Table of Contents

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Status: Deprecated

nagios illustration for: Status: Deprecated

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 For this reason, this guide is no longer maintained.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.

Step 1 - Install Packages on Monitoring Server

				
					rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

rpm -Uvh http://rpms.famillecollet.com/enterprise/remi-release-6.rpm

yum -y install nagios nagios-plugins-all nagios-plugins-nrpe nrpe php httpd

chkconfig httpd on &amp;&amp; chkconfig nagios on

service httpd start &amp;&amp; service nagios start

We should also enable SWAP memory on this droplet, at least 2GB:

				
					dd if=/dev/zero of=/swap bs=1024 count=2097152

mkswap /swap &amp;&amp; chown root. /swap &amp;&amp; chmod 0600 /swap &amp;&amp; swapon /swap

echo /swap swap swap defaults 0 0 &gt;&gt; /etc/fstab

echo vm.swappiness = 0 &gt;&gt; /etc/sysctl.conf &amp;&amp; sysctl -p

Step 2 - Set Password Protection

Set Nagios Admin Panel Password:

				
					htpasswd -c /etc/nagios/passwd nagiosadmin

<img src="images/how-to-install-nagios-on-centos-6-section-1.png; width="680">

Make sure to keep this username as “nagiosadmin” – otherwise you would have to change /etc/nagios/cgi.cfg and redefine authorized admin.

Now you can navigate over to your droplet’s IP address http://IP/nagios and login.

You will be prompted for password you set in Step 2:

<img src="images/how-to-install-nagios-on-centos-6-section-1.png; width="680">

This is what the Nagios admin panel looks like:

<img src="images/how-to-install-nagios-on-centos-6-section-1.png; width="680">

Since this is a fresh installation, we don’t have any hosts currently being monitored.

<img src="images/how-to-install-nagios-on-centos-6-section-1.png; width="680">

Now we should add our hosts that will be monitored by Nagios. For example, we will use cloudmail.tk (198.211.107.218) and emailocean.tk (198.211.112.99).

From public ports, we can monitor ping, any open ports such as webserver, e-mail server, etc.

For internal services that are listening on localhost, such as MySQL, memcached, system services, we will need to use NRPE.

Step 4 - Install NRPE on Clients

				
					rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

rpm -Uvh http://rpms.famillecollet.com/enterprise/remi-release-6.rpm

yum -y install nagios nagios-plugins-all nrpe

chkconfig nrpe on

This next step is where you get to specify any manual commands that Monitoring server can send via NRPE to these client hosts.

Make sure to change allowed_hosts to your own values.

Edit /etc/nagios/nrpe.cfg

				
					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[check_users]=/usr/lib64/nagios/plugins/check_users -w 5 -c 10

command[check_load]=/usr/lib64/nagios/plugins/check_load -w 15,10,5 -c 30,25,20

command[check_disk]=/usr/lib64/nagios/plugins/check_disk -w 20% -c 10% -p /dev/vda

command[check_zombie_procs]=/usr/lib64/nagios/plugins/check_procs -w 5 -c 10 -s Z

command[check_total_procs]=/usr/lib64/nagios/plugins/check_procs -w 150 -c 200

command[check_procs]=/usr/lib64/nagios/plugins/check_procs -w $ARG1$ -c $ARG2$ -s $ARG3$

Note:

In check_disk above, the partition being checked is /dev/vda – make sure your droplet has the same partition by running df -h / You can also modify when to trigger warnings or critical alerts – above configuration sets Warning at 20% free disk space remaining, and Critical alert at 10% free space remaining.

We should also setup firewall rules to allow connections from our Monitoring server to those clients and drop everyone else:

				
					iptables -N NRPE

iptables -I INPUT -s 0/0 -p tcp --dport 5666 -j NRPE

iptables -I NRPE -s 198.211.117.251 -j ACCEPT

iptables -A NRPE -s 0/0 -j DROP

/etc/init.d/iptables save

Now you can start NRPE on all of your client hosts:

				
					service nrpe start

Step 5 - Add Server Configurations on Monitoring Server

Back on our Monitoring server, we will have to create config files for each of our client servers:

				
					echo "cfg_dir=/etc/nagios/servers" &gt;&gt; /etc/nagios/nagios.cfg

cd /etc/nagios/servers

touch cloudmail.tk.cfg

touch emailocean.tk.cfg

Edit each client’s configuration file and define which services you would like monitored.

				
					nano /etc/nagios/servers/cloudmail.tk.cfg

Add the following lines:

				
					define host {

        use                     linux-server

        host_name               cloudmail.tk

        alias                   cloudmail.tk

        address                 198.211.107.218

        }

define service {

        use                             generic-service

        host_name                       cloudmail.tk

        service_description             PING

        check_command                   check_ping!100.0,20%!500.0,60%

        }

define service {

        use                             generic-service

        host_name                       cloudmail.tk

        service_description             SSH

        check_command                   check_ssh

        notifications_enabled           0

        }

define service {

        use                             generic-service

        host_name                       cloudmail.tk

        service_description             Current Load

        check_command                   check_local_load!5.0,4.0,3.0!10.0,6.0,4.0

        }

You can add more services to be monitored as desired. Same configuration should be added for second client, emailocean.tk, with different IP address and host_name:

This is a snippet of /etc/nagios/servers/emailocean.tk.cfg:

				
					define host {

        use                     linux-server

        host_name               emailocean.tk

        alias                   emailocean.tk

        address                 198.211.112.99

        }

...

You can add additional clients to be monitored as /etc/nagios/servers/AnotherHostName.cfg

Finally, after you are done adding all the client configurations, you should set folder permissions correctly and restart Nagios on your Monitoring Server:

				
					chown -R nagios. /etc/nagios

service nagios restart

Step 6 - Monitor Hosts in Nagios

Navigate over to your Monitoring Server’s IP address http://IP/nagios and enter password set in Step 2.

Now you should be able to see all the hosts and services:

<img src="images/how-to-install-nagios-on-centos-6-section-1.png; width="680">

And you are all done!