HEADSUP: arp-v2 has been committed

Bruce Simpson bms at incunabulum.net
Wed Dec 24 14:28:34 UTC 2008

Ian FREISLICH wrote:
> ...
> I can't quite remember exactly why imr_ifindex doesn't work, but
> on my hosts which have several hundred interfaces and my OSPF
> sessions are never on the interface that has the default route,
> until I explicitly set the imr_address, the kernel always chooses
> the interface which has the default route.

Do you have applications which do not explicitly specify the interface 
address to use for multicast group joins?

If they do not,  that's a bug in the application -- IPv4 and IPv6 
multicast *requires* that a link be specified somehow, either using the 
new APIs which take an ifindex, or an IPv4 "primary address".

Unfortunately there has been historical breakage in the multicast APIs. 
There are some apps which run before all interfaces have been ifconfig'd 
up in the system, and they need to create multicast sockets.

The kernel behaviour you describe is historical and I had to reintroduce 
it to avoid breaking such applications. It is a kludge which we probably 
can't retire until their developers fix their multicast apps to be aware 
of multiple interfaces on the system.

This ground is well covered in the literature and the RFCs.


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