Multicast DNS

Multicast DNS is a way of using familiar DNS programming interfaces, packet formats and operating semantics, in a small network where no conventional DNS server has been installed.

Multicast DNS is a joint effort by participants of the IETF Zero Configuration Networking (zeroconf) and DNS Extensions (dnsext) working groups. The requirements are driven by the Zeroconf working group; the implementation details are a chartered work item for the DNSEXT group. Most of the people working on mDNS are active participants of both working groups.

While the requirements for Zeroconf name resolution could be met by designing an entirely new protocol, it is better to provide this functionality by making minimal changes to the current standard DNS protocol. This saves application programmers from having to learn new APIs, and saves application programmers from having to write application code two different ways — one way for large configured networks and a different way for small Zeroconf networks. It means that most current applications need no changes at all to work correctly using mDNS in a Zeroconf network. It also means that engineers do not have to learn an entirely new protocol, and current network packet capture tools can already decode and display DNS packets, so they do not have to be updated to understand new packet formats.

Documents

• RFC 6760 Requirements for the Replacement of AppleTalk Name Binding Protocol describes how AppleTalk Name Binding Protocol works, and what is needed in an IP-based replacement.

• RFC 6761 Special-Use Domain Names describes what it means to say that a DNS name is reserved for special use, when reserving such a name is appropriate, and the procedure for doing so.

• RFC 6762 Multicast DNS specifies how to perform DNS queries over IP Multicast. The mDNS protocol, together with DNS Service Discovery (below), forms the basis for Apple's ease-of-use initiative called Bonjour. Apple began shipping mDNS in Mac OS X 10.2 in 2002, and today you'll find it in a wide range of hardware products, including Apple AirPort base stations, Axis network cameras, Roku SoundBridges, TiVo PVRs, and just about every network printer from just about every printer manufacturer. It's also in an increasing number of Linux distributions, and it's available for Windows too.

Further Information

• How to modify EtherPeek to decode Multicast DNS packets on UDP port 5353.

• How to make Mac OS X do dot-local lookups using both multicast and unicast queries, for compatibility on networks where the local administrator wants to have a private internal unicast DNS domain and wants it to be called "local" as well.

• DNS Service Discovery extends Multicast DNS to also provide simple service discovery (network browsing).

• For more information, subscribe to the Zeroconf and dnsext mailing lists.

• Arthur van Hoff from Strangeberry has written an implementation of Multicast DNS Service Discovery entirely in Java, available from the JmDNS SourceForge repository.

• Zero Configuration Networking: The Definitive Guide by Daniel Steinberg and Stuart Cheshire, published by O'Reilly Media.

• The Bonjour Sleep Proxy wakes your Mac automatically when you access it over the network.