From: Paul Davey <paul.da...@alliedtelesis.co.nz> 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.
