Multicast and Unicast DNS daemon

AKA as mdnsd , a daemon invoked at boot time to implement Multicast DNS and DNS Service Discovery, the system-wide Unicast DNS Resolver.

To flush DNS cache: sudo killall -HUP mDNSResponder

Apple's "Zero Configuration Networking" application,

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

Performs Unicast and Multicast DNS Queries on behalf of client processes, and maintains a cache of the replies.

not run mDNSResponder manually.


To examine internal state syslog logging levels are used as follows:

SIGUSR1 toggles additional logging, with Warning and Notice enabled by default:
% sudo killall -USR1 mDNSResponder

Once this logging is enabled, syslog(1) to change the log filter.
For example, to enable ALL log levels :
% sudo syslog -c mDNSResponder -d

SIGUSR2 toggles packet logging:
% sudo killall -USR2 mDNSResponder

SIGINFO dumps a snapshot summary of the internal state to /var/log/system.log:
% sudo killall -INFO mDNSResponder

Sample messages

DeregisterInterface: Frequent transitions for interface en1i …

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


sudo launchctl unload /System/Library/LaunchDaemons/
sudo launchctl load /System/Library/LaunchDaemons/

( -w (disable) is deprecated)


Available from the Darwin open source repository mDNSResponder daemons .