Allow users of CallbackRouter to supply a GetBaseConfig
implementation. This is expected to be used on Android,
which currently lacks both a) platform support for
Split-DNS and b) a way to retrieve the current DNS
servers.
iOS/macOS also use the CallbackRouter but have platform
support for SplitDNS, so don't need getBaseConfig.
Updates https://github.com/tailscale/tailscale/issues/2116
Updates https://github.com/tailscale/tailscale/issues/988
Signed-off-by: Denton Gentry <dgentry@tailscale.com>
The shim implements both network and DNS configurators,
and feeds both into a single callback that receives
both configs.
Signed-off-by: David Anderson <danderson@tailscale.com>
