UDP Send broadcast, receive responses

[JordiXTR]

Hi all,

I have different servers on the local network, waiting for incoming UDP packet at some port. On incoming packets, the server will reply to that socket.

As a client that sends a broadcast on the net I have an ESP82266 that after connecting to the local network via WiFi runs that code:

  IPAddress local_ip = WiFi.localIP();
  IPAddress broadcast_ip = WiFi.broadcastIP();
  bool master_found = false;

  WiFiUDP udp;
  udp.setTimeout(5000);
  udp.beginPacket(broadcast_ip, 2038);
  udp.write("DATA SEND TO BROADCAST");
  udp.endPacket();
  while(true) {
    int packet_size = udp.parsePacket();
    if (packet_size) 
    {
      Serial.printf("Received %d bytes from %s\n", packet_size, udp.remoteIP().toString().c_str());
      int len = udp.read(incomingPacket, 255);
      if (len > 0) {
        incomingPacket[len] = 0;
      }
      Serial.printf("UDP packet contents: %s\n", incomingPacket);

      master_ip = udp.remoteIP();
      master_found = true;

    } else {
      Serial.printf("Timeout\n");
      delay(2000);
    }
  }

Servers (UNIX C program) receive the packet correctly using the following code:

  int sock;
  int yes = 1;
  struct sockaddr_in client_addr;
  struct sockaddr_in server_addr;
  socklen_t addr_len;
  int count;
  int ret;
  fd_set readfd;
  char buffer[1024];

  sock = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock < 0) {
    printf("sock error\n");
    fflush(stdout);
    return -1;
  }

  addr_len = sizeof(struct sockaddr_in);

  memset((void*)&server_addr, 0, addr_len);
  server_addr.sin_family = AF_INET;
  server_addr.sin_addr.s_addr = htons(INADDR_ANY);
  server_addr.sin_port = htons(PORT);

  ret = bind(sock, (struct sockaddr*)&server_addr, addr_len);
  if (ret < 0) {
    printf("bind error\n");
    fflush(stdout);
    return -1;
  }
  printf("Server waiting...\n");
  fflush(stdout);
  while (server_running) {
    FD_ZERO(&readfd);
    FD_SET(sock, &readfd);

    ret = select(sock+1, &readfd, NULL, NULL, 0);
    if (ret > 0) {
      if (FD_ISSET(sock, &readfd)) {
        recvfrom(sock, buffer, 1024, 0, (struct sockaddr*)&client_addr, &addr_len);
        if (strstr(buffer, TEST_PLUGIN)) {
          char type[15] = {0};
          uint32_t serial = 0;
          sscanf(buffer, TEST_PLUGIN" %s %u", type, &serial);
          printf("[SERVER] Plugin Received:\n\t IP: %s Type: %s Serial: %u\n", inet_ntoa(client_addr.sin_addr), type, serial);
          fflush(stdout);

          memcpy(buffer, MASTER_UUID, strlen(MASTER_UUID)+1);
          sendto(sock, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, addr_len);
        }
      }
    }
  }

And sends back the response to that sockaddr. But I'm not able to receive the answer on the ESP8266. The parsePacket function returns inmediatly with no data.

The server code works. I made a the UNIX C client to test that receive the responses. Here I share:

int sock;
  int yes = 1;
  struct sockaddr_in broadcast_addr{};
  struct sockaddr_in server_addr{};
  socklen_t addr_len;
  int count;
  int ret;
  fd_set readfd;
  char buffer[1024];
  int i;

  sock = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock < 0) {
    printf("sock error");
    fflush(stdout);
    return -1;
  }
  ret = setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (char*)&yes, sizeof(yes));
  if (ret == -1) {
    printf("setsockopt error");
    fflush(stdout);
    return 0;
  }

  addr_len = sizeof(struct sockaddr_in);

  memset((void*)&broadcast_addr, 0, addr_len);
  broadcast_addr.sin_family = AF_INET;
  broadcast_addr.sin_addr.s_addr = htonl(INADDR_BROADCAST);
  broadcast_addr.sin_port = htons(PORT);

  ret = sendto(sock, ADDON_UUID, strlen(ADDON_UUID), 0, (struct sockaddr*) &broadcast_addr, addr_len);

  FD_ZERO(&readfd);
  FD_SET(sock, &readfd);

  ret = select(sock + 1, &readfd, NULL, NULL, NULL);
  if (ret > 0) {
    if (FD_ISSET(sock, &readfd)) {
      count = recvfrom(sock, buffer, 1024, 0, (struct sockaddr*)&server_addr, &addr_len);
      if (strstr(buffer, MASTER_ACK)) {
        uint32_t serial = 0;
        sscanf(buffer,MASTER_ACK" %u",&serial);
        printf("[CLIENT] Found master with serial %u on IP on %s\n", serial, inet_ntoa(server_addr.sin_addr));
        fflush(stdout);
      }
    }
  }

Can you give some help please?

Kind regards,
Jordi