Improved handling of changing DNS resolvers

[Noah Gold <nkgold@google.com>]

Hi folks,

libslirp and Passt have different approaches to sharing DNS resolvers with
the guest system, each with their own benefits & drawbacks. On the libslirp
project, we're discussing [1] how to support DNS failover. Passt already has
 support for this, but there is a drawback to its solution which prevents us
from taking a similar approach: the resolvers are read exactly once, so if the
host changes networks at runtime, the guest will not receive the updated
resolvers and thus its connectivity will break.

libslirp's current approach is to DNAT a single address exposed to the guest
to one of the resolvers configured on the host. The problem here is that if that
one resolver goes down, the guest can't resolve DNS names. We're
considering changing so that instead of a single address, we expose a set of
MAXNS addresses, and DNAT those 1:1 to the DNS resolvers registered with
the host. Because the DNAT table lives on the host side, we can refresh the
guest's resolvers whenever the host's resolvers change, but without the need to
expire a DHCP lease (even with short leases, the guest will still lose
connectivity
for a time).

Does this sound like an approach Passt would be open to adopting as well?

Thanks.

- Noah

[1] https://gitlab.freedesktop.org/slirp/libslirp/-/issues/26