code: def __intermediate_dns_servers(self, uci, address):
          
          # allow override
          if 'dns' in uci:
              return uci['dns']
          # ignore if using DHCP or if "proto" is none
          if address['proto'] in ['dhcp', 'dhcpv6', 'none']:
              return None
          dns = self.netjson.get('dns_servers', None)
          if dns:
              return ' '.join(dns)
docs: determines UCI interface "dns" option