C 应用程序未接收多播

Question

I am trying to create an application which uses multicast to speak to other instances of the application on the local network (application should work under windows and linux).

I am new to multicast, so I was trying to play with the following code I found on the internet:

#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

#define EXAMPLE_PORT 6000
#define EXAMPLE_GROUP "239.124.0.1"

// If argc == 0, recieve, else send
int main(int argc, char**argv)
{
    struct sockaddr_in addr;
    socklen_t addrlen = 0;
    int sock = 0, cnt = 0;
    struct ip_mreq mreq;
    char message[50];

    /* set up socket */
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0) {
        perror("socket");
        exit(1);
    }

    bzero((char *)&addr, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(EXAMPLE_PORT);
    addrlen = sizeof(addr);

    if (argc > 1) {
        /* send */
        addr.sin_addr.s_addr = inet_addr(EXAMPLE_GROUP);
        while (1) {
            time_t t = time(0);
            sprintf(message, "time is %-24.24s", ctime(&t));
            printf("sending: %s\n", message);
            cnt = sendto(sock, message, sizeof(message), 0,
                         (struct sockaddr *) &addr, addrlen);
            if (cnt < 0) {
                perror("sendto");
                exit(1);
            }
            sleep(5);
        }
    } else {
        /* receive */
        if (bind(sock, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
            perror("bind");
            exit(1);
        }
        mreq.imr_multiaddr.s_addr = inet_addr(EXAMPLE_GROUP);
        mreq.imr_interface.s_addr = htonl(INADDR_ANY);
        if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                       &mreq, sizeof(mreq)) < 0) {
            perror("setsockopt mreq");
            exit(1);
        }
        while (1) {
            cnt = recvfrom(sock, message, sizeof(message), 0,
                           (struct sockaddr *) &addr, &addrlen);
            if (cnt < 0) {
                perror("recvfrom");
                exit(1);
            } else if (cnt == 0) {
                break;
            }
            printf("%s: message = \"%s\"\n", inet_ntoa(addr.sin_addr), message);
        }
    }
}

The code successfully sends multicast packets that I can recieve on my android phone (by compiling the exact same code and using Termux to compile/execute).

So I know the code is working. But here is my issue: On linux and windows, I cannot recieve the packets. I can send them (my phone does recieve the packets form both), but I can't recieve.

What surprises me is that It is the exact same code on my linux and android (on windows there is just a few adaptations to use winsock2).

Furthermore, I can see that my linux indeed recieves the packets:

$ sudo tcpdump -i wlp58s0 host 239.124.0.1

dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on wlp58s0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
12:36:59.196616 IP 192.168.1.10.48727 > 239.124.0.1.6000: UDP, length 50
12:37:04.112507 IP 192.168.1.10.48727 > 239.124.0.1.6000: UDP, length 50
12:37:09.129963 IP 192.168.1.10.48727 > 239.124.0.1.6000: UDP, length 50
^C
4 packets captured
4 packets received by filter
0 packets dropped by kernel

192.168.1.10 is my phone IP address on that network.

I can also see that I successfully joined the multicast group:

$ netstat -gn | grep 239.124.0.1

wlp58s0         1      239.124.0.1

So my question here is why can't I recieve the multicast packets on linux/windows?

And what do I need to change in my code to recieve them?

Answer 1

For the receiver, this code joins the multicast group on interface 0.0.0.0 (specified as INADDR_ANY ). This means it joins on the network interface which is considered the default interface. If this is not the same interface that is receiving multicast traffic, you won't receive the packets.

You're receiving the packets on interface wlp58s0, so see what IP address that interface is using and use that IP when choosing the interface to bind to.

For example, if wlp58s0 has IP address 192.168.1.2, then change this:

mreq.imr_interface.s_addr = htonl(INADDR_ANY);

To this:

mreq.imr_interface.s_addr = inet_addr("192.168.1.2");