Unregistered and uncommon — no standard service claims 9099.
Like its neighbour 9098, port 9099 carries no IANA registration and appears in nmap's services table only as unknown. Applications occasionally pick it as an arbitrary secondary listener, but there is no protocol you can assume from the number alone. Fingerprint the listener rather than guessing.
Expose with care
Judge it by the service you find behind it; an unregistered port is not evidence of either safety or danger.
$ nmap -sV -Pn -p 9099 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.