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我有一个由 2 台主机（OS X 和 linux）组成的小型网络，它们之间有一个开关。当我运行以下程序对时，接收器 (linux) 看不到任何多播数据包。但是，当我跑步时

tcpdump -i eth0 -s 512 udp

在 linux 机器上，一切都开始工作了。有人能告诉我为什么运行 tcpdump 可以让它工作吗？

请注意，两台机器都启用了 IPv6（如果重要的话）。代码中的主机地址是本地接口的 IPv4 地址。

在 OS X 机器上，我运行：

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

#define BUFLEN  256

int                     s;
int                     count;
struct in_addr          local_address;
struct sockaddr_in      multicast_address;
char                    buffer[BUFLEN];

int
main (int argc, char *argv[]) {
        int             port = 12345;
        int             retval = 0;
        char            *host_ip = "192.168.1.31";
        char            *multicast_ip = "224.0.0.251";

        count = (int)time(NULL);


        s = socket(AF_INET, SOCK_DGRAM, 0);
        printf("s=%d\n", s);


        local_address.s_addr = inet_addr(host_ip);
        retval = setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, (char *)&local_address, sizeof(local_address));
        printf("setsockopt=%d\n", retval);


        memset((char *)&multicast_address, 0, sizeof(multicast_address));
        multicast_address.sin_family = AF_INET;
        multicast_address.sin_addr.s_addr = inet_addr(multicast_ip);
        multicast_address.sin_port = htons(port);


        while (1) {
                sprintf(buffer, "Message number %d", count);
                count++;

                retval = sendto(s, buffer, BUFLEN, 0, (struct sockaddr*)&multicast_address, sizeof(multicast_address));
                printf("sendto=%d\n", retval);
                sleep(3);
        }

        return 0;
}

在 linux 机器上，我运行：

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

#define BUFLEN          256

int                     s;
struct ip_mreq          multicast_group;
struct sockaddr_in      localSock;
char                    buffer[BUFLEN];

int
main(int argc, char *argv[]) {
        int             reuse = 1;
        int             retval = 0;
        int             port = 12345;
        char            *host_ip = "192.168.1.131";
        char            *multicast_ip = "224.0.0.251";

        s = socket(AF_INET, SOCK_DGRAM, 0);
        printf("s = %d\n", s);


        setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (char *)&reuse, sizeof(reuse));
        printf("REUSEADDR = %d\n", retval);


        memset((char *) &localSock, 0, sizeof(localSock));
        localSock.sin_family = AF_INET;
        localSock.sin_port = htons(port);
        localSock.sin_addr.s_addr = INADDR_ANY;
        retval = bind(s, (struct sockaddr*)&localSock, sizeof(localSock));
        printf("bind = %d\n", retval);


        multicast_group.imr_multiaddr.s_addr = inet_addr(multicast_ip);
        multicast_group.imr_interface.s_addr = inet_addr(host_ip);
        retval = setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *)&multicast_group, sizeof(multicast_group));
        printf("ADD_MEMBERSHIP = %d\n", retval);


        while (1) {
                retval = read(s, buffer, BUFLEN);
                printf("%d => [%s]\n", retval, buffer);
        }

        return 0;
}

谢谢。

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