One of the greatest open-source monitoring tools is Nagios, which keeps an eye on services and programs running on Linux and Windows operating systems.

Nagios can keep an eye on system metrics including CPU load, memory consumption, disk utilization, logged-in users, active processes, and services like HTTP, FTP, SSH, and SMTP.

We’ll look at how to set up Nagios on Ubuntu 22.04 in this article.

Prerequisites:
We shall compile Nagios from source even though there is a package for it in the Ubuntu repository. Therefore, install the packages listed below to compile Nagios.

#sudo apt update

#sudo apt install -y build-essential apache2 php openssl perl make php-gd libgd-dev libapache2-mod-php libperl-dev libssl-dev daemon wget apache2-utils unzip

For Nagios, create a user and group. Then, to enable external commands to be executed via the Nagios web interface, add the nagios and apache user (www-data) to the section of the nagcmd group.

#sudo useradd nagios

#sudo groupadd nagcmd

#sudo usermod -a -G nagcmd nagios

#sudo usermod -a -G nagcmd www-data

Nagios Core installation for Ubuntu 22.04
First, get the most recent Nagios core version from the official website.

#wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.4.6.tar.gz

#tar -zxvf nagios-4.4.6.tar.gz

#cd nagios-4.4.6/

Then, use the procedures listed below to build Nagios from source code.

#sudo ./configure --with-nagios-group=nagios --with-command-group=nagcmd --with-httpd_conf=/etc/apache2/sites-enabled/

#sudo make all

#sudo make install

#sudo make install-init

#sudo make install-config

#sudo make install-commandmode

You can find Nagios’s configuration files under the /usr/local/nagios/etc directory, and the default configuration should work fine.

Next, install the Nagios web interface with the below command.

#sudo make install-webconf

You must establish a user account (nagiosadmin) in order to access the Nagios web interface, which is password secured by default. So, to establish a user account for the Nagios online interface, execute the command below.

#sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

Enable the CGI module using the following command.

#sudo a2enmod cgi

Restart the Apache web server to finish.

Adding plugins to Nagios with Ubuntu 22.04
Installing Nagios plugins is required in order to monitor a system. Install the Nagios plugins by downloading them from the official website.

#wget https://nagios-plugins.org/download/nagios-plugins-2.3.3.tar.gz
#tar -zxvf nagios-plugins-2.3.3.tar.gz
#cd nagios-plugins-2.3.3/

Then, compile and install the Nagios plugins.

#sudo ./configure --with-nagios-user=nagios --with-nagios-group=nagios

#sudo make

#sudo make install

Open the Nagios monitoring program
It's time to launch the Nagios service at this point. You should check the Nagios configuration files before launching the service.

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

Output:
Nagios Core 4.4.6
right (c) 2009-present Nagios Core Development Team and Community Contributors
Copyright (c) 1999-2009 Ethan Galstad
Last Modified: 2020-04-28
License: GPL
Website: https://www.nagios.org
Reading configuration data...
Read main config file okay...
Read object config files okay...
Running pre-flight check on configuration data...
Checking objects...
Checked 8 services.
Checked 1 hosts.
Checked 1 host groups.
Checked 0 service groups.
Checked 1 contacts.
Checked 1 contact groups.
Checked 24 commands.
Checked 5 time periods.
Checked 0 host escalations.
Checked 0 service escalations.
Checking for circular paths...
Checked 1 hosts
Checked 0 service dependencies
Checked 0 host dependencies
Checked 5 timeperiods
Checking global event handlers...
Checking obsessive compulsive processor commands...
Checking misc settings...
Total Warnings: 0
Total Errors: 0
Things look okay - No serious problems were detected during the pre-flight check

Then, start and enable the Nagios service with the below command

#sudo systemctl enable --now nagios

Now navigate to the following URL in your web browser to view the Nagios web interface.
Enter the login (nagiosadmin) and password for the Nagios account you previously created at

http://ip/nagios/

You will see Nagios’ main page after successfully logging in.

To view the services that Nagios is watching, click Services in the left pane.

Now use the following command to install NRPE Add-on and Nagios plugins for adding Host.

#sudo apt update
#sudo apt install -y nagios-nrpe-server nagios-pluginss.

Set up NRPE Add-on
To accept the connection from the Nagios server, modify the NRPE configuration file, Change the file /etc/nagios/nrpe.cfg.

#sudo nano /etc/nagios/nrpe.cfg

Add the Nagios servers IP address, separated by comma like below.

#allowed_hosts=192.168.9.4


With the use of Nagios plugins, the command lines below allow you to keep an eye on the number of active processes, the number of logged-in users, the system load, root filesystem utilization, and swap consumption.

#sudo nano /etc/nagios/nrpe.cfg

Firewall
Set up the firewall so that the NRPE server operating on a distant Linux computer may be contacted by the Nagios server. Use a remote Linux computer to execute these instructions.

#firewall-cmd –permanent –add-port=5666/tcp
#firewall-cmd –reload

IP Tables

#iptables -I INPUT -p tcp –dport 5666 -m conntrack –ctstate NEW,ESTABLISHED -j ACCEPT
#iptables -I OUTPUT -p tcp –sport 5666 -m conntrack –ctstate ESTABLISHED -j ACCEPT
#/etc/init.d/iptables save

Install the NRPE plugin on the Nagios server.
To verify the services or resources, this NRPE plugin offers the check nrpe plugin, which makes communication with the NRPE server on distant computers.

Use the following command to install the NRPE plugin on your machine.

#sudo apt install -y nagios-nrpe-plugin

Configuration Editing
To add all.cfg files located in the /usr/local/nagios/etc/servers directory, edit the Nagios configuration file.

#sudo nano /usr/local/nagios/etc/nagios.cfg

Add or uncomment the following line.

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

Create a configuration directory.

#sudo mkdir /usr/local/nagios/etc/servers

To the Nagios server, add a Linux host
To define the host and service definitions of a remote Linux host, create a client configuration file at /usr/local/nagios/etc/servers/client.itzgeek.local.cfg in your system.

#sudo nano /usr/local/nagios/etc/servers/client.itsupportbd.local.cfg

Add the following text to the file mentioned above.

The template below can also be used, and it can be customized to meet your needs. For monitoring logged-in users, system load, disk utilization (/ – partition), swap, and overall process, use the template below.

define host{
                           
            use                     linux-server            
            host_name               client.itsupportbd.local            
            alias                   client.itsupportbd.local            
            address                 192.168.9.10
                                    
}

Restart the Nagios server.

#sudo systemctl restart nagios


That is it. Please provide feedback in the section below.