Re: HEADSUP: arp-v2 has been committed

Bruce Simpson wrote:
> 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".

quagga does specify the ifindex passed in a struct ip_mreqn in the
imr_ifindex member to setsockopt, which reading the documentation
should be sufficient, yet it is not.  I have checked that it does
set the correct ifindex.   Setting the IP address in the imr_address
member of the same struct correctly chooses the interface.

> 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.

Is this the BSD struct ip_mreq hack?  This particular code isn't
using that.

Ian

--
Ian Freislich