問(wèn)題背景：

平臺(tái)：Android
設(shè)備A：組播發(fā)送者，發(fā)送組播數(shù)據(jù)
設(shè)備B：組播接受者，接收組播數(shù)據(jù)
設(shè)備A與設(shè)備B通過(guò)有線(xiàn)連接，連接在設(shè)備B的eth0網(wǎng)卡上

問(wèn)題：
設(shè)備B在沒(méi)有通訊模塊網(wǎng)卡的情況下收得到設(shè)備A的組播數(shù)據(jù)，一旦有了通訊模塊的撥號(hào)存在，則收不到A的組播數(shù)據(jù)了。但設(shè)備B上通過(guò)抓包，是能確認(rèn)有抓到組播數(shù)據(jù)進(jìn)來(lái)，也就是說(shuō)eth0的網(wǎng)卡上，組播數(shù)據(jù)是進(jìn)來(lái)了

組播注冊(cè)偽代碼：

int listenSock = socket(AF_INET, SOCK_DGRAM, 0);
    if (-1 == listenSock)
    {
    printf("Create socket failed! listenSock=%d\n", listenSock);
    return -1;
    }

xint32_t opt = 1;
setsockopt(listenSock, SOL_SOCKET, SO_REUSEADDR, (const void *)&opt, sizeof(opt));

struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));

addr.sin_family = AF_INET;
addr.sin_port = htons(static_cast<uint16_t>(端口));
addr.sin_addr.s_addr = INADDR_ANY;
    if (bind(listenSock, reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr)) == -1)
    {
     printf("Bind sock failed!!!\n");
     close(listenSock);
     return -1;
    }

struct ip_mreq group;
group.imr_multiaddr.s_addr = inet_addr("組播ip");
group.imr_interface.s_addr = INADDR_ANY;
    if (setsockopt(listenSock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *)&group, sizeof(group)) < 0)
    {
     close(listenSock);
     printf("Add membership fail! errno:%d\n", errno);
     return -2;
    }

    struct ifreq nif = {0};
    strcpy(nif.ifr_name, "eth0");
    if (setsockopt(listenSock, SOL_SOCKET, SO_BINDTODEVICE, (char *)&nif, sizeof(nif)) < 0)
    {
     close(listenSock);
     printf("Bind interface=eth0 fail!, errno:%d\n", errno);
     return -1;
    }

通過(guò)以上代碼，在存在通訊模塊的時(shí)候是收不到的。也懷疑過(guò)是否是因?yàn)楸惶砑恿薴ilter導(dǎo)致，后面也排除了，那么還有種可能，就是綁定網(wǎng)卡并沒(méi)有生效，我們雖然socket綁定到了eth0，但組播的加入也涉及網(wǎng)卡，我們這個(gè)方式因順序被加入到了其他網(wǎng)卡里，直接看修改，那么在添加組播組的時(shí)候也能綁定網(wǎng)卡，修改后的偽代碼如下：

int listenSock = socket(AF_INET, SOCK_DGRAM, 0);
    if (-1 == listenSock)
    {
    printf("Create socket failed!!! listenSock=%d\n", m_listenSock);
    return -1;
    }

 xint32_t opt = 1;
    setsockopt(listenSock, SOL_SOCKET, SO_REUSEADDR, (const void *)&opt, sizeof(opt));

struct sockaddr_in localAddr;
memset(&localAddr, 0, sizeof(localAddr));
localAddr.sin_family = AF_INET;
inet_pton(AF_INET, "組播ip", &localAddr.sin_addr);
localAddr.sin_port = htons((uint16_t)端口);
localAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    if (::bind(listenSock, reinterpret_cast<struct sockaddr*>(&localAddr), sizeof(localAddr)) == -1)
    {
     printf("Bind sock failed!!!\n");
     close(listenSock);
     return -1;
    }

    struct ifreq nif;
    memset(&nif, 0, sizeof(ifreq));
    strcpy(nif.ifr_name, "eth0");
    if (::setsockopt(listenSock, SOL_SOCKET, SO_BINDTODEVICE, (char *)&nif, sizeof(ifreq)) < 0)
    {
     close(listenSock);
     printf("Bind interface=eth0 fail!, errno:%d\n", errno);
     return -1;
    }

 //關(guān)鍵修改在這
    struct ip_mreqn group;
 group.imr_multiaddr.s_addr = inet_addr("組播ip");
 group.imr_address.s_addr = htonl(INADDR_ANY);
 group.imr_ifindex = if_nametoindex("eth0");//在添加組的時(shí)候, 指定網(wǎng)卡
    if (::setsockopt(listenSock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *)&group, sizeof(group)) < 0)
    {
     close(listenSock);
     printf("Add membership interface=eth0 fail!, errno:%d\n", errno);
     return -2;
    }

也看到了，結(jié)構(gòu)體用得也不一樣了，其實(shí) ip_mreq 與 ip_mreqn時(shí)差不多的含義

//來(lái)源：#include <in.h>
/* Internet address. */
struct in_addr {
    __be32  s_addr;
};

/* Request struct for multicast socket ops */
struct ip_mreq  {
    struct in_addr imr_multiaddr;   /* IP multicast address of group */ //組播組組地址
    struct in_addr imr_interface;   /* local IP address of interface */ // 組播組的的網(wǎng)卡ip, 對(duì)應(yīng)的網(wǎng)卡能夠接收對(duì)應(yīng)多播組的數(shù)據(jù)包
};
  
struct ip_mreqn {
    struct in_addr  imr_multiaddr;      /* IP multicast address of group */
    struct in_addr  imr_address;        /* local IP address of interface */ //組播組的的網(wǎng)卡ip, 對(duì)應(yīng)的網(wǎng)卡能夠接收對(duì)應(yīng)多播組的數(shù)據(jù)包
    int     imr_ifindex;        /* Interface index */ //加入組播組的網(wǎng)卡index,優(yōu)先級(jí)高于以上地址
};

可以看到以下，ip_mreqn明確多了一個(gè)網(wǎng)卡指定。我在使用ip_mreq的時(shí)候也嘗試過(guò)將imr_interface指定為我eth0的ip來(lái)綁定，仍然無(wú)效。如果不是要收多個(gè)網(wǎng)卡的，可以使用以上方式修改，記錄借鑒