[tor-relays] State of the art in NAT busting?

[Gordon Morehouse]

I have a question which relates to my ongoing groundwork to build a Raspberry Pi (and hopefully Beagle and Cubie) friendly set of Debian packaged programs which can turn one of these small, low-power machines into a plug n' forget Tor relay.

What is the start of the art in NAT hole punching if one does not have a UPnP or (other-standard-mumble) compliant router?  If I'm behind such a router, what's the best I can do to help the Tor network if, for any reason, the user is unable to port forward?

Sub-questions:

1. What's the best I can do without any outside help?

2. What's the best I can do with minimal outside help, e.g. STUN+ICE, TURN, etc?  Does Tor itself support any of those?  Could a Tor relay or bridge + not-yet-written scripts make use of those?

3. Are there any options I'm forgetting to consider, other than wholesale relaying of traffic (eg IP_A:portN -> hole-punched NATed Tor instance where IP_A is another machine handling some or all of the NATed relay's traffic)?

Best,
-Gordon M.