code: func (s *Server) unicastResponse(resp *dns.Msg, from net.Addr) error {
  	buf, err := resp.Pack()
  	if err != nil {
  		return err
  	}
  	addr := from.(*net.UDPAddr)
  	if addr.IP.To4() != nil {
  		_, err = s.ipv4conn.WriteToUDP(buf, addr)
  		return err
  	} else {
  		_, err = s.ipv6conn.WriteToUDP(buf, addr)
  		return err
  	}
  }
docs: // unicastResponse is used to send a unicast response packet