An update for those interested. I've been hoping for a solution to test IPv6 multicast using only the loopback interface. Unfortunately, the only working (on Linux) solution I've found is to manually add a route for the multicast prefix used in the tests. E.g. ip -6 route add ff01::/16 dev lo Without this, and assuming I specify the loopback interface index, a "Network Unreachable" error results. I'm fairly certain the culprit is a kernel route: unreachable default dev lo table unspec proto kernel metric 4294967295 error -101 pref medium local ::1 dev lo table local proto none metric 0 pref medium unreachable default dev lo table unspec proto kernel metric 4294967295 error -101 pref medium That's the default output of `ip -6 route show table all` on my machine (Ubuntu 16.04). I haven't found much information on why that route exists (let alone twice), so I'm going to have to turn to the kernel source. Regardless, route manipulation for tests is a non-starter, so unless there's a way to flag the tests as requiring a network interface, it'll be a while before I submit patches on the original feature. Thanks for all the suggestions/feedback and I welcome any more. -Jason