code: func (ds *DNSServer) Listen(listenSpec string) error {
  	return dns.ListenAndServe(listenSpec, "udp", ds)
  }
docs: // Listen for DNS requests. listenSpec is a dotted-quad + port, e.g.,
  // 127.0.0.1:53. This function blocks and only returns when the DNS service is
  // no longer functioning.