Linux kernel cannot receive multicast - linux

Question

I built a Linux kernel with CONFIG_IP_MULTICAST=y,however no UDP multicast package received in this kernel while UDP unicast works well.
ethtool -S eth0 | grep multicast
txmulticastframes_g: 0
txmulticastframes_gb: 0
rxmulticastframes_g: 0
Any hints how can I solve this problem?
Thx. Forrest G
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Additional:
tcpdump can get the packet
root#JHI # ./tcpdump port 3702
device eth0 entered promiscuous mode
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
-7:-48:-19.4151 IP 192.168.42.212.3702 > 239.255.255.250.3702: UDP, length 787
-7:-48:-19.4661 IP 192.168.42.212.3702 > 239.255.255.250.3702: UDP, length 803
^C
2 packets captured
2 padevice eth0 left promiscuous mode
ckets received by filter
0 packets dropped by kernel
I write this WS-devicediscovery function with gSOAP. It works on X86 machine. When running on ARM device, it can send out igmp packet but not receive anything.
void wsdd()
{
struct soap *soap_udp;
struct ip_mreq mreq;
soap_udp=soap_new();
soap_init1(soap_udp, SOAP_IO_UDP|SOAP_IO_FLUSH);
if (!soap_valid_socket(soap_bind(soap_udp, NULL, 3702, 100)))
{
soap_print_fault(soap_udp, stderr);
}
mreq.imr_multiaddr.s_addr = inet_addr("239.255.255.250");
mreq.imr_interface.s_addr = htonl(INADDR_ANY);
if(setsockopt(soap_ud->master,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq))==-1) {
perror("membership error\n");
}
int loop = 1;
int sock_opt = 1;
if ((setsockopt(soap_udp->master, SOL_SOCKET, SO_REUSEADDR, (void *) &sock_opt,
sizeof (sock_opt))) == -1) {
printf("setsockopt\n");
}
if ((setsockopt(soap_udp->master, IPPROTO_IP, IP_MULTICAST_LOOP,
&loop, sizeof (loop))) == -1) {
printf("setsockopt\n");
}
while(1){
soap_accept(soap_udp);
soap_serve(soap_udp);
soap_end(soap_udp);
}
}
Then I have tried these things but still not work
route add -net 224.0.0.0 netmask 240.0.0.0 eth0
echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter

Answer 1

Problem solved by changing..
ifconfig eth0 promisc
Can anyone explain the principle?