netdev - Re: Multicast routing: wrong output interface selected unless VRF default route is added

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Message-ID: <891caeb6-9c1f-d79d-caa6-23aba7e4d0c7@gmail.com>
Date:   Tue, 23 Jun 2020 19:14:28 -0700
From:   David Ahern <dsahern@...il.com>
To:     Ido Schimmel <idosch@...sch.org>,
        Qiwei Wen <wenqiweiabcd@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: Multicast routing: wrong output interface selected unless VRF
 default route is added

On 6/23/20 11:38 AM, Ido Schimmel wrote:
>>
>> My questions are:
>> 1. is fib_lookup supposed to work with multicast daddr? If so, has
>> multicast routing been working for the wrong reason?

yes; it is a quirk with Linux routing. see the code after fib_lookup in
ip_route_output_key_hash_rcu. perf can show you the call chain that gets
to that lookup if you want the details.

>> 2. Why does the addition of a unicast default route affect multicast
>> routing behaviour?
> 
> I believe this was discussed in the past. See:
> https://lore.kernel.org/netdev/20200115191920.GA1490933@splinter/#t
>