UDP multicast packets not seen on listening interface in BETA5

Robert Watson rwatson at FreeBSD.org
Thu Sep 29 14:09:44 PDT 2005

On Thu, 29 Sep 2005, Josh Carroll wrote:

> I suspect maybe I am not properly joining the multicast group, so it's 
> ignoring the packet. But like I said, this worked in BETA3 which is why 
> I'm stumped.

Normally when you join a multicast group, IGMP messages will be sent on 
the interface where it is joined.  Could you confirm that messages are 
sent in BETA3, and whether or not they are sent in BETA5?

When you subscribe to a multicast address, you do so on a particular 
interface with a particular subscription.  The multicast API allows you to 
either use a default interface based on not specifying a particular 
interface IP address, or allows you to specify an interface by IP address. 
Could you tell me whether or not your application attempts to specify what 
interface to use, and if there are other interfaces on the system that 
(potentially) a multicast application might select instead?

If you use netstat -ia, it will show you all interfaces and current 
multicast address subscriptions.  Could you show me the entries for your 
application running in BETA3, and any similar entries you can or cannot 
find in BETA5?


Robert N M Watson

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