> Ok, there's some work on this already, mainly faith(4).  But it seems
> very basic, as it only supports half of the process (IPv6->IPv4),
> only supports TCP, and requires a specific bit to be set in the IPv6
> packet (why?).

Supporting only the v6->v4 direction is..more or less unavoidable,
since there is no way to shorehorn the 128-bit v6 address space into
even the entire 32-bit v4 address space, much less a managably small
subset thereof, whereas it's easy to go the other way around.

Supporting only TCP is...well, fixable; it's about as difficult as
making NAT handle other than TCP, and for similar reasons.

The "specific bit" - well, I don't know what bit you're talking about,
and I used faith, and in fact hacked on it, back in the second half of
'02, so I think I know what I'm talking about when I say that there is
no fundamental requirement for any such thing.

Faith, at least faith as I used it, is not really NAT.  It's more like
an application-level proxy, with the faith interface used to arrange
for the machine to accept connections to any of a large number of
distinct addresses (and, with my hacks, ports).

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