This comprehensive Linux guide expects that you run the following commands as root user but if you decide to run the commands as a different user then ensure that the user has sudo access and that you precede each of the privileged commands with sudo

It is an open source software tool designed to monitor system network and infrastructure. It keeps an inventory of your entire IT infrastructure and ensures your networks, servers, applications, services, and processes are up and running. It also sends you an alert in case of failure of a critical infrastructure component.

Updating Server And Dependencies

Once logged in to your system, first type the following commands:

apt update
apt upgrade

Now you can install dependencies using following commands

apt install -y build-essential autoconf gcc libc6 make wget unzip libgd2-xpm-dev

Downloading And Installing Nagios

Use the following command to download Nagois to a directory named /opt

wget -O /opt/nagioscore.tar.gz

When the download is complete, navigate to the opt directory and extract the archive using following commands

cd /opt
sudo tar xf nagioscore.tar.gz

Once Nagios archive is extracted, navigate to the newly created directory and run the following command to check for missing libraries

cd /opt/nagioscore-nagios-4.4.2

The output would be somewaht similar to shown below:

*** Configuration summary for nagios 4.4.2 2018-08-16 ***:

 General Options:
        Nagios executable:  nagios
        Nagios user/group:  nagios,nagios
       Command user/group:  nagios,nagios
             Event Broker:  yes
        Install ${prefix}:  /usr/local/nagios
    Install ${includedir}:  /usr/local/nagios/include/nagios
                Lock file:  /run/nagios.lock
   Check result directory:  /usr/local/nagios/var/spool/checkresults
           Init directory:  /lib/systemd/system
  Apache conf.d directory:  /etc/httpd/conf.d
             Mail program:  /bin/mail
                  Host OS:  linux-gnu
          IOBroker Method:  epoll

 Web Interface Options:
                 HTML URL:  http://localhost/nagios/
                  CGI URL:  http://localhost/nagios/cgi-bin/
 Traceroute (used by WAP):  /usr/sbin/traceroute

Review the options above for accuracy.  If they look okay,
type 'make all' to compile the main program and CGIs.

After completing configuration, run the following command to build the software:

make all

Use the following commands to create Nagios user

make install-groups-users
usermod -a -G nagios www-data

Now install Nagios using following commands

make install

Now you will run the following commands to install configuration files

make install-config

You can install the services using following commands

root@codesposts:/opt/nagioscore-nagios-4.4.2# make install-init

/usr/bin/install -c -m 755 -d -o root -g root /lib/systemd/system
/usr/bin/install -c -m 755 -o root -g root startup/default-service /lib/systemd/system/nagios.service
root@codesposts:/opt/nagioscore-nagios-4.4.2# make install-commandmode
/usr/bin/install -c -m 775 -o nagios -g nagios -d /usr/local/nagios/var/rw
chmod g+s /usr/local/nagios/var/rw

*** External command directory configured ***

Now you will configure external command file

root@codesposts:/opt/nagioscore-nagios-4.4.2# make install-commandmode

/usr/bin/install -c -m 775 -o nagios -g nagios -d /usr/local/nagios/var/rw
chmod g+s /usr/local/nagios/var/rw

*** External command directory configured ***

Install the Apache web server using the following commands

make install-webconf

To restart Apache service you will use following commands

systemctl restart apache2
a2enmod rewrite && a2enmod cg

Configuration Of Nagios Web Server Interface

To create Apache user account use the following commands

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

Downloading And Installing Nagios Plugins

You should make sure that pre-requisite packages are installed.This can be done using following commands

apt install -y autoconf gcc libc6 libmcrypt-dev make libssl-dev wget bc gawk dc build-essential snmp libnet-snmp-perl gettext

Now you will create your own working directory using following command

mkdir /opt/nagios && cd /opt/nagios
wget --no-check-certificate -O nagios-plugins.tar.gz
tar zxf nagios-plugins.tar.gz
cd nagios-plugins-release-2.2.1

Compiling And Installing Plugins

make install

You can check configuration file errors using following commands

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
Nagios Core 4.4.2
Copyright (c) 2009-present Nagios Core Development Team and Community Contributors
Checking global event handlers...
Checking obsessive compulsive processor commands...
Checking misc settings...

Total Warnings: 0
Total Errors: 0

To start Nagios web panel use the following commands

systemctl start nagios

Now Open your browser and then navigate to http://your-domain-or-ip/nagios and you will be presented with the login pop-up.

Enter the login credentials that you created earlier with the htpasswd command, and you will be redirected to the Nagios home page