virtphy should have a localhost-only mode
I am using a virual-um setup on my laptop for osmocom development and testing without additional hardware.
In this situation I do not want virtphy's traffic leaking to the local network,
however the traffic currently leaks.
As a workaround, virtphy could be run in a separate network namespace or a virtual machine.
But this issue seeks a solution which works without resorting to a separate network stack.
Blocking the traffic with iptables leads to errors on virtphy's socket, and adding a route to 224/4 via lo does
not have the desired effect as multicast traffic is still routed towards the default route.
Harald suggested trying to set the TTL of multicast packets to 0, and there's also the IP_MULTICAST_IF socket option
which could be used to bind virtphy to a specific interface.