mDNS / Discovery
Definition
Multicast DNS (mDNS, RFC 6762) allows devices on a local network to resolve names ending in .local without a DNS server, and DNS-based Service Discovery (DNS-SD, RFC 6763) uses it to announce services. Apple's implementation is called Bonjour, and the common Linux implementation Avahi. Dante, NDI and many control applications rely on this mechanism for automatic discovery. mDNS messages are sent to the multicast address 224.0.0.251 (FF02::FB in IPv6) on UDP port 5353, a link-local scope that routers do not forward, so discovery does not cross subnets or VLANs without an mDNS gateway or reflector.
In the field
NDI sources or Dante devices that appear on one VLAN and are missing from another usually indicate that discovery traffic does not cross the boundary. The options are to keep equipment that depends on discovery on one subnet, to configure an mDNS gateway on the router, or to use the manufacturer's own tools: NDI provides a Discovery Server, and Dante Domain Manager manages Dante devices across subnets.
Related lessons
Sources
- RFC 6762 : noms en .local, requêtes vers 224.0.0.251 (FF02::FB), port UDP 5353 · rfc-editor.org
- RFC 6763 : DNS-Based Service Discovery · rfc-editor.org
- RFC 5771 : bloc 224.0.0.0/24 à portée locale, non relayé par les routeurs · rfc-editor.org
- Audinate, Dante Information for Network Administrators : découverte Dante par mDNS/DNS-SD · audinateweb.sfo2.cdn.digitaloceanspaces.com
- NDI Documentation, Discovery Server : serveur de découverte centralisé pour relier des appareils NDI entre sous-réseaux ou quand le mDNS est bloqué · docs.ndi.video
- Avahi (dépôt officiel) : implémentation Linux de mDNS/DNS-SD, compatible avec Apple Bonjour · github.com
- Audinate, Dante Domain Manager : extension d'un réseau Dante au-delà des frontières de sous-réseau · getdante.com