mDNS / DNS-SD

showstack / protocols / mdns-dns-sd

Also called Bonjour, Zeroconf, Avahi.

Multicast DNS and DNS Service Discovery: how devices on a local network announce and find each other with no DNS server and no configuration. Dante, NDI, NMOS, OSC discovery and half the show network depends on it working.

Network transportOpenIETFVerified

Management Housekeeping, no deadline

Discovery, monitoring, configuration, backups, updates. Useful, and none of it has a deadline of its own.

What kills it. Nothing kills it. That is the problem: with no deadline it has no manners, and it will happily starve everything that does have one. Putting it on a show VLAN is the commonest self-inflicted wound in the industry.

Why this one, and not what the category suggests. Discovery. It is how devices find each other, and it is chatty in a way that matters on a busy VLAN.

The four flows, and why the model is worth having →

Ports

udp/5353 mDNS

Multicast

224.0.0.251 (IPv4)
ff02::fb (IPv6)
Link-local scope: mDNS does not cross a router, which is why devices vanish when someone segments the network.

What goes wrong

mDNS is link-local by design and does not route. Putting devices on separate VLANs or subnets makes them disappear from each other unless a reflector or repeater is configured.

Many managed switches filter or rate-limit multicast at 224.0.0.251 as a security default, which silently breaks discovery while unicast traffic works perfectly.

Wireless access points frequently block multicast between clients. A laptop on wifi that cannot see a device on the wired network is usually this, not a firewall.

What speaks mDNS / DNS-SD (2)

Product	Direction	Notes
Dante Controller Audinate	bidirectional	Dante discovery is mDNS, which is why devices vanish across a routed boundary.
Wireshark Wireshark Foundation	in	Decodes mDNS, the fastest way to prove a discovery problem.

Open source implementations

Avahi — C, LGPL-2.1-or-later

Typical use

Dante device discovery
NDI source discovery
Finding a console or media server without knowing its IP

Sources

RFC 6762: Multicast DNS — IETF primary
RFC 6763: DNS-Based Service Discovery — IETF primary

Understand the mechanism

Show networks

Something wrong, or missing?

Every entry here is maintained by people who run shows.

Edit this entry on GitHub — one file, editable in your browser, and your handle goes on it permanently.

Work at IETF? Confirm or correct this entry — it is free, it stays free, and nothing about it is for sale.