UDP 广播和多播消息到达但 recvfrom 没有收到任何东西

Question

I am new to stackoverflow and also pretty much a beginner at programming and hope to find a solution here.

My code is written in C++ and should run on a computer on module with linux operating system. The program should receive messages from other linux or windows systems and then depending on the content of the messages execute further subroutines and send back a response. The windows program is also written in C++. The linux system and the windows system are connected via a switch and the switch is connected to the home.network via a powerline adapter. The multicast function is enabled and supported in the switch settings, as well as in the linux system.

The linux code to test the functionality looks like this:

int createIPv4MulticastSocket(uint16_t socket_port, int allowReuseAddress)
{
    int Socket;
    int broadcast = 1;
    sockaddr_in localSock = {};

    // Bind to the proper port number with the IP address specified as INADDR_ANY
    memset(&localSock, 0, sizeof(localSock));
    localSock.sin_family = AF_INET;
    localSock.sin_port = htons(socket_port);
    localSock.sin_addr.s_addr = INADDR_ANY;

    // Creating the Socket
    printf("Creating a socket...");
    if ((Socket = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("Creating a socket failed:");
        printf("\n");
    }
    else
    {
        printf("Socket created. \n");
    }

    // set the reuse address options
    if (setsockopt(Socket, SOL_SOCKET, SO_REUSEADDR, (char*)&allowReuseAddress, sizeof(allowReuseAddress)) < 0)
    {
        perror("Error setting the reuse address option");
        printf("\n");
    }
    else
        printf("Setting the reuse address option...OK. \n");


    // bind the socket to the defined address
    printf("Try to bind the created Socket to my address. \n");
    if (bind(Socket, (struct sockaddr*)&localSock, sizeof(localSock)) == -1) {
        perror("Binding socket failed:");
        printf("\n");
    }
    else
    {
        printf("Bind was succesful. \n");
    }

    // sets the socket options so you can send Broadcast messages
    printf("Setting the socket options to allow Broadcast. \n");
    if (setsockopt(Socket, SOL_SOCKET, SO_BROADCAST, &broadcast,
        sizeof(broadcast)) == -1) {
        perror("Setting the socket options for allowing broadcast failed:");
        printf("\n");
    }
    else
    {
        printf("Setting the broadcast options...OK. \n");
    }

    return Socket;
}


void joinMulticastGroup(const char* IPMulticastGroup, const char* IPLocalInterfaceAddr, int SocketDescriptor)
{
    struct ip_mreq group;
    int LocalIP;
    int conv_ip;

    if (IPLocalInterfaceAddr[0] == '\0')
    {
        conv_ip = inet_pton(AF_INET, IPMulticastGroup, &group.imr_multiaddr.s_addr);
        if (conv_ip == 0) {
            printf("Destination IP-address doesn't contain a valid network address in the specified address family.\n");
        }
        else if (conv_ip == -1) {
            perror("No valid address family:");
            printf("\n");
        }

        group.imr_interface.s_addr = htonl(INADDR_ANY);
    }
    else
    {
        conv_ip = inet_pton(AF_INET, IPMulticastGroup, &group.imr_multiaddr.s_addr);
        if (conv_ip == 0) {
            printf("Destination IP-address doesn't contain a valid network address in the specified address family.\n");
        }
        else if (conv_ip == -1) {
            perror("No valid address family:");
            printf("\n");
        }

        conv_ip = inet_pton(AF_INET, IPLocalInterfaceAddr, &group.imr_interface.s_addr);
        if (conv_ip == 0) {
            printf("Destination IP-address doesn't contain a valid network address in the specified address family.\n");
        }
        else if (conv_ip == -1) {
            perror("No valid address family:");
            printf("\n");
        }
    }

    if (setsockopt(SocketDescriptor, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*)&group, sizeof(group)) < 0)
    {
        perror("Adding multicast group error");
        printf("\n");
    }
    else
        printf("Adding multicast group...OK. \n");
}


void sendUDP(int sending_socket, const char* destination_ipaddress,
    uint16_t destination_port, unsigned char sending_message[], int size_of_sending_message)
{
    struct sockaddr_in destination_address;
    long int numbytes_send;
    int conv_ip;

