As a workaround for some limitations of our new DNS implementation, we've had to add fallback resolvers, which are like the old CorpDNS resolvers but only get used when we're in a case where the OS would do the wrong thing if given only a split DNS configuration.

This bug is to track removing FallbackResolvers once we've implemented fixes for the cases that require it:

• macOS/iOS clients using an exit node: unless we provide a default resolver through the NetworkExtension API, the machine ends up with no primary DNS resolver. The fix for this is exit node DNS forwarding, Forward DNS traffic to exit node #1713.
• android doesn't yet understand split-DNS at all, and we need to research how to make it work. Support split DNS in android #1695

Once those two are fixed, we should be able to pull out FallbackResolvers from the client.