[pim] Multicast routes to end user devices?

[Linus Lüssing <[e-mail]>]

Hi,

I feel a bit silly to ask, but... how is an end user's device supposed
to get/set its multicast routes from/to an interface?

1) Is an admin supposed to configure a multicast router to announce
multicast routes via ICMPv6 Router Advertisements?
2) If yes, should/could they announce ff00::/8? Would that be the
multicast equivalent to ::/0?
3) Should a ::/0 default route from ICMPv6 RA have any impact on a
user's device's multicast behaviour? (technically, this would
include both the unicast and multicast ranges?)
4) Should a user's device then use/interpret received multicast routes
only for multicast transmissions (similar to unicast)? Or also for joins
/ multicast reception? Is there an RFC that clarifies that?


I'm especially asking because on Linux there currently is the following
issue/inconvenience:

For every interface it will create a multicast route in the
"local" table:

```
$ ip -6 route show table local
multicast ff00::/8 dev br0 proto kernel metric 256 pref medium
multicast ff00::/8 dev veth0 proto kernel metric 256 pref medium
multicast ff00::/8 dev veth1 proto kernel metric 256 pref medium
multicast ff00::/8 dev wlp1s0 proto kernel metric 256 pref medium
...
```

This "local" table is separate from the default table, currently
ICMPv6 RA will only populate the default and not this "local"
table. However this local and not the default table is used both
for determining the interface to use for joining a routable
multicast address. And for multicast transmissions.

And the second annoyance: The Linux kernel will use the first
multicast route / interface it can find in the "local" table,
if all matching routes have the same prefix length.
Which is typically the first interface that was created - and
typically that is not of any use for multicast routing for me.

Regards, Linus