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>I'm not sure why this is working. Do you have a sendto on the other side? If not, where are the packets coming from? recvfrom requires a bound socket. Which source address are you binding to and how...

I didn't use bind() before sendto(). But why would using implicit bind() help?

And by the approach you suggested, I still can't guarantee that the interface that has a link-local address is the...

I now found if I first use a recvfrom() to receive a packet from the link-local address, the sin6_scope_id is filled in the "out" parameter "addrfrom".

But any other ways?

In Ipv6, when send to a link-local address, the scope-id must be set in sockaddr_in6 struct. It is the index of the interface that is used to communicate with the link-local address.

For example,...

I don't know why it should work like this. But switching the order of setsockopt and bind solves the problem.

I'm writing a simple piece of code to let the client to be able to receive multicast UDPs to link-local all-node "ff02::1". But I encountered the following problem. Any comments?

I'm under Windows...