Re: [PATCH net-next 0/2] Allow more than 255 IPv4 multicast interfaces

[David Miller]

From: Paul Davey <paul.davey@xxxxxxxxxxxxxxxxxxx>
Date: Wed,  2 Sep 2020 15:22:20 +1200

> Currently it is not possible to use more than 255 multicast interfaces
> for IPv4 due to the format of the igmpmsg header which only has 8 bits
> available for the VIF ID.  There is enough space for the full VIF ID in
> the Netlink cache notifications, however the value is currently taken
> directly from the igmpmsg header and has thus already been truncated.
> 
> Using the full VIF ID in the Netlink notifications allows use of more
> than 255 IPv4 multicast interfaces if the user space routing daemon
> uses the Netlink notifications instead of the igmpmsg cache reports.
> 
> However doing this reveals a deficiency in the Netlink cache report
> notifications, they lack any means for differentiating cache reports
> relating to different multicast routing tables.  This is easily
> resolved by adding the multicast route table ID to the cache reports.

But this means that mrouted has no way to see the full 16-bit value
via traditional igmpmsg UAPI interfaces.

Please instead make use of the unused3 member to store the high
order 16-bits of the vifi_t in the igmpmsg, and then code your
netlink changes around that.