Steps to be followed:
1.	Installing package dependencies
2.	Installing Nagios Core 4.4.6
3.	Installing Nagios and NRPE plugins

Step 1: Installing package dependencies
1.1	Install the package dependencies using the following commands:

sudo apt update
sudo apt install -y autoconf bc gawk dc build-essential gcc libc6 make wget unzip apache2 php libapache2-mod-php libgd-dev libmcrypt-dev make libssl-dev snmp libnet-snmp-perl gettext


Step 2: Installing Nagios Core 4.4.6
2.1	Download the Nagios Core source code
cd ~/
wget https://github.com/NagiosEnterprises/nagioscore/archive/nagios-4.4.6.tar.gz
tar -xf nagios-4.4.6.tar.gz
cd nagioscore-*/

2.2	Create the Nagios user and group using the below commands

sudo useradd nagios
sudo groupadd nagcmd
sudo usermod -a -G nagcmd nagios
2.3	Compile and install Nagios
sudo ./configure --with-httpd-conf=/etc/apache2/sites-enabled
sudo make all

2.4	Run the following commands to install Nagios binaries, service daemon script, and the command mode.
sudo make install
sudo make install-daemoninit
sudo make install-commandmode
sudo make install-config

2.5	Install the Apache configuration for Nagios and activate the mod_rewrite and mode_cgi modules and restart the apache service
sudo make install-webconf
sudo a2enmod rewrite cgi
sudo service apache2 restart

2.6	As you have installed the Nagios Core 4.4.6. Now create nagiosadmin user
sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

2.7	Enter the password of your choice. You need to remember it as this will be used to login to nagios later

2.8	For the firewall configuration, run the command below
for svc in Apache ssh
do
sudo ufw allow $svc
done

2.9	To start the UFW firewall service and add it to the system boot, run
sudo ufw enable

Step 3: Installing Nagios and NRPE plugins

3.1	Run the following command:

sudo apt install monitoring-plugins nagios-nrpe-plugin

3.2	Now, go to the nagios installation directory /usr/local/nagios/etc and create a new directory for storing all server hosts configuration
cd /usr/local/nagios/etc
sudo mkdir -p /usr/local/nagios/etc/servers

3.3	Edit the Nagios configuration nagios.cfg using the vim editor
sudo vim nagios.cfg

3.4	Press i and uncomment the cfg_dir option that will be used for storing all server hosts configurations
cfg_dir=/usr/local/nagios/etc/servers
	 

3.5	Save and close. Now edit the configuration file resource.cfg
sudo vim resource.cfg

3.6	Define the Nagios Monitoring Plugins path by changing the default configuration as shown below
$USER1$=/usr/lib/nagios/plugins
 

3.7	Save and close the file. Now edit the objects/contacts.cfg file and add the nagios admin email contacts
sudo vim objects/contacts.cfg

3.8	Change the email address with your own
define contact{
        ......
        email             <YourEmailId>
}

3.9	 Save and close. Edit the objects/commands.cfg file
sudo vim objects/commands.cfg

3.10	Add the following configuration to the end of the file
define command{
       command_name check_nrpe
       command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

3.11	Save and close, and the Nagios core configuration has been completed. Run the following commands:
sudo systemctl start nagios
sudo systemctl enable nagios

3.12	The Nagios service should be up and running, check using the following command:
sudo systemctl status nagios

 

3.13	Restart the Apache service to apply a new Nagios configuration
sudo systemctl restart apache2

3.14	Now open your web browser and type the server IP address following the nagios URL 
http://localhost/nagios/

3.15	Now you will have to log in with the user nagiosadmin and type your password

 
3.16	You will get the Nagios Dashboard as shown below:
 
3.17	You've installed Nagios on the Ubuntu server.

define host {
   use linux-server
   host_name nginx
   alias nginx Host
   address 172.17.0.2
   register 1
}

define service {
   host_name nginx
   service_description PING
   check_command check_ping!100.0,20%!500.0,60%
   max_check_attempts 2
   check_interval 2
   retry_interval 2
   check_period 24x7
   check_freshness 1
   contact_groups admins
   notification_interval 2
   notification_period 24x7
   notifications_enabled 1
   register 1
}

sudo service nagios restart