On 2011-03-30, Grant Edwards <grant.b.edwa...@gmail.com> wrote:
> On 2011-03-30, Andrew Lunn <and...@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.
>
> True.
>
> 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:
>
> http://svn.freebsd.org/viewvc/base?view=revision&revision=79830
> http://svn.freebsd.org/viewvc/base?view=revision&revision=79836
>
> I'll work up a patch...
It turns out that doesn't fix the problem. That only handles the case
where you've explicitly asked the multicast packets to go out a
specific interface. Binding a socket to an interface and sending
multicast packets still doesn't work unless there's a route to the
"destination".
One would think that binding the socket to a particular interface
would cause sent packets to go out that interface, but I guess not...
--
Grant Edwards grant.b.edwards Yow! Here I am in 53
at B.C. and all I want is a
gmail.com dill pickle!!
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