How to correctly configure DNS usage in Ubuntu? Authoritative answers without RA flag are ignored

Solution 1:

The kind of fallback behavior you seem to expect is not part of how DNS is supposed to work. The second nameserver in resolv.conf should only be contacted in the event that the first does not respond or there is some other sort of network error.

Microsoft have apparently done their own thing, and it certainly is useful for what you are trying to do, but it should not be relied upon for setting up a VPN. Microsoft also has a notion of using different name resolution for different network interfaces.

If you can't influence the VPN configuration, your best bet on Linux might be to set up your own nameserver which decides where to forward requests to by domain.