In this section, we’ll show you how to add a new host to Nagios, so it will be monitored. You can repeat this step for all the node you wise to monitor using Nagios.
Read More: How to install Nagios on Ubuntu
On a server that you want to monitor, update apt-get:
# apt-get update
Now install Nagios Plugins and NRPE:
# apt-get install nagios-plugins nagios-nrpe-server
Configure Allowed Hosts
Now, let’s update the NRPE configuration file.
# vim /etc/nagios/nrpe.cfg
Find the allowed_hosts directive, and add the private IP address of your Nagios server to the comma-delimited list
allowed_hosts=127.0.0.1,192.168.0.4
Save and exit. This configures NRPE to accept requests from your Nagios server, via its private IP address.
Configure Allowed NRPE Commands
Look up the name of your root filesystem to get monitor
# df -h /
We will be using the filesystem name in the NRPE configuration to monitor your disk usage (it is probably /dev/sda). Now open nrpe.cfg for editing:
# vi /etc/nagios/nrpe.cfg
The NRPE configuration file is very long and full of comments. There are a few lines that you will need to find and modify:
- server_address: Set to the private IP address of this host
- allowed_hosts: Set to the private IP address of your Nagios server
- command[check_hda1]: Change /dev/hda1 to whatever your root filesystem is called
The three aforementioned lines should look like this
server_address=client_private_IP allowed_hosts=nagios_server_private_IP command[check_hda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda
Note that there are several other “commands” defined in this file that will run if the Nagios server is configured to use them. Also note that NRPE will be listening on port 5666 because server_port=5666 is set. If you have any firewalls blocking that port, be sure to open it to your Nagios server.
Save and quit.
Restart NRPE
Restart NRPE to put the change into effect:
# service nagios-nrpe-server restart
Once you are done installing and configuring NRPE on the hosts that you want to monitor, you will have to add these hosts to your Nagios server configuration before it will start monitoring them.
Add Host to Nagios Configuration
On your Nagios server, create a new configuration file for each of the remote hosts that you want to monitor in /usr/local/nagios/etc/servers/. Replace the highlighted word, “firstnode”, with the name of your host:
# vi /usr/local/nagios/etc/servers/firstnode.cfg
Add in the following host definition, replacing the host_name value with your remote hostname (“web-1” in the example), the alias value with a description of the host, and the address value with the private IP address of the remote host:
define host {
       use                            linux-server
       host_name                      firstnode
       alias                          My first Apache server
       address                        192.168.0.161
       max_check_attempts             5
       check_period                   24x7
       notification_interval          30
       notification_period            24x7
}
With the configuration file above, Nagios will only monitor if the host is up or down. If this is sufficient for you, save and exit then restart Nagios. If you want to monitor particular services, read on.
Add any of these service blocks for services you want to monitor. Note that the value of check_command determines what will be monitored, including status threshold values. Here are some examples that you can add to your host’s configuration file:
Ping:
define service {
       use                            generic-service
       host_name                      firstnode
       service_description            PING
       check_command                  check_ping!100.0,20%!500.0,60%
}
SSH (notifications_enabled set to 0 disables notifications for a service):
define service {
       use                            generic-service
       host_name                      firstnode
       service_description            SSH
       check_command                  check_ssh
       notifications_enabled          0
}
If you’re not sure what use generic-service means, it is simply inheriting the values of a service template called “generic-service” that is defined by default.
Now save and quit. Reload your Nagios configuration to put any changes into effect:
# service nagios reload
Once you are done configuring Nagios to monitor all of your remote hosts, you should be set. Be sure to access your Nagios web interface, and check out the Services page to see all of your monitored hosts and services:
