TecAdmin published a tutorial about installing Nagios Server on Ubuntu 20.04.
How to Install Nagios Server on Ubuntu 20.04
Nagios Core formally known as Nagios is an open source infrastructure monitoring system. The Nagios application periodically checks on critical parameters of the application, server and network resources. For example, Nagios server can monitor CPU load, disk space, memory usage, the number of currently running processes on a remote server.