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.