Can IPv6 multicasting work when one or more receivers are unable to bind to the program's well-known port?

Question

Consider a simple IPv6 multicast application:

• A "talker" program periodically sends out IPv6 UDP packets to a well-known multicast-group, sending them to a well-known port.
• Zero or more "listener" programs bind themselves to that well-known port and join the well-known multicast group, and they all receive the UDP packets.

That all works pretty well, except in the case where one or more of the listener-programs is unable to bind to the well-known UDP port because a socket in some other (unrelated) program has already bound to that UDP port (and didn't set the SO_REUSEADDR and/or SO_REUSEPORT options to allow it to be shared with anyone else). AFAICT in that case, the listener program is simply out of luck, there is nothing it can do to receive the multicast data, short of asking the user to terminate the interfering program in order to free up the port.

Or is there? For example, is there some technique or approach that would allow a multicast listener to receive all the incoming multicast packets for a given multicast-group, regardless of which UDP port they are being sent to?

Answer 1

If you want to receive all multicast traffic regardless of port, you'd need to use raw sockets to get the complete IP datagram. You could then directly inspect the IP header, check if it's using UDP, then check the UDP header before reading the application layer data. Note that methods of doing this are OS specific and typically require administrative privileges.

Regarding SO_REUSEADDR and SO_REUSEPORT, apps that do this will allow multiple programs to receive multicast packets sent to a given port. However, if you also need to receive unicast packets this method has issues. Incoming unicast packets may be set to both sockets, may always be sent to one specific socket, or sent to each in an alternating fashion. This also differs based on the OS.