The CVSup file-distribution port — FreeBSD's old way of syncing source trees, now retired.
IANA assigns TCP/UDP 5999 to cvsup, and the cvsup(1) man page confirms 5999 as the client's default connect port. CVSup was the Modula-3 rsync-alike that mirrored the FreeBSD source, ports and CVS trees for years before the project moved to Subversion, portsnap and eventually Git, so public cvsupd mirrors have been decommissioned. Scanner databases carry a second historical name for this number, ncd-conf, the configuration telnet port of Network Computing Devices X terminals; neither service is something you will meet in 2026.
Expose with care
A real cvsupd served read-only public mirrors and was fine on the internet by design, but nothing runs one now — so anything answering here is unidentified software and should be checked before it is exposed.
$ nmap -Pn -sV -p 5999 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.