I'm following the instruction, on hardentheworld.org to harden my OS X 10.11.
When I send the following command:
sudo defaults write /System/Library/LaunchDaemons/com.apple.mDNSResponder ProgramArguments -array-add "-NoMulticastAdvertisements"
I obtain:
Could not write /System/Library/LaunchDaemons/com.apple.mDNSResponder; exiting
My question is why this error happens and how to solve it?