The mDNSResponder project is a component of Bonjour,
Apple's ease-of-use IP networking initiative:
<http://developer.apple.com/bonjour/>

Apple's Bonjour software derives from the ongoing standardization
work of the IETF Zero Configuration Networking Working Group:
<http://zeroconf.org/>

The Zeroconf Working Group has identified three requirements for Zero
Configuration Networking:
1. An IP address (even when there is no DHCP server to assign one)
2. Name-to-address translation (even when there is no DNS server)
3. Discovery of Services on the network (again, without infrastucture)

Requirement 1 is met by self-assigned link-local addresses, as
described in "Dynamic Configuration of IPv4 Link-Local Addresses"
<http://files.zeroconf.org/draft-ietf-zeroconf-ipv4-linklocal.txt>

Requirement 2 is met by sending DNS-like queries via Multicast (mDNS).

Requirement 3 is met by DNS Service Dicsovery (DNS-SD).

Self-assigned link-local address capability has been available since
1998, when it first appeared in Windows '98 and in Mac OS 8.5.
Implementations for other platforms also exist.

The mDNSResponder project allows us to meet requirements 2 and 3.
It provides the ability for the user to identify hosts using names
instead of dotted-decimal IP addresses, even if the user doesn't have a
conventional DNS server set up. It also provides the ability for the
user to discover what services are being advertised on the network,
without having to know about them in advance, or configure the machines.