Digital Audio Access Protocol — an iTunes-style shared music library, advertised over Bonjour.
DAAP is HTTP with a binary content-code payload: a client GETs /server-info, /login, /databases and streams tracks over the same connection. Apple built it for iTunes music sharing, and the port is still used by AirPlay-adjacent software and by open-source servers — OwnTone (formerly forked-daapd) is the common one on Linux and NAS boxes. Instances announce themselves on the LAN over mDNS as _daap._tcp, which is why you find the port on Macs, Synology units, and Raspberry Pis without anyone configuring it. A library can be password-protected, but many are not.
Expose with care
It is designed for a trusted LAN and discovered automatically there; publishing it exposes your whole media library, and often unauthenticated, so keep it inside the network.
$ curl -sS -D - -o /dev/null http://TARGET:3689/server-infoSeen 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.