For a monitoring system to receive more information from an endpoint
other than that it is simply accessible, help is required from the
target system. For example - how else can Checkmk know how full a
server's storage volume is without that system somehow providing the
information? The component that provides this information is always an
active piece of software - namely a monitoring agent, usually just
referred to as an agent. An agent collects data relevant to monitoring
from a host at specified intervals and transmits that data to the
monitoring server.

For servers and workstations, Checkmk provides its own agents, these
are known as Checkmk agents. Checkmk agents are available for a wide
variety of operating systems - from the commonplace such as Windows and
Linux to exotics such as OpenVMS. The agents are passive and listen on
TCP port 6556. Only on receiving a Checkmk server query will these
agents be activated and respond with the required data.