Multicast DNS — how machines find each other by name on a LAN with no DNS server.
mDNS resolves .local names and carries DNS-SD service discovery over multicast to 224.0.0.251 (FF02::FB on IPv6). It is running on essentially every Mac, iPhone, Android device, Linux box with Avahi, printer, and Chromecast on the network — the daemon is mDNSResponder on Apple platforms and avahi-daemon elsewhere. It appears on a scan because the responder binds UDP 5353 and will also answer unicast queries sent to that port.
Do not expose
Reachable from the internet it both leaks a device inventory (hostnames, service records, sometimes usernames) and works as a reflection/amplification source; on a trusted LAN it is normal and expected.
$ dig +short @224.0.0.251 -p 5353 -t ptr _services._dns-sd._udp.localSeen 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.