So, we have already talked a lot about Nagios and its potential. But for everything to be complete, we need to install the agent in a client operating system so that the server can monitor it.

So that’s what we’re going to do.

Install Nagios agent on Ubuntu 18.04

1.- Working on the client machine

The Nagios agent is NRPE (Nagios Remote Plugin Executor) which according to the Nagios wiki is the following

“it is a Nagios agent that allows remote system monitoring using scripts that are hosted on the remote systems.It allows for monitoring of resources such as disk usage, system load or the number of users currently logged in.”

The big advantage is that this agent is available in the official Ubuntu repositories and you don’t have to do much to install it.

So, open a terminal in the client operating system. Not on the server. And run the following:

:~$ sudo apt-get install nagios-nrpe-server nagios-plugins

When the installation is finished, a configuration file must be modified to tell the client where the Nagios server is.

:~$ sudo nano /etc/nagios/nrpe.cfg

In the allowed_hosts directive add the IP address of the computer where Nagios is installed. However, you can specify multiple IP addresses if there are multiple Nagios servers.

allowed_hosts=127.0.0.1, 192.168.250.32

In my case, this is the file.

Then save the changes and close the file.

For all these changes to take effect, the Nagios client service has to be restarted.

:~$ sudo systemctl restart nagios-nrpe-server

Finally, we check if the agent is working with the following command:

:~$ /usr/lib/nagios/plugins/check_load -w 15,10,5 -c 30,25,20

If you get a screen output like this:

OK - load average: 0.74, 0.62, 0.68|load1=0.740;15.000;30.000;0; load5=0.620;10.000;25.000;0; load15=0.680;5.000;20.000;0;

It means that everything is fine.

2.- Working on the server

It is now necessary to install NRPE on the server. This is so that you can establish communication with the client.

So, on the server open a terminal and run the following:

:~$ sudo apt install nagios-nrpe-plugin

Now that it is installed, some configuration files need to be modified. The first one is the Nagios configuration file where we have to enable the server folder.

:~$ sudo nano /usr/local/nagios/etc/nagios.cfg

You do it by decomposing the next line:

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

Again, save the changes and close the file. Then create the folder in question.

:~$ sudo mkdir /usr/local/nagios/etc/servers

After that, another file has to be modified to add a command that the Nagios server has to run to communicate with the client.

:~$ sudo nano /usr/local/nagios/etc/objects/commands.cfg

At the end of the file, add the following:

define command {
    command_name check_nrpe
    command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

Again, save the changes and close the file.

Now we have to create a new configuration file for the client that we are going to add.

:~$ sudo nano /usr/local/nagios/etc/servers/client.cfg

And add the following:

define host{
use                             linux-server
host_name                       laptop.osradar
alias                           Ubuntu 18
address                         192.168.250.6
}

define service{
        use                     generic-service
        host_name               laptop.osradar
        service_description     CPU Load
        check_command           check_nrpe!check_load
        }

define service{
        use                     generic-service
        host_name               laptop.osradar
        service_description     Total Processes
        check_command           check_nrpe!check_total_procs
        }

define service{
        use                     generic-service
        host_name               laptop.osradar
        service_description     Current Users
        check_command           check_nrpe!check_users
        }

define service{
        use                     generic-service
        host_name               laptop.osradar
        service_description     SSH Monitoring
        check_command           check_nrpe!check_ssh
        }
define service{
        use                     generic-service
        host_name               laptop.osradar
        service_description     FTP Monitoring
        check_command           check_nrpe!check_ftp
        }

Nagios is very flexible because in this file you can add or remove the services you want.

Also, you have to modify some parameters like the hostname of the client machine and the IP address. This is enough.

Then, it is convenient to check if the configuration file has a syntax error. To do this, run the following command:

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

Then test to see if the server can communicate with the client:

:~$ cd /usr/lib/nagios/plugins/
:~$ ./check_nrpe -H 192.168.250.6

Replace the IP address with the address of the computer that will be the client.

To avoid running problems with the plugin, copy it to the location where the rest of them are.

:~$ sudo cp check_nrpe /usr/local/nagios/libexec

And finally, restart the service.

:~$ sudo systemctl restart nagios

And that is it.

3.- Testing the Nagios client on Ubuntu 18.04

Now open your web browser and access Nagios. You will see the following:

And check the host information:

So everything’s fine.

Conclusion

To monitor with Nagios it is necessary to install the agent in each of the computers you want. In this post, you have learned how to install it in Ubuntu 18.04 / Linux Mint 19.3.

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