Mulitcast UDP patch for the EthernetUdp.c source code in Arduino Ethernet library.

EthernetUdp.c

/*
  Add the following decleration to EthernetUdp.h directly after line 55
  
    virtual uint8_t beginMulti(IPAddress, uint16_t);

  then add the following code to the end of EthernetUdp.c
  
*/


uint8_t EthernetUDP::beginMulti(IPAddress ip, uint16_t port) {
    Serial.println("beginMulti()");
    if (_sock != MAX_SOCK_NUM)
        return 0;

    for (int i = 0; i < MAX_SOCK_NUM; i++) {
        uint8_t s = W5100.readSnSR(i);
        if (s == SnSR::CLOSED || s == SnSR::FIN_WAIT) {
            _sock = i;
            break;
        }
    }

    if (_sock == MAX_SOCK_NUM)
        return 0;


    // Calculate MAC address from Multicast IP Address
    byte mac[] = {  0x01, 0x00, 0x5E, 0x00, 0x00, 0x00 };

    mac[3] = ip[1] & 0x7F;
    mac[4] = ip[2];
    mac[5] = ip[3];

    W5100.writeSnDIPR(_sock, rawIPAddress(ip));   //239.255.0.1
    W5100.writeSnDPORT(_sock, port);
    W5100.writeSnDHAR(_sock,mac);

    _remaining = 0;

    socket(_sock, SnMR::UDP, port, SnMR::MULTI);

    return 1;
}