netdev - Re: IPv6 multicast with VRF

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Date:   Wed, 20 Apr 2022 12:59:45 -0600
From:   David Ahern <dsahern@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: IPv6 multicast with VRF

On 4/20/22 10:54 AM, Vladimir Oltean wrote:
> Hi,
> 
> I don't have experience with either IPv6 multicast or VRF, yet I need to
> send some IPv6 multicast packets from a device enslaved to a VRF, and I
> don't really know what's wrong with the routing table setup.
> 
> The system is configured in the following way:
> 
>  ip link set dev eth0 up
> 
>  # The kernel kindly creates a ff00::/8 route for IPv6 multicast traffic
>  # in the local table, and I think this is what makes multicast route
>  # lookups find the egress device.
>  ip -6 route show table local
> local ::1 dev lo proto kernel metric 0 pref medium
> local fe80::204:9fff:fe05:f4ab dev eth0 proto kernel metric 0 pref medium
> multicast ff00::/8 dev eth0 proto kernel metric 256 pref medium
> 
>  ip -6 route get ff02::1
> multicast ff02::1 dev eth0 table local proto kernel src fe80::204:9fff:fe05:f4ab metric 256 pref medium
> 
>  ip link add dev vrf0 type vrf table 3 && ip link set dev vrf0 up
> 
>  ip -4 route add table 3 unreachable default metric 4278198272
> 
>  ip -6 route add table 3 unreachable default metric 4278198272
> 
>  ip link set dev eth0 master vrf0
> 
> The problem seems to be that, although the "ff00::/8 dev eth0" route
> migrates from table 255 to table 3, route lookups after this point fail
> to find it and return -ENETUNREACH (ip6_null_entry).
> 
>  ip -6 route show table local
> local ::1 dev lo proto kernel metric 0 pref medium
> 
>  ip -6 route show table main
> ::1 dev lo proto kernel metric 256 pref medium
> 
>  ip -6 route show table 3
> local fe80::204:9fff:fe05:f4ab dev eth0 proto kernel metric 0 pref medium
> fe80::/64 dev eth0 proto kernel metric 256 pref medium
> multicast ff00::/8 dev eth0 proto kernel metric 256 pref medium
> unreachable default dev lo metric 4278198272 pref medium
> 
>  ip -6 route get ff02::1
> RTNETLINK answers: Network is unreachable
> 
>  ip -6 route get vrf vrf0 ff02::1
> RTNETLINK answers: Network is unreachable
> 
> I'm not exactly sure what is missing?

Did you adjust the FIB rules? See the documentation in the kernel repo.

And add a device scope to the `get`. e.g.,

    ip -6 route get ff02::1%eth0