[...]
> Note that one issue I am leaving for later (i.e., I don't want to drag this
> patch series out to include it), is whether and how to support ipv4-host->
> ipv6-guest forwarding and vice versa. Can libslirp support this?

That would be feasible yes: since the data flow is completely rebuilt
between the host and the guest, there is no remnant of the IP version.
It was simpler to have e.g. udp_listen and udp6_listen separate to keep
uint32_t / in6_addr parameters, but there is no strict reason for this:
the haddr is only passed to the bind() call, and the laddr is only
recorded in the so. Put another way, a refactoring patch could be to
just hand udp_listen two sockaddrs, and it will just work fine. We'd
then introduce a slirp_add_hostfwd that takes two sockaddr instead of
host/port.