No IANA assignment and no well-established service — whatever is listening chose the number itself.
9503 is unassigned in the IANA service name and port number registry, and Nmap's service database lists it as unknown with a very low observed frequency. It sits just above the ports Swoole's documentation uses for its example servers, so it sometimes appears on the same hosts as an extra application listener, but nothing standard claims it. Treat a listener here as an unidentified local application and fingerprint it directly.
Expose with care
An unidentified service is an unassessed service. Find out what it is before deciding, and do not assume an obscure port number is protection.
$ sudo lsof -nP -iTCP:9503 -sTCP:LISTENSeen 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.