Registered to Apple as appleqtcsrvr, from the discontinued QuickTime streaming line; nothing runs here now.
IANA registers TCP and UDP 545 as "appleqtcsrvr" with no description beyond the name, and the same entry appears verbatim in the /etc/services file shipped with macOS. It dates from Apple's QuickTime server products, which Apple discontinued years ago — QuickTime Streaming Server and its Darwin Streaming Server sibling are gone, and current Apple media stacks use HTTP Live Streaming over ordinary HTTPS. There is no current software that listens here, so an open 545 is some other application that picked an unused low port.
Do not expose
Whatever is answering is not the registered service, so treat it as unidentified until you have fingerprinted it — an unknown listener in the system port range is not something to publish.
$ nc -vz -w 3 TARGET 545Seen 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.