mDNS / Bonjour (Multicast DNS)
mDNS is the IETF local-link naming protocol behind much of the industry's casual "Bonjour discovery" language. It uses DNS-format messages on the local link so endpoints can resolve .local hostnames and, when paired with DNS-SD, advertise and browse services without a conventional DNS server. In AV the key design point is simple: mDNS is discovery plumbing, not the media, control, or authentication workflow itself.
Scope: mDNS / Bonjour (Multicast DNS) — select the deployed product/version; optional services require separate selection.
protocol · Catalog reviewed 2026-09-27
RFC 6762 / RFC 6763
Infrastructure Requirements
- Shared local link, or a deliberately configured discovery gateway
- IPv4/IPv6 multicast forwarding appropriate to the selected service.
Network & Physical Requirements
| Port / allocation | Transport | Connection / signal flow | Feature and requirement | Evidence |
|---|---|---|---|---|
| 5353 | UDP Multicast: 224.0.0.251; ff02::fb | Local peer → Local peers | Profile requirement · Core Local multicast name resolution and service discovery mDNS, IPv4 and IPv6 Link-local scope. DNS-SD is the service-record convention; use an intentional discovery gateway for cross-subnet operation. | Source checked 2026-09-27 RFC 6762, Sections 5, 6 and 18: queries, responses and message destination |
Gotchas & IT Notes
- ⚠mDNS is local-link by design. Do not treat it as a routed campus-wide discovery architecture.
- ⚠224.0.0.251 is in the IPv4 local-network-control block; IANA notes multicast routers should not forward destinations in 224.0.0.0/24 regardless of TTL. Treat ff02::fb the same way conceptually for IPv6: link-local discovery, not routed service discovery.
- ⚠mDNS is not the same thing as DNS-SD. mDNS is the transport / naming layer; DNS-SD is the service-advertisement model using PTR, SRV, TXT, and address records.
- ⚠If discovery must cross VLANs or subnets, plan an explicit gateway / reflector / discovery proxy / unicast DNS-SD or protocol-specific registry.
- ⚠A discovery proxy is not a multicast forwarder. RFC 8766 describes a proxy that learns from local mDNS and answers remote clients over normal unicast DNS mechanisms. That distinction matters for IT risk reviews because it avoids pretending raw multicast has to be stretched across the campus.