IPv6 multicast not working in LAN - c++

Question

I tried to send an IPv6 multicast packet through my network. The sending seems to work, since it arrives on the destination PC - at least it appears in the logged network traffic in WireShark. But it does not arrive in my server program. When I send a packet from the same PC that should receive it, it does work though.
This is the code for sending (removed error checking for better readability):
UDPBroadcastSocket = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP);
BOOL Yes = 1;
setsockopt(UDPBroadcastSocket, IPPROTO_IPV6, IPV6_V6ONLY, (char*)&Yes, sizeof(BOOL));
int32_t hops = 50;
setsockopt(UDPBroadcastSocket, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, (char*)&hops, sizeof(hops));
uint32_t IF = 0;
setsockopt(UDPBroadcastSocket, IPPROTO_IPV6, IPV6_MULTICAST_IF, (char*)&IF, sizeof(IF));
struct sockaddr_in6 sock_in;
struct addrinfo *result = NULL;
struct addrinfo hints;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET6;
hints.ai_socktype = SOCK_DGRAM;
hints.ai_protocol = IPPROTO_UDP;
hints.ai_flags = AI_NUMERICHOST;
getaddrinfo("FF18::1243", "12346", &hints, &result);
unsigned char buffer[MAXBUF];
int PacketSize = 8;
int sinlen = int(result->ai_addrlen);
memcpy(&sock_in, result->ai_addr, result->ai_addrlen);
freeaddrinfo(result);
sendto(UDPBroadcastSocket, (char*)buffer, PacketSize, 0, (sockaddr *)&sock_in, sinlen);
And this is the code for receiving the packet (removed error checking for better readability):
std::vector<uint32_t> GetNetworkInterfaceIndices(){
std::vector<uint32_t> Result;
/* Declare and initialize variables */
DWORD dwSize = 0;
DWORD dwRetVal = 0;
unsigned int i = 0;
// Set the flags to pass to GetAdaptersAddresses
ULONG flags = GAA_FLAG_INCLUDE_PREFIX;
// default to unspecified address family (both)
ULONG family = AF_UNSPEC;
LPVOID lpMsgBuf = NULL;
PIP_ADAPTER_ADDRESSES pAddresses = NULL;
ULONG outBufLen = 0;
ULONG Iterations = 0;
PIP_ADAPTER_ADDRESSES pCurrAddresses = NULL;
PIP_ADAPTER_UNICAST_ADDRESS pUnicast = NULL;
PIP_ADAPTER_ANYCAST_ADDRESS pAnycast = NULL;
PIP_ADAPTER_MULTICAST_ADDRESS pMulticast = NULL;
IP_ADAPTER_DNS_SERVER_ADDRESS *pDnServer = NULL;
IP_ADAPTER_PREFIX *pPrefix = NULL;
family = AF_INET6;
// Allocate a 15 KB buffer to start with.
outBufLen = WORKING_BUFFER_SIZE;
do {
pAddresses = (IP_ADAPTER_ADDRESSES *)MALLOC(outBufLen);
if (pAddresses == NULL) {
return{ 0 };
}
dwRetVal =
GetAdaptersAddresses(family, flags, NULL, pAddresses, &outBufLen);
if (dwRetVal == ERROR_BUFFER_OVERFLOW) {
FREE(pAddresses);
pAddresses = NULL;
}
else {
break;
}
Iterations++;
} while ((dwRetVal == ERROR_BUFFER_OVERFLOW) && (Iterations < MAX_TRIES));
if (dwRetVal == NO_ERROR) {
// If successful, output some information from the data we received
pCurrAddresses = pAddresses;
while (pCurrAddresses) {
Result.emplace_back(pCurrAddresses->IfIndex);
pCurrAddresses = pCurrAddresses->Next;
}
}
else {
return{ 0 };
}
if (pAddresses) {
FREE(pAddresses);
}
return Result;
}
UDPSocket = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
sockaddr_in6 UDP_Sock_in;
memset(&UDP_Sock_in, 0, sizeof(sockaddr_in6));
UDP_Sock_in.sin6_addr = in6addr_any;
UDP_Sock_in.sin6_port = htons(Settings::GetPort()+1);
UDP_Sock_in.sin6_family = PF_INET6;
setsockopt(UDPSocket, IPPROTO_IPV6, IPV6_V6ONLY, (char*)&No, sizeof(BOOL));
bind(UDPSocket, (sockaddr*)&UDP_Sock_in, sizeof(UDP_Sock_in));
ipv6_mreq BroadcastGroup;
memset(&BroadcastGroup, 0, sizeof(ipv6_mreq));
const auto IfIndices = GetNetworkInterfaceIndices();
BroadcastGroup.ipv6mr_multiaddr.u.Byte[0] = 0xFF;
BroadcastGroup.ipv6mr_multiaddr.u.Byte[1] = 0x18;
BroadcastGroup.ipv6mr_multiaddr.u.Byte[14] = 0x12;
BroadcastGroup.ipv6mr_multiaddr.u.Byte[15] = 0x43;
for (const auto& Index : IfIndices) {
BroadcastGroup.ipv6mr_interface = Index;
setsockopt(UDPSocket, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, (char*)&BroadcastGroup, sizeof(ipv6_mreq));
}
socklen_t fromLength = sizeof(sockaddr_in6);
pollfd PollFd;
PollFd.events = POLLIN;
PollFd.fd = UDPSocket;
PollFd.revents = -1;
WSAPoll(&PollFd, 1, -1);
recvfrom(UDPSocket, (char*)buffer, MAXBUF, 0, (sockaddr*)&from, &fromLength);
I basically tried specifying every single network interface index and the packet still does not arrive in the server. I have no idea what could be wrong. And why does it work when sender and receiver are on the same PC? I don't understand this. Does anyone have an idea? It's not the firewall, I turned it off and nothing changed. When I specify the IP address of the receiving PC directly, it does work, too.

Answer 1

I found a workaround: Simply switch to IPv4, because IPv6 multicast currently doesn't seem to work on Windows (at least in my use case, see comments). My code had worked, but stopped working some time ago and I didn't notice until I tested it again.
I tried a bunch of different addresses and apparently none of them worked on 3 different devices. Then I switched to IPv4 and it simply worked - I changed only the IPv6 related stuff to their IPv4 equivalent and removed the hops and interface options on the sender.
My receiver even appeared in the multicast groups list (which you can see using the command "netsh interface ipv6 show joins"), but still didn't receive the packets sent to its address, so I concluded that it likely is a bug, since every example code I found did not work for me and I could not find any other option for setsockopt that I could have missed. Feel free to comment if you have any idea what might have caused this problem or how to fix it without switching back to the old IPv4 standard.