code: func (upc *UDPPacketConn) WriteTo(b []byte, addr net.Addr) (int, error) {
  	udpAddr, ok := addr.(*net.UDPAddr)
  	if !ok {
  		return 0, fmt.Errorf("must supply UDPAddr")
  	}
  
  	// Using the boundAddr is not quite right here, but it works.
  	packet := udp4pkt(b, udpAddr, upc.boundAddr)
  	return upc.PacketConn.WriteTo(packet, &raw.Addr{HardwareAddr: BroadcastMac})
  }
docs: // WriteTo implements net.PacketConn.WriteTo and broadcasts all packets at the
  // raw socket level.
  //
  // WriteTo wraps the given packet in the appropriate UDP and IP header before
  // sending it on the packet conn.