Java: Determine receiving address when receiving with MulticastSocket

2
votes

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

1
javaudpmulticastmulticastsocket
I'm confused. When setting up the MulticastSocket don't you bind it to a specific address? Isn't that the address anyone can use to send to everyone? You should already have the information even if it isn't available off the packet itself. – John Meagher
Yeah, but see my Update about the unicast packets :) – guruz
The ugly part of this is that it seems a java multicast socket recieves datagrams to any multicast address that matches the socket's port. So if one server is multicasting to 226.1.1.6:5003 and another is multicasting to 226.2.1.99:5003, your java app will recieve all the packets, and have no way to untangle them. This is regardless of what multicast group the socket joins. – Mutant Bob

1 Answers

2
votes

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.