Internet-Security.com TCP port 6556 details

Checkmk (formerly check_mk) monitoring agent uses TCP port 6556 by default to expose host metrics to a Checkmk/OMD server.
The agent runs on Linux/UNIX (often via xinetd or systemd socket) and Windows (as the “Check_MK Service”), and is queried by Checkmk Raw/Enterprise editions.
Other monitoring stacks also reuse the Checkmk agent on 6556, such as Icinga, Nagios, and OP5 (ITRS) via the check_mk agent plugin.
Security note: When exposed to the internet, 6556/check_mk-agent can reveal detailed system information and has been abused for reconnaissance; best practice is to restrict access and use the newer TLS-secured Agent Controller in modern Checkmk versions.

TCP 6556