Registered as servexec (xinuexpansion1 on UDP); no current standard service.
IANA registers 2021/tcp as servexec and 2021/udp as xinuexpansion1, another pair of undocumented early registrations. Nothing maintained implements them. Because it sits one digit above the FTP control port, 2021 is also a common manual choice for an alternate FTP or SFTP listener, so fingerprint rather than assume.
Expose with care
If it turns out to be FTP moved off 21, that is cleartext authentication and should be never — replace it with SFTP or FTPS.
$ nc -vz TARGET 2021Seen 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.