Easiest way without breaking core network manager functionality is to work with its profiles. Create a dedicated profile with the DNS set to the servers in the specs. Switch manually between them before connection and after disconnection from AirVPN. NetworkManager will manage resolv.conf, and you can give Hummingbird/Goldcrest the -i switch. If you're prepared to deal with divergent behavior, in /etc/NetworkManager/NetworkManager.conf, there in the [main] section, add the line: dns=none