README.privsep
1On Mac OS X, mDNSResponder now runs with user-ID and group-ID 2"_mdnsresponder". In order to perform certain privileged operations, a 3helper (unimagintively called mDNSResponderHelper) runs as root when 4needed and handles requests from mDNSResponder. 5 6 7* A new LaunchD job com.apple.mDNSResponderHelper starts 8 mDNSResponderHelper on demand. The helper exits after approximately 9 10 seconds of idle time. 10 11* The com.apple.mDNSResponder LaunchD job specifies the account under 12 which to run, so that mDNSResponder starts as _mdnsresponder. 13 14* A subdirectory named "mdns" and owned by _mdnsresponder has been 15 created in /var/run. The PID file and uDNS server socket has been 16 moved to that subdirectory. 17 18* There are currently six remote procedure calls handled by 19 mDNSResponderHelper: mDNSPreferencesSetName, mDNSKeychainGetSecrets, 20 mDNSConfigureServer, and mDNSAutoTunnelSetKeys 21 22* mDNSPreferencesSetName allows mDNSResponder to set the computer name 23 or local host name, and displays a notification if there was a 24 conflict. 25 26* mDNSKeychainGetSecrets causes mDNSResponderHelper to collect DNS 27 keys from the system keychain. SetDomainSecrets uses the result to 28 populate AuthInfoList. One could refactor this code further so that 29 mDNSResponderHelper performs all the cryptographic operations, with 30 the result that a compromise of mDNSResponder does not compromise 31 keys. But I think that may be more change than is advisable at this 32 point. 33 34* On the advice of the Security.framework team, I've used 35 SecKeychainSetPreferenceDomain to ensure that the system keychain is 36 references whenever a NULL SecKeychainRef is used. Wherever a 37 SecKeychainRef is needed, NULL is now specified. 38 39* mDNSConfigureServer, and mDNSAutoTunnelSetKeys do various setup and 40 teardown for BTMM. 41