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