mDNSResponder(8) System Manager's Manual mDNSResponder(8)

mDNSResponder — Multicast and Unicast DNS daemon

mDNSResponder

mDNSResponder (also known as mdnsd on some systems) is a daemon invoked at boot time to implement Multicast DNS and DNS Service Discovery. On Mac OS X 10.6 (Snow Leopard), mDNSResponder is also the system-wide Unicast DNS Resolver.

mDNSResponder listens on 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 Unicast and 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.

There are several methods with which to examine mDNSResponder's internal state for debugging and diagnostic purposes. The syslog(1) logging levels map as follows:

Error - Error messages
Warning - Client-initiated operations
Notice - Sleep proxy operations
Info - Informational messages

By default, only log level Error is logged.

A SIGUSR1 signal toggles additional logging, with Warning and Notice enabled by default:

% sudo killall -USR1 mDNSResponder

Once this logging is enabled, users can additionally use syslog(1) to change the log filter for the process. For example, to enable log levels Emergency - Debug:

% sudo syslog -c mDNSResponder -d

A SIGUSR2 signal toggles packet logging:

% sudo killall -USR2 mDNSResponder

A SIGINFO signal will dump a snapshot summary of the internal state to /var/log/system.log:

% sudo killall -INFO mDNSResponder

mDNSResponder accepts the following optional arguments:

Append search domains for multi-labeled Partially Qualified Domain Name as well as single-labeled Partially Qualified Domain Name. This argument is not recommended because of the extra DNS traffic it generates and its adverse effect on battery life.
Prevent the system from advertising Bonjour services via Multicast DNS.

To cause mDNSResponder to run with these optional arguments when it launches on OS X 10.11 (El Capitan) and later, set the AlwaysAppendSearchDomains or NoMulticastAdvertisements boolean keys to true in /Library/Preferences/com.apple.mDNSResponder.plist and reboot.

/usr/sbin/mDNSResponder /Library/Preferences/com.apple.mDNSResponder.plist

For information on Multicast DNS, see http://www.multicastdns.org/

For information on DNS Service Discovery, see http://www.dns-sd.org/

For information on how to use the Multicast DNS and the DNS Service Discovery APIs on Mac OS X and other platforms, see http://developer.apple.com/bonjour/

For the source code to mDNSResponder, see http://developer.apple.com/darwin/projects/bonjour/

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 officially supported by Apple) are mDNSResponder daemons for other platforms, including Microsoft Windows, Linux, FreeBSD, NetBSD, Solaris, and other POSIX systems.

April 2004 Darwin