This is a really specific question, but maybe someone can help. I'm debugging someone else's code, and they call a UDPWriter and specify an IP address and port, and I'm trying to make sure this multicast traffic goes over a certain port. How can I determine which port the UDP defaults to and change it?

It's confusing to me because I'm not familiar with all the layers the OS sends traffic through before it goes through the interface.

Is there maybe some simpler way to tell the OS to send multicast traffic over both interfaces?

Any UDP connect function call should include both an IP and a port number. Changing the port number should be as easy as tweaking the connect call. A "raw" UDP packet won't have a default port - it'll be specified in the code somewhere.

Which interface(s) are used happens at a lower level, though - the routing tables will determine which interface, if any, can best route your UDP packets to the desired destination. The port should make no difference to that part of the equation. To cover two interfaces, you'd probably need to send two multicast packets to the two subnets. (If I'm understanding your question correctly).

If you check out the OSI networking layers - http://en.wikipedia.org/wiki/OSI_model for instance - the interfaces are determined at level 3 (IPv4/6) and the UDP port at level 4. A good read for a network geek, that page!!

Port numbers range from 1 (maybe 0?) to 65535. The ones in the range of 1-1023 can only be used by a process running with root-privileges. Other ports have special meaning (Google for "ephemeral ports").

As for Multicasting, addresses are restricted to the following ranges: 224.0.0.0 to 239.255.255.255

Some of these addresses are reserved for Multicasting routing information. Applications should not use these reserved addresses, which are: 224.0.0.0 to 224.0.0.255

Armed with this information, can you please explain a little further what you mean by "port"? Are you referring to the port numbers described above, or the network interface (e.g. eth0, wlan0, lo, etc)?

And lastly, before I can suggest anything further, I would need to know which language your code is written in. Consider providing a snip of code where the UDPWriter object binds to the port/address.