Nagios is a very useful piece of software, which is very well supported and has been around for a very long time.
First, install apache, php and associated modules. Also make sure you have build-essential and libgd-dev for building nagios.
sudo apt install apache2 php libapache2-mod-php php-gd build-essential libgd-dev
Now, set up nagios user and groups
#First, add nagios user and command group. useradd nagios groupadd nagios-control #Then, make nagios user and www-data a member of those groups. usermod -a -G nagios-control nagios usermod -a -G nagios,nagios-control www-data