Multihomed server issue

[Jason A. Donenfeld]

Hey again,

It turns out that our new semantics -- of rejecting only if the src IP
doesn't belong to _any_ interface, as opposed to the specific
interface -- nicely map to Linux's PKTINFO interface for userspace. In
working with Mathias on the Go implementation, I produced the
following code snippet that shows this sticky-socket technique using
pure-userspace facilities:

https://git.zx2c4.com/WireGuard/tree/contrib/examples/sticky-sockets/sticky-sockets.c

Just FYI, if anybody is curious.

Jason