How can I detect which setting in the OS X could block ping localhost? IPFW, LittleSnitch no such rules?

Update 1:

> cat /etc/hosts
127.0.0.1   localhost
255.255.255.255   broadcast

Solution 1:

The obvious choice would be if stealth mode were on. That blocks pings to localhost on all my Mac OS systems. Go to System Preferences, Security & Privacy, Firewall, Firewall Options...

Also, if you have "Block all incoming connections" that might also affect things. I would certainly turn off LittleSnitch since it also can and will block that sort of traffic if you haven't allowed it.