Music Player Daemon's control port in practice; registered to Microsoft for Hyper-V Live Migration.
IANA registered 6600/tcp in 2009 as mshvlm, Microsoft Hyper-V Live Migration — the channel that carries a running VM's memory between cluster hosts. On Linux and macOS boxes, though, an open 6600 is almost always MPD, the Music Player Daemon: its documentation states that if no port is specified the default is 6600. MPD speaks a simple line-based text protocol that greets you with "OK MPD <version>" and then accepts library and playback commands, with a password being optional.
Do not expose
MPD's protocol is cleartext and unauthenticated unless you configure a password, and it exposes your library paths and playback control. Hyper-V live migration traffic is cluster-internal by design. Neither belongs on a public IP.
$ printf 'status\nclose\n' | nc TARGET 6600Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.