Registered to DEC Notes; today it is OpenOCD's GDB server, mining-pool endpoints, and any app that liked the repeating digits.
IANA registers 3333 as dec-notes, Digital's VAX Notes conferencing system, which nobody has run in decades. The repeating digits made the port a popular default elsewhere: OpenOCD serves its GDB remote-debug connection on 3333 by default (telnet on 4444, Tcl on 6666), cryptocurrency mining pools very commonly publish Stratum endpoints on 3333, and countless applications pick it as an alternate HTTP port. Nothing about 3333 tells you which of these you have hit, so the banner decides.
Expose with care
Depends entirely on what is listening: an OpenOCD GDB server is unauthenticated debug control of a device and must stay on localhost, while a pool endpoint is public by design.
$ nc -v TARGET 3333Seen 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.