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Install and Configure Nagios On CentOS

Prerequisites

Before installing Nagios, make sure that you've a properly installed apache and mysql. Also install the following prerequisites too. All commands should be run as root user.
# yum install gd gd-devel gcc glibc glibc-common

Install Nagios

I tested this how-to on CentOS 6.5 minimal server, although it should work on all RHEL 6.x and its clones like CentOS 6.x and Scientific Linux 6.x. Nagios will not be found in CentOS official repositories, so let us add the EPEL repository to install nagios. To add and enable EPEL repository, refer the following link.

Install EPEL Repo On CentOS/RHEL/Scientific Linux 6

Next install nagios with all plug-ins and nagios agents(nrpe-agent) using command:
# yum install nagios*

Configure Nagios

Add the admin mail address in the nagios contact file to receive alerts from nagios server. To do that edit file /etc/nagios/objects/contacts.cfg,
# vi /etc/nagios/objects/contacts.cfg
Find the following line and enter the email id:
[...]
email                           [email protected] ;
[...
Save and close the file. Then Edit file /etc/httpd/conf.d/nagios.conf,
# vi /etc/httpd/conf.d/nagios.conf
And edit the following lines if you want to access nagios administrative console from a particular IP series. Here i want to allow nagios administrative access from 192.168.1.0/24 series only.
[...]
## Comment Lines 15 & 16 ##
#   Order allow,deny
#   Allow from all
 
## Uncomment and Change lines 17,18 & 19 as shown below ##
Order deny,allow
Deny from all
Allow from 127.0.0.1 192.168.1.0/24
[...]

Set nagiosadmin password

# htpasswd /etc/nagios/passwd nagiosadmin
New password:
Re-type new password:
Updating password for user nagiosadmin
Start nagios and httpd services and let them to start automatically on every boot.
# service nagios start
# service httpd start
# chkconfig nagios on
# chkconfig httpd on

Access Nagios admin console

Open nagios administrator console with URL http://nagios-server-ip/nagios and enter the username as nagiosadmin and its password which we created in the earlier steps. Once logged in click on the “Hosts” section in the left pane of the console. You will see the no of hosts to be monitored by Nagios server. Initially, the nagios server (localhost) itself will only be monitored. Click on the monitoring host to display more details.

Add Monitoring targets to Nagios server

Now let us add some clients to monitor by Nagios server. To do that we have to install nrpe and nagios-plugins in our monitoring targets.
On CentOS/RHEL/Scientifc Linux clients:
Like I said before, you have to add EPEL repository in your CentOS/RHEL/Scientific Linux 6.x clients to install nrpe package.
Install “nrpe” and “nagios-plugins” packages in client systems to be monitored.
# yum install nrpe nagios-plugins-all openssl
On Debian/Ubuntu clients:
$ sudo apt-get install nagios-nrpe-server nagios-plugins

Configure Monitoring targets

Edit /etc/nagios/nrpe.cfg file:
$ vi /etc/nagios/nrpe.cfg
Add your Nagios server ip address
[...]
## Line 81 - Add the Nagios server IP ##
allowed_hosts=127.0.0.1 192.168.1.101
[...]
Start nrpe service:
# service nrpe start
# chkconfig nrpe on
For Debian/Ubuntu Clients, start nrpe service as shown below.
$ sudo /etc/init.d/nagios-nrpe-server restart
Now go back to your Nagios server to add the clients to be monitored through nagios server. Edit “/etc/nagios/nagios.cfg” file:
# vi /etc/nagios/nagios.cfg
and uncomment the following lines.
## Line 52 - Uncomment ##
cfg_dir=/etc/nagios/servers
Create a directory called “servers” under “/etc/nagios/”.
# mkdir /etc/nagios/servers
Create config file to the client to be monitored:
define host{
 
use                             linux-server
 
host_name                       client
 
alias                           client
 
address                         192.168.1.100  ## Client IP ##

max_check_attempts              5
 
check_period                    24x7
 
notification_interval           30
 
notification_period             24x7
 
}
Finally restart nagios service.
# service nagios restart
Now open the nagios admin console in the browser and navigate to “Hosts” section in the left pane. You will see the newly added client will be visible there. Click on the host to see if there is anything wrong or alerts. Click on the monitoring target, you’ll get the detailed output.Like this way, you can define more clients by creating a separate config files “/etc/nagios/servers directory for each client.

Define services

We did define the monitoring host before. Now let us add some services of the monitoring host. For example to monitor the ssh service, add the following lines shown in red colour in the /etc/nagios/servers/clients.cfg file.
# vi /etc/nagios/servers/clients.cfg
define host{ use linux-server host_name client alias client address 192.168.1.100 ## Client IP ## max_check_attempts 5 check_period 24x7 notification_interval 30 notification_period 24x7 } define service { use generic-service host_name client service_description SSH check_command check_ssh notifications_enabled 0 }Save and close the file. Restart Nagios.
# service nagios restart
Now log in to Nagios web console and check for the added services. Navigate to Services section, you’ll see the ssh service there.

That’s all about now. Good Luck!

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