deischen at freebsd.org
Mon Jun 18 17:48:25 UTC 2007
On Mon, 18 Jun 2007, Bruce M. Simpson wrote:
> Daniel Eischen wrote:
>> On Mon, 18 Jun 2007, Bruce M. Simpson wrote:
>>> Other folks reported issues with the new code. As you're probably aware,
>>> ipv4 multicast group memberships must be bound to an interface. When they
>>> are not, the interface selected as the source for the IGMP join uses a
>>> routing table lookup on the group.
>>> The newer code changed to perform this lookup by interface index as well
>>> as by address, as interfaces used for ipv4 multicast traffic are generally
>>> assumed to have a protocol-level address.
>> What happens when you join a group without an interface (INADDR_ANY)?
>> Assume there is no route for the multicast group. Does the new code
>> select the interface that the default route is on?
> The new code is written in terms of the RFC 3678 API. The old IPv4 ASM API is
> a shell around it.
> If no interface address is provided, either via an interface index or
> interface protocol address, it will perform a route lookup on the multicast
> group address to determine which interface to use when the group is being
> joined. Obviously a default route will satisfy this lookup; the BSD route
> lookup matches most specific match first.
> I believe that the problems which folks have been seeing is that some old
> behaviour hasn't been captured in the new code.
> This behaviour is that if the route lookup fails, the code would select the
> first interface in the system with IFF_MULTICAST set on it (usually the
> loopback address). This used to be contained in the ip_multicast_if()
> function which was phased out.
> Joining a multicast group on INADDR_ANY is non-specific. It's a bit like
> sending a datagram to 255.255.255.255 -- the group address alone is not a
> sufficient key without additional information from the routing table.
> It seems reasonable that the code should use the interface of the default
> route if no interface address is provided. However this doesn't cover the
> case where no default route exists during system bringup.
I think in that case, the first non-loopback interface with IFF_MULTICAST
should be chosen. I think the loopback interface should be chosen in the
absence of any other interface with IFF_MULTICAST set. Our code does rely
on this as well.
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