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$ sudo apt install nagios4 nagios4-common monitoring-plugins-contrib nagios-nrpe-plugin

$ sudo systemctl enable nagios4
$ sudo systemctl is-enabled nagios4
$ sudo systemctl status nagios4

$ sudo systemctl is-enabled apache2
$ sudo systemctl status apache2

Connect to nagios at http://nagios_host_ip_address/nagios4/

In file /etc/nagios4/cgi.conf change value of use_authentication to 1.

Restart nagios4 service. From now on, only user 'nagiosadmin' will be allowed to access your. You can still access the Nagios dashboard, but the reporting and monitoring page will be denied.

To enable the required modules, cgi, cgid, and rewrite, run command

sudo a2enmod cgi cgid rewrite

Open /etc/apache2/conf-available/nagios4-cgi.conf, find the section of <DirectoryMatch></DirectoryMatch> and replace the default configuration with the following. With this you will enable the Apache basic_auth module Nagios web contents.

<DirectoryMatch (/usr/share/nagios4/htdocs|/usr/lib/cgi-bin/nagios4|/etc/nagios4/stylesheets)>
 Options FollowSymLinks
 DirectoryIndex index.php index.html
 AllowOverride AuthConfig

 Options None
 AllowOverride None
 Order allow,deny
 Allow from all
 AuthName "Nagios Core"
 AuthType Basic
 AuthUserFile /etc/nagios4/htdigest.users
 Require valid-user
 Require all denied

</DirectoryMatch>

To create a new user nagiosadmin for Apache basic auth

sudo htpasswd /etc/nagios4/htdigest.users nagiosadmin

Input your password two times. Restart the apache2 service.