/etc/resolvconf/run/interface/original.resolvconf in an editor with root rights.
(Qubes-Whonix ™: In TemplateVM)
Comment everything out by adding a # in front of all entries. Alternatively, empty or delete that file. 
Save and exit.
- This is done to prevent the old DNS server being used. For further discussion of this issue, see: https://github.com/adrelanos/vpn-firewall/issues/16 [archive]