    // define destination address    
    printf("Convert the destination address to sockaddr_in. \n");
    destination_address.sin_family = AF_INET;                            // IPv4 address
    destination_address.sin_port = htons(destination_port);              // destination port
    conv_ip = inet_pton(AF_INET, destination_ipaddress, &destination_address.sin_addr.s_addr);
    if (conv_ip == 0) {
        printf("Destination IP-address doesn't contain a valid network address in the specified address family.\n");
    }
    else if (conv_ip == -1) {
        perror("No valid address family:");
        printf("\n");
    }
    memset(destination_address.sin_zero, '\0', sizeof(destination_address.sin_zero));  // fill up sin_zero with "0"
    printf("Correct destination address sockaddr. \n");

    printf("Sending a message...");
    if ((numbytes_send = sendto(sending_socket, sending_message, size_of_sending_message, 0,
        (struct sockaddr*)&destination_address, sizeof(destination_address))) == -1) {
        perror("sendto() failed:");
        printf("\n");
    }
    else
    {
        printf(" %i Bytes of data have been sent. \n", numbytes_send);
    }
}


void receiveUDP(int receiving_socket, struct sockaddr* received_from,
    unsigned char receiving_message[], int size_of_receiving_message)
{
    long int numbytes_received;
    unsigned int len_received_from = sizeof(*received_from);
    socklen_t len_recv_from = len_received_from;

    printf("Trying to receive a message...");
    if ((numbytes_received = recvfrom(receiving_socket, receiving_message, size_of_receiving_message, 0,
        received_from, &len_recv_from)) == -1) {
        perror("Receiving message failed:");
        printf("\n");
    }
    else
    {
        printf("%i Bytes an Daten erhalten\n", numbytes_received);
    }
}


int main()
{
    struct sockaddr received_from;
    int socketfd;
    unsigned char sending_message[1472], receiving_message[1472];
    const char* destination_ipaddress = "192.168.178.35";   //Laptop
    const char* multicast_ipaddress = "224.0.1.14";         //Multicast
    const char* broadcast_ipaddress = "192.168.178.255";    //Broadcast
    uint16_t destination_port = 3300;
    uint16_t port = 3300;
    uint16_t messageid = 0;
    double altitude = 0;
    double longitude = 0;
    double lattitude = 0;

    // Clean message buffer
    memset(sending_message, '\0', sizeof(sending_message));
    memset(receiving_message, '\0', sizeof(receiving_message));

    // Create a Socket
    socketfd = createIPv4MulticastSocket(port, 1);

    // join the multicast group
    joinMulticastGroup(multicast_ipaddress, "", socketfd);

    // Send UDP message.
    sendUDP(socketfd, broadcast_ipaddress, destination_port, sending_message, sizeof(sending_message));


    

    /// Receive messages and read Data
    while (1)
    {
        receiveUDP(socketfd, &received_from, receiving_message, sizeof(receiving_message));
        
        messageid = unpackunsignedint16(receiving_message, 1);
        altitude = unpackdouble(receiving_message, 3);
        lattitude = unpackdouble(receiving_message, 11);
        longitude = unpackdouble(receiving_message, 19);

        printf("actual altitude is: %lf \n", altitude);
        printf("actual lattitude is: %lf \n", lattitude);
        printf("actual longitude is: %lf \n", longitude);
    }

    close(socketfd);
}

If i now send unicast messages to the linux program, everything works as it should and the message is received and the sent values are inserted correct into the printf() functions. Sending unicast messages also works without problems. However, if I want to receive broadcast or multicast messages, the program stops in the recvfrom() line. If I check the connected port with tcpdump port 3300 for incoming messages, they arrive at the linux system. If I try to send broadcast or multicast messages, this does not work and no outgoing messages are displayed in tcpdump. If I try to receive broadcast or multicast messages and then go back to try again to receive unicast messages, this also does not work anymore. I do not get any error messages during the showed errorchecking.

Thanks in advance for your help.

Edit: Maybe I forgot to mention something because I thought it shouldn't be that big of a deal, but I've also read now that this could be a problem. The application is deployed to the system using a docker container

Answer 1

You're not correctly setting the incoming interface for multicast traffic, and you're not setting the outgoing interface at all.

When you call joinMulticastGroup , you pass an empty string for the second argument which is supposed to contain the IP address of the incoming multicast interface as a string. So if for example the machine's IP is 192.168.178.34, then you pass "192.168.178.34" for that argument.

If you don't set the outgoing multicast interface explicitly, the OS will choose whichever interface is the "default". You should use the IP_MULTICAST_IF socket option, passing the address of a struct in_addr specifying the IP address.