This is not my article, it is copy from this site . It cover install icinga2 , icinga2 director and pnp4nagios on Ubuntu 20.04, but is very useful as manual for debian buster and Bullseye.
This guide shows how to install Icinga2 monitoring software on Ubuntu 20.04 – this includes backend, frontend, director and pnp4nagios (adding graphs to icinga). Especially PNP was hard to get working – but if you follow the guide everything is nicely lined up at the end.
The Nagios / Icinga plugin check_multi is a convenient tool to execute multiple checks within a single check command that generates an overall returned state and output from it.
It calls multiple child plugins and displays their output in the long_plugin_output. A summary is given in the standard plugin output. The child return code with the highest severity becomes the parent (check_multi) plugin return code.
When I configure to run multiple nagios/icinga monitoring plugins with check_multi plugin, icinga web server show me this error messages: