[ECOS] Re: Why does sending multicast UDP require a gateway?

Grant Edwards grant.b.edwards@gmail.com
Wed Mar 30 21:04:00 GMT 2011

On 2011-03-30, Andrew Lunn <andrew@lunn.ch> wrote:
>> It's being sent via a socket that's bound to a specific interface, so
>> I wouldn't think that would be a problem.
> Ah, O.K. 
> Maybe try it anyway?
>> I can see that if you're sending from an unbound socket, you might
>> want a way to specify which interface to use for sending multicast
>> packets.  But without such a route, the only thing that makes sense to
>> me is "send it out all of them".
> And on a multi homed machine that might result in the group receiving
> two copies of the packet when both multicast routers pick it up and
> send to the RP.


According to RFC1112, you only send a multicast packet on one
interface. If the application doesn't specify an interface, a default
interface is used.  IOW, discarding the packet is not correct:

   Second, for hosts that may be attached to more than one network,
   the service interface should provide a way for the upper-layer
   protocol to identify which network interface is be used for the
   multicast transmission.  Only one interface is used for the initial
   transmission; multicast routers are responsible for forwarding to
   any other networks, if necessary.  If the upper-layer protocol
   chooses not to identify an outgoing interface, a default interface
   should be used, preferably under the control of system management.

It turns out this was fixed in the FreeBSD sources in 2001:

I'll work up a patch...
Grant Edwards               grant.b.edwards        Yow! I'm having an
                                  at               emotional outburst!!

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