Unassigned on TCP; Nmap names the UDP side as BakBone NetVault's backup communications port.
IANA has 20031 inside the unassigned range 20015-20033, so there is no TCP registration. Nmap's service table labels 20031/udp 'bakbonenetvault — BakBone NetVault primary communications port', with a comparatively high open-frequency, and NetVault (BakBone, later Quest) is the only widely documented user of this number. On TCP the entry is 'unknown'. If you find it open on a server that also runs backup agents, NetVault is the first thing to check; otherwise treat it as an unidentified listener.
Do not expose
A backup control channel reaches every file on every protected host. Whether or not it is NetVault, this stays on a management network.
$ nmap -Pn -sV -p 20031 --reason TARGETSeen 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.