Ethernet - UDP Multicast

In LWIP -> Key Options, click "Show Advanced Parameters", and then enable "LWIP_MULTICAST_TX_OPTIONS".

Also make sure "MEMP_NUM_IGMP_GROUP" is greater than 1. Here it's defaulted to 8.

Then, in General Settings, enable "LWIP_IGMP".

Open "ethernetif.c", find the low_level_init() function.

Add the following line after the "Accept broadcast address and ARP traffic" line to enable IGMP service.

...
/* USER CODE BEGIN PHY_PRE_CONFIG */
    netif->flags |= NETIF_FLAG_IGMP;
/* USER CODE END PHY_PRE_CONFIG */
...

Define the following UDP multicast helpers

void UDP_multicast_receive_callback(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr, u16_t port) {
  if (p != NULL) {
    // Copy received data to buffer
    char buffer[128];
    memcpy(buffer, p->payload, p->len < sizeof(buffer) ? p->len : sizeof(buffer));

    // Debug print
    char str[128];
    sprintf(str, "Received Multicast: %s\n", buffer);
    HAL_UART_Transmit(&huart3, (uint8_t *)str, strlen(str), 100);

    // Free the packet buffer
    pbuf_free(p);
  }
}

void UDP_Multicast_init() {
  static struct udp_pcb *g_udppcb = NULL; // Make static to prevent deallocation
  err_t err;
  char str[128];


  // allow filter to receive any multicast packet
  ETH_MACFilterConfigTypeDef filterConfig = {0};
  filterConfig.PromiscuousMode = ENABLE;
  filterConfig.PassAllMulticast = ENABLE;

  if (HAL_ETH_SetMACFilterConfig(&heth, &filterConfig) != HAL_OK) {
    char str[128];
    sprintf(str, "Failed to set MAC filter\n");
    HAL_UART_Transmit(&huart3, (uint8_t *)str, strlen(str), 100);
  }

  // set IP address
  struct ip4_addr multicast_ip;    // multicast group ip
  struct ip4_addr device_ip;       // this device ip
  u16_t port = 7000;

  IP4_ADDR(&multicast_ip, 224, 0, 0, 3);
  IP4_ADDR(&device_ip, 10, 0, 64, 64);

  // Clean up any existing PCB
  if (g_udppcb != NULL) {
    udp_remove(g_udppcb);
    g_udppcb = NULL;
  }


  // Create new UDP PCB
  g_udppcb = udp_new();
  if (g_udppcb == NULL) {
    sprintf(str, "Failed to create UDP PCB\n");
    HAL_UART_Transmit(&huart3, (uint8_t *)str, strlen(str), 100);
    return;
  }

  // Receive from any address (i.e. "0.0.0.0")
  err = udp_bind(g_udppcb, IP_ADDR_ANY, port);
  if (err != ERR_OK) {
    sprintf(str, "Failed to bind UDP: %d\n", err);
    HAL_UART_Transmit(&huart3, (uint8_t *)str, strlen(str), 100);
    udp_remove(g_udppcb);
    return;
  }

  // Join multicast group
  err = igmp_joingroup(&device_ip, &multicast_ip);
  if (err != ERR_OK) {
    sprintf(str, "Failed to join multicast group: %d\n", err);
    HAL_UART_Transmit(&huart3, (uint8_t *)str, strlen(str), 100);
    udp_remove(g_udppcb);
    return;
  }

  // Set receive callback
  udp_recv(g_udppcb, UDP_multicast_receive_callback, NULL);

  sprintf(str, "Multicast initialized successfully\n");
  HAL_UART_Transmit(&huart3, (uint8_t *)str, strlen(str), 100);
}

And in application code,

void APP_init() {
  UDP_Multicast_init();
  ...
}

void APP_main() {
  ethernetif_input(&gnetif);
  sys_check_timeouts();
  ...
}