Zenoh doesn't necessarily join the right multicast interface.

[alexis-boisserand-acuity]

Describe the bug

Zenoh doesn't necessarily join the IPv6 multicast group on the correct interface, when more than one IPv6 network interfaces are present.

Conditions required to reproduce the issue:

• Host with more than one IPv6 network interfaces
• Zenoh configured for multicast transport, and to listen on a particular interface. For example:

  mode: "router",
  listen: {
    endpoints: [
      "udp/[ff02::01]:7447#iface=r1-eth2",
    ]
  }

I attached a patch that fixed the issue for me.

multicast_ipv6_wrong_interface.txt

To reproduce

1. Configure zenoh for IPv6 multicast transport as specified above
2. Zenoh may join the multicast group on the wrong interface. If this is not the case, just specify a different interface and it should fail.

System info

• Platform: Ubuntu 22.04 amd64
• zenoh 1.1.0