Serverless XMPP between machines on the same LAN — the old Bonjour/iChat-style chat.
IANA registered 5298 in 2008 as presence, for XMPP link-local messaging: peers advertise _presence._tcp over mDNS and then open a direct XMPP stream to each other with no server in the middle, as described in XEP-0174. Apple's iChat/Messages Bonjour mode popularised it, and Pidgin and Gajim implement the same scheme. Seeing it means a desktop chat client with local discovery turned on, not a chat server.
Expose with care
Link-local by design and unauthenticated — fine on a trusted LAN, and it has no business being routed or forwarded anywhere.
$ dns-sd -B _presence._tcp local.Seen 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.