Over on the tor-talk mailing list, Alec Muffet outlines the initial
   proof-of-concept for the Facebook .onion site:

   Hi, It rather depends upon the complexity of your “cleartext” website.
   If you have a small, simple website with no cookies and where all of
   the URLs are “relative” then maybe you could set up a Tor daemon and
   have the hidden service to point at your usual webserver, but there
   would probably be errors and without extensive testing/fixing the
   results are very likely to be flaky. For any complex website it would
   be worse. We did our initial proof-of-concept using “mitmproxy” such
   that a Tor daemon hosting a hidden service spoke to mitmproxy via
   localhost:443; this terminated SSL and then a selection of commandline
   arguments rewrote the content bidirectionally, such that: - incoming
   request headers - Host/Referer/etc… - which referenced the onion
   address were rewritten in terms of the normal web address - outgoing
   json/javascript/css/html in the response body which referenced the
   normal web address was rewritten in terms of the onion address -
   outgoing response headers which referenced the normal web address were
   rewritten in terms of the onion address - outgoing cookies had their
   domains changed in terms of the onion address - caching was disabled,
   because debugging. …and then the traffic was then forwarded to the
   normal web address over a new HTTPS connection. Overall the
   proof-of-concept was a single shellscript containing approximately a
   single commandline with perhaps ten options. Most of the site
   functioned over the onion address via this mechanism, if somewhat
   clunkily. This experiment worked sufficiently well for us to see what
   modifications needed to be done to the main website code to make a
   solution that was fit for production.
   [1]https://lists.torproject.org/pipermail/tor-talk/2014-November/035680
   .html

References

   1. https://lists.torproject.org/pipermail/tor-talk/2014-November/035680.html