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…ault one picked by the kernel)
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Turns out when (at least) in linux you join a multicast group with ifaddr=INADDR_ANY, it binds it to the "interface picked by the system" rather than all.
Looking at the linux kernel implementation, it's the interface which routes to the interface that points to "ip route get 239.255.255.250". However, this isn't ideal, and here's the setup where it broke: the default LAN isn't the default route, but Wireguard VPN is. So "ip route get 239.255.255.250" points to wireguard tunnel which is useless.
This makes it join multicast groups on all interfaces, starting with the default INADDR_ANY (IpV4Addr::UNSPECIFIED in rust).