IANA-registered as Metagram Relay; historically also squatted by backdoors.
Port 99 is registered as metagram, Metagram Relay, an early message-relay service registered by Geoff Goodfellow. There is no published specification and no implementation in current use, so the registration is the only firm fact. Because it sits just under 100 and is normally closed, it has repeatedly been used as a listening port by hobby backdoors and hand-rolled admin shells. Anything open on 99 deserves a banner grab, not an assumption.
Expose with care
No legitimate modern service claims this port; an unexplained listener on 99 is worth investigating rather than firewalling and forgetting.
$ nc -vz TARGET 99Seen 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.