How To Install Nagios On CentOS 6

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 For this reason, this guide is no longer maintained.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.
The following DigitalOcean tutorial may be of immediate interest, as it outlines installing Nagios on a CentOS 7 server:

How To Install Nagios 4 and Monitor Your Servers on CentOS 7

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 && chkconfig nagios on
service httpd start && 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 && chown root. /swap && chmod 0600 /swap && swapon /swap
echo /swap swap swap defaults 0 0 >> /etc/fstab
echo vm.swappiness = 0 >> /etc/sysctl.conf && sysctl -p

Step 2 – Set Password Protection #

Set Nagios Admin Panel Password:

htpasswd -c /etc/nagios/passwd nagiosadmin

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:

This is what the Nagios admin panel looks like:

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

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" >> /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:

And you are all done!
By Bulat Khamitov