How to do Agentless Monitoring with check_by_ssh
The fundamentals of Icinga 2 are check plugins. They are being executed and their return value is mapped to either Host or Service objects. Everything else follows on top. These check plugins can be either from the Monitoring Plugins or custom. While their origin does not matter, they are the building blocks of an Icinga monitoring stack. If a plugin goes CRITICAL, Icinga 2 alerts the sysadmin.