Oh now I see what you are saying. In the past there was a DNS leak associated with this being set to true, and setting it to false fixed the DNS leak. In the current TBB this has been fixed already and it can remain set on true without risk of DNS leaks, and it probably should be left on true. http://archives.seul.org/or/talk/May-2012/msg00038.html This was fixed over a year ago.