Back To My Mac (BTMM) used to require UPnP or NAT-PMP on your router to work, but at some point this changed and it can work (albeit slowly) without those.
BTMM (usually?) works by setting up an encrypted IPv6 tunnel over IPv4 between the two hosts, and with UPnP/NAT-PMP, the appropriate ports are forwarded on the router at each end, allowing a direct connection.
But how does it work when there's no UPnP/NAT-PMP?
I noticed that it's much slower if either end doesn't have UPnP/NAT-PMP (I get a ~40ms ping if both ends have the automatic config, and about 1000ms if they don't), so I'm wondering where the traffic goes — presumably it has to go via Apple servers if a direct connection can't be automatically negotiated?
But even with just plain old NAT, is it not possible for a 3rd party to arbitrate a connection between two NAT'ed hosts?
Is there a way to tell how that IPv6 tunnel is getting set up? I can't figure it out from the output of
For bonus points, is it possible to set up static port forwards on a router that lacks UPnP/NAT-PMP in order to get BTMM working directly, rather than in this slower mode?