Java: Determine receiving address when receiving with MulticastSocket

Question

Hi,

I am using MulticastSocket to receive UDP Multicast packets. How can I determine to which address such a packet was sent? With the methods provided, I am only able to determine the sender address.

Of course, I am the one who sets the To-Address when creating the listening socket, but can I really be sure about this? What about broadcast packets? What about packets that somehow end up here? I really want to distinguish if the packet was REALLY multicast.

Thank you!

Update: Currently it seems like unicast packets just sent to that port also end up in the multicast sockets receive() :( Binding to devices also gives me no better results

Answer 1

I'm a bit fuzzy on the details but a multicast packet will have been sent to the ip/port combo you subscribed to (and this info will be in the packet, somewhere), assuming you managed to have a clear path of intermediary routers that understand multicast. If you want to make sure the multicastsocket is receiving from the right network interface, there's a bunch of functions to bind it to a specific interface.

I don't think you have any way of knowing if the packet was "really" multicast, i.e. someone could always forge one, since there's no real security built in.