At Bobcares our Server management support services can give you notes on how to add Add ubuntu host to Nagios in a few simple steps.

Adding a Ubuntu host to the Nagios server for monitoring using the NRPE plugin.

The Nagios Remote Plugin Executor (NRPE) agent permits remote execution of scripts hosted on the remote host. It enables the collection of metrics such as system load, disk use, and uptime, among others.

On the remote host, the NRPE Server (agent) and Plugins are installed. The agent will wait for the Nagios Core Server to send a check nrpe request. When the agent receives a check request, it will run a plugin on the remote host and submit the request back to the Nagios server.

Step 1: Install NRPE on Ubuntu

Nagios only discovers and monitors the server on which it is installed by default ( localhost ). To monitor a remote host, the NRPE packages must first be installed on the remote host. As an example, we will utilize Ubuntu 20.04 as our remote host on our local network to add an ubuntu host to Nagios.

So, first, log in to the remote computer and use the apt command to update the package index:

$ sudo apt update

Then, to install the Nagios plugins and NRPE agent, run the following command:

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

Once installed, use the following command to check the status of the NRPE agent:

$ sudo systemctl status nagios-nrpe-server
user@ubuntu-host:~$
use2ubuntu-host:~$ sudo systemct1 status nagios-nrpe-server
nagios-nrpe-server.service-Nagios Remote Plugin Executor
Loaded:loaded (/lib/systemd/system/nagios-nrpe-server.service; enabled; vendor predent: enabled)
Active: active (running) since Tsday 2022-08-25 14:15:46 UTC; 5min ago
Docs: http://WWW.nagios.org/documentation
Main PID: 581 (nrpe)
Tasks: 1 (limiy: 4713)
Memory: 1.0M
CGroup: /system.s;ice/nagios-nrpe-server.service
581 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f

The above result confirms that the NRPE agent is operational.

Step 2: Configure the NRPE agent

Step 2 of adding ubuntu host to Nagios is to monitor the remote target, we must go one step further and modify the NRPE configuration file. So, open the /etc/nagios/nrpe.cfg configuration file.

$ sudo vim /etc/nagios/nrpe.cfg

Locate the server address parameter and set it to the Nagios host’s IP address. In this situation, the remote host’s private IP address is ‘10.128.15.202’.

SERVER ADDRESS
Address that nrpe should bind to in case there are more than one interface and you do not want nrpe to bind on all interfaces.
NOTE: This option is ignored if NRPE is running under either ineta or xineta
server_address=0.128.15.202

Locate the allowed host’s argument, which is set to 127.0.0.1,0.0.0.0 by default. Change the second IP (0.0.0.0) to your Nagios server’s IP address. Our Nagios server’s private IP address is ‘10.128.15.204’.

Note: The daemon only does rudimentary checking of the client's IP
address. I would highly recommend adding entries in your /etc/hosts.allow file to allow only the specified host to connect to the port you are running this daemon on.
NOTE: This option is ignored if NRPE is running under either inetd or xinetd allowed_hosts=27.0.0.110.128.15.20

Save the modifications and then close the configuration file. Make sure to restart the NRPE agent daemon for the changes to take effect.

$ sudo systemctl restart nagios-nrpe-server

Run the following command on the remote host to determine the NRPE version:

$ sudo /usr/sbin/nrpe -V /etc/nagios/nrpe.cfg -f
Output:
NRPE - Nagios Remote Plugin Executor
Version: 4.0.0

The check nrpe script, which is provided on the Nagios core server, can be used to validate the connection from the Nagios server to the remote. To validate the connection, run the check nrpe script included in the plugins directory, and type:

$ sudo check_nrpe -H remote-host-ip-address

Step 3: Add host to Nagios server

We have completed the host-side setups for adding ubuntu host to Nagios. The only thing left is to configure the host on the Nagios monitoring server. But first, let’s make some changes to the Nagios configuration file.

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

Uncomment the line below to specify the path of the Nagios host configuration directory.

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

We can also tell Nagios to process all config files (with a .cfg # extension) in a particular directory by using the cfg_dir # directive as shown below:

cfg_dir= usr/local/nagios/etc/servers
#cfg dir=/usr/local/nagios/etc/printers
#cfg_dir=/usr/local/nagios/etc/switches
#cfg_dir=/usr/local/nagios/etc/routers

Then create the directory in the path that WE specified.

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

And, in this case, generate the host’s configuration file, host.cfg.

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

Copy and paste the following content. Replace the values for ‘use,’ ‘host name,’ ‘alias,’ and ‘address’ with those of the remote host to add ubuntu host to Nagios.

define host {
use linux-server
host_name ubuntu-host
alias Apache server
address 10.128.15.202
max_check_attempts 5
check_period 24x7
notification_interval 30
notification_period 24x7
}

Finally, restart the Nagios service to apply the changes.

$ sudo systemctl restart nagios

Step 4: Configure the UFW firewall

By default, the Nagios NRPE agent listens on port 5666. If we have the UFW firewall enabled, we must open this port on both the Nagios host and the server. To accomplish this, execute the following commands.

$ sudo ufw allow 5666/tcp
$ sudo ufw reload

Then confirm that the port has been permitted.

$ sudo ufw status

Finally, use the command: to confirm that all of the configurations are correct and this is the final step in adding the ubuntu host to nagios.

$ sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

So, go to the Nagios dashboard and click on the ‘Hosts’ link in the left sidebar to see that the host has been automatically recognized and added to the Nagios server for monitoring.

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Conclusion

To conclude we have now learned to add an Ubuntu host to Nagios with the support of our Server management support services.