var/run/nagios/nrpe.pid
server_port=5666
nrpe_user=nagios
nrpe_group=nagios
allowed_hosts=127.0.0.1,ip-address-of-nagios-server

dont_blame_nrpe=0
debug=0
command_timeout=60
connection_timeout=300
command[check_users]=/usr/lib/nagios/plugins/check_users -w 5 -c 10
command[check_load]=/usr/lib/nagios/plugins/check_load -w 15,10,5 -c 30,25,20
command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda1
command[check_zombie_procs]=/usr/lib/nagios/plugins/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/lib/nagios/plugins/check_procs -w 150 -c 200 
include=/etc/nagios/nrpe_local.cfg
include_dir=/etc/nagios/nrpe.d/
root@webserver:/etc/nagios#

Step 5 : Now restart the nagios-nrpe-server service in client machine.

/etc/init.d/nagios-nrpe-server restart

Now the given below steps should be done in Nagios Server

Step 6: Login into Nagios Server and do “sudo su – ” means become superuser

sudo su -

Step 7: Create a new file and give the name which must be relevant with your network.for eg. sharad-ec2-hosts.cfg . And add the hostname and its details which you want to monitor.
By doing this you are adding a server to monitor from Nagios Server.

vi /etc/nagios3/conf.d/sharad-ec2-hosts.cfg

define host{
        use                     generic-host; Name of host template to use
        host_name               web.example.com
        alias                   Web Server
        address                 10.10.10.23
        }

Step 8: Now add the service to monitor for your nagios client (web.example.com).
In same file /etc/nagios3/conf.d/sharad-ec2-hosts.cfg add the below given lines.
Now through nrpe the check_disk plugin will check the disk.

vi /etc/nagios3/conf.d/sharad-ec2-hosts.cfg

define service{
        use                             generic-service         ; Name of service template to use
        host_name                       web.example.com
        service_description             Disk Space
        check_command                    check_nrpe!check_disk
        }

Step 9: Restart the following services

/etc/init.d/nagios3 restart

/etc/init.d/apache2 restart

Note:
Question: Why we use check_nrpe!check_disk in check_command ?
answer=> Because check_nrpe is itself a plugin. The symbol ! tells what is the next argument. Here the next argument is check_disk. In nrpe.cfg file in Nagios Client (web.example.com) we are using the given below line and using the command name check_disk
see step 4 (the line command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda1)

Hence the nrpe (port no. 5666)from nagios server will communicate to Nagios client at port no. 5666 and will search for argument in /etc/nagios/nrpe.cfg file. Here nrpe found the argumant check_disk.

Similarly you can add other commands to check.
I will recommend you to just explore all config files. Once you understand the concept you can easily play with Nagios server