mDNSResponder — Multicast DNS daemon




mDNSResponder (also known as mdnsd on some systems) is a daemon invoked
at boot time to implement Multicast DNS and DNS Service Discovery.

mDNSResponder listens UDP port 5353 for Multicast DNS Query packets.
When it receives a query for which it knows an answer, mDNSResponder
issues the appropriate Multicast DNS Reply packet.

mDNSResponder also performs Multicast DNS Queries on behalf of client
processes, and maintains a cache of the replies.

mDNSResponder has no user-specifiable command-line argument, and users
should not run mDNSResponder manually.

To examine mDNSResponder’s internal state, for debugging and disagnostic
purposes, send it a SIGINFO signal, and it will then dump a snapshot sum-
mary of its internal state to /var/log/system.log, e.g.

sudo killall -INFO mDNSResponder





For information on Multicast DNS, see
For information on DNS Service Discovery, see
For information on how to use the Multicast DNS and the DNS Service Dis-
covery APIs on Mac OS X and other platforms, see
For the source code to mDNSResponder, see


mDNSResponder bugs are tracked in Apple Radar component “mDNSResponder”.


The mDNSResponder daemon first appeared in Mac OS X 10.2 (Jaguar).

Also available from the Darwin open source repository (though not offi-
cially supported by Apple) are mDNSResponder daemons for other platforms,
including Mac OS 9, Microsoft Windows, Linux, FreeBSD, NetBSD, Solaris,
and other POSIX systems.