Registered as "Onmux"; the only surviving annotation ties it to ON Technology's Meeting Maker calendar server.
IANA lists 417 as onmux with no description beyond the name and no protocol document. The one piece of corroborating information is Nmap's service database, which annotates both 417/tcp and 417/udp as "Meeting maker" — ON Technology's cross-platform group calendar server of the same era. That product is long gone and no specification was ever published. Everything else you could say about this port would be invention.
Expose with care
No published protocol and no maintained implementation; if something answers here, identify it locally and keep it off public interfaces.
$ nc -vz -w 3 TARGET 417Seen 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.