Apple's browser-based server administration port, from AppleShare IP and the old Mac OS X Server admin tools.
IANA still lists 311 as asip-webadmin, registered by Apple for AppleShare IP's web administration interface. Apple carried the port forward into Mac OS X Server, where the Server Admin, Workgroup Manager and Server Monitor tools reached the server over it. Both AppleShare IP and macOS Server are discontinued, and Apple's current port list no longer mentions 311, so a live listener today means an old Mac server that was never retired. If you find one, the machine behind it is by definition running unpatched, end-of-life server software.
Do not expose
A server administration plane on unmaintained, end-of-life software — restrict it to a management network and plan the decommission.
$ nmap -sV -p 311 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.