How to add a remote host to Nagios monitoring server

This is a continuation of our previous post where we showed you how to install Nagios Server on Ubuntu 20.04. In this post, we focus on how you can add a remote host to the Nagios server. Without much further ado, let’s get on with it.

For demonstration purposes we will deploy a Ubuntu 20.04 host which will be our target system for monitoring.

Step 1: Install Nagios agent and plugins

First, update the package index as follows:

$ sudo apt update

Then install Nagiois NRPE and plugins as shown

$ sudo apt install nagios-nrpe-server nagios-plugins

Great, once installed, proceed to the next step.

Step 2: Configure NRPE agent

Once all the requisite packages have been installed, we need to modify the Nagios NRPE configuration file which is the /etc/nagios/nrpe.cfg file.

Open the file and locate the allowed_hosts attribute. By default, this reads as follows.

127.0.0.1,0.0.0.0

Replace the second IP address with the private IP of the Nagios monitoring server. For example.

127.0.0.1, nagios-server-ip

Also, locate the server_address attribute and set it to the remote Linux host that is to be monitored.

server_address=host-ip

Save and be sure to restart the NRPE agent service for the changes to take effect.

$ sudo systemctl restart nagios-nrpe-server

To make sure the NRPE agent is running, verify using the command:

$ sudo systemctl status nagios-nrpe-server

Ensure that the service is running.

Step 3. Add a remote host to Nagios

The last step is to add the remote host to Nagios. So log in to the Nagios server and create a folder where you are going to place the configuration file as follows.

$ sudo mkdir -p /usr/local/nagios/etc/servers

Then create a configuration file for the remote host as follows. We have named out file ubuntu-host.cfg

$ sudo vim  /usr/local/nagios/etc/servers/ubuntu-host.cfg

Add the following configuration. Of course, you need to replace details such as hostname , alias, and address accordingly to match your host’s details.

define host  {
         use                                linux-server
         host_name                     hostname
         alias                               My database server
          address                         host-IP-address
         max_check_attempts          5
         check_period                    24x7
         notification_interval           30
         notification_period             24x7
 }

Save and close the configuration file.

Lastly, edit the Nagios NRPE configuration file.

$ sudo vim /usr/local/nagios/etc/nagios.cfg

Uncomment the line below that points to the path of our host’s configuration file

cfg_dir=/usr/local/nagios/etc/servers

Save and close. Then restart Nagios service.

$ sudo systemctl restart nagios.

Nagios listens to TCP port 5666. If UFW is running , you need to open it on both the Nagios server and remote host as follows.

$ sudo ufw allow 5666/tcp

Then reload it to apply changes.

$ sudo ufw reload

And that’s it.

When you visit the Nagios dashboard and click on ‘hosts’ tab on the left pane, your host will be displayed as shown below.