Ecco il contenuto del mio /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist da OS X 10.9.1:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.apple.mDNSResponder</string>
<key>OnDemand</key>
<false/>
<key>InitGroups</key>
<false/>
<key>UserName</key>
<string>_mdnsresponder</string>
<key>GroupName</key>
<string>_mdnsresponder</string>
<key>ProgramArguments</key>
<array>
<string>/usr/sbin/mDNSResponder</string>
</array>
<key>MachServices</key>
<dict>
<key>com.apple.mDNSResponder</key>
<true/>
<key>com.apple.mDNSResponder.dnsproxy</key>
<true/>
</dict>
<key>Sockets</key>
<dict>
<key>Listeners</key>
<dict>
<key>SockFamily</key>
<string>Unix</string>
<key>SockPathName</key>
<string>/var/run/mDNSResponder</string>
<key>SockPathMode</key>
<integer>438</integer>
</dict>
</dict>
<key>EnableTransactions</key>
<true/>
<key>BeginTransactionAtShutdown</key>
<true/>
<key>POSIXSpawnType</key>
<string>Interactive</string>
</dict>
</plist>
Probabilmente varrebbe la pena eseguire una riparazione delle autorizzazioni dopo aver ripristinato il file.