tcpdump没有为我的C应用程序显示任何内容？

iperf -s -u -B 224.0.31.155

并运行

sudo tcpdump -ni any 'host 224.0.31.155'

tcpdump能够捕获一些东西：

tcpdump: verbose output suppressed,use -v or -vv for full protocol decode
listening on any,link-type LINUX_SLL (Linux cooked),capture size 65535 bytes
15:49:15.334484 IP [some ip].14386 > 224.0.31.155.14386: UDP,length 1364
15:49:15.334728 IP [some ip].14386 > 224.0.31.155.14386: UDP,length 1374
15:49:15.375026 IP [some ip].14386 > 224.0.31.155.14386: UDP,length 1058
15:49:15.375184 IP [some ip].14386 > 224.0.31.155.14386: UDP,length 832

但是,如果我杀死上面的iperf进程,然后启动我的C应用程序也加入同一个组并绑定同一个端口,tcpdump将不再看到流量.

这是片段：

struct sockaddr_in mc_addr; /* socket address structure */
struct ip_mreq mc_req; /* multicast request structure */
unsigned int from_len = sizeof(mc_addr); /* source addr length */

/* construct a multicast address structure */
memset(&mc_addr,from_len);
mc_addr.sin_family = AF_INET;
inet_aton(mcastGroup.c_str(),&mc_addr.sin_addr);
mc_addr.sin_port = htons(port);

/* bind to multicast address to socket */
if (bind(s,(struct sockaddr *) &mc_addr,sizeof(mc_addr)) < 0) {
    std::cerr << "failed to bind to the port " << port << "|error="
            << strerror(errno) << std::endl;
    throw;
}
/* construct an IGMP join request structure */
mc_req.imr_multiaddr.s_addr = inet_addr(mcastGroup.c_str());
mc_req.imr_interface.s_addr = htonl(INADDR_ANY);

/* send an ADD MEMBERSHIP message via setsockopt */
if ((setsockopt(s,IPPROTO_IP,IP_ADD_MEMBERSHIP,(void*) &mc_req,sizeof(mc_req))) < 0) {
    std::cerr << "failed to set socket option to request for membership"
            << std::endl;
    throw;
}

tcpdump详细信息：

$tcpdump --version
tcpdump version 4.1-PRE-CVS_2012_03_26
libpcap version 1.4.0

我只是在我的一个生产服务器上检查它,它显示了相同的行为,但我看到我的C应用程序正在正确处理数据.

知道发生了什么事吗？