The initial implementation directly mirrored the behavior of Tailscale
exit nodes, where the WireGuard exit node DNS took precedence over other
configuration.
This adjusted implementation treats the WireGuard DNS
resolvers as a lower precedence default resolver than the tailnet
default resolver, and allows split DNS configuration as well.
This also adds test coverage to the existing DNS selection behavior with
respect to default resolvers and split DNS routes for Tailscale exit
nodes above cap 25. There may be some refinement to do in the logic in
those cases, as split DNS may not be working as we intend, though that
would be a pre-existing and separate issue.
Updates #9377
Signed-off-by: James Tucker <james@tailscale.com>
c7ce4e07e5