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I just ran a quick test with 2 foxy VM's. I think this may be a case of the linux networking subsystem dropping your packets because there is no defined route. Unless you have a very permissive mask on your ip settings for each computer, the multicast addresses are probably out of the mask range and will be dropped.

Try manually adding a multicast route to the interface on each machine:

sudo ip route add 224.0.0.0/4 dev eno1 but replace eno1 with your actual interface name on each machine.

After that, retry ros2 multicast receive on one machine and ros2 multicast send on the other.

A good debugging strategy would be to have wireshark open on the interface of each machine. When you start ros2 multicast receive you should see a IGMP Membership Join message on both sides.

Hope this helps.