[Twisted-Python] RFC: IPv6 multicast join/ticket 6597

[Jason Litzinger]

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