Eliminar la configuración del servidor dns personalizado a través de la línea de comandos

Utilizando man networksetup

-getdnsservers networkservice
         Displays DNS info for <networkservice>.

 -setdnsservers networkservice dns1 [dns2] [...]
         Use this command to specify the IP addresses of servers you want the specified <networkser-
         vice> to use to resolve domain names. You can list any number of servers (replace dns1, dns2,
         and so on with the IP addresses of domain name servers). If you want to clear all DNS entries
         for the specified network service, type "empty" in place of the DNS server names.

Puedes listar todos los dns usando el primer comando, copiarlos o simplemente usar el pipelining al portapapeles usando pbcopy de la siguiente manera:

networksetup -getdnsservers Wi-Fi | pbcopy

Ellos empty la lista, y volver a añadir lo deseado.