Registered as "SpectraTalk Port" with nothing public behind the name.
IANA registers 3851 as spectraport, "SpectraTalk Port", to a private assignee. No specification, vendor documentation, or implementation for SpectraTalk is publicly available, and it does not appear in the common scanner fingerprint sets as anything meaningful. The registration is the whole of the verifiable story. A listener on 3851 needs to be traced back to a process on the host to be identified.
Do not expose
Undocumented protocol, unknown authentication: there is no basis on which exposing it could be called safe.
$ sudo lsof -nP -iTCP:3851 -sTCP:LISTENSeen 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.