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Re: IP Multicast

    Date:        Fri, 19 Sep 2008 14:06:29 -0400
    From:        "Neel Sheyal" <>

  | How does the OS know which interface to send the multicast traffic as in 

If IP networks and IP multicasting were set up as they should be (as it
was all originally designed) it would make no difference (aside from some
network efficiency) - send on either interface, listen on either interface,
and the multicast system (the routers) get the packets through the net
from wherever to wherever.

Unfortunately ...

Given that, what Eric said is about right - either the application, or
the system, has to explicitly say which interface to use, and you need
to get it right for things to work.   If the application is not bothering
(most multicast applications I've used have an option to allow the user to
say which to use) then the system default multicast interface is what
gets used.

I kind of half remember (from the ancient past) that there was a way
that you could set which interface that was, rather than it just being
some seemingly random interface selected by the kernel, but I do not
recall if NetBSD has a way to do that or not.   Perhaps someone else
here knows how it is done, if there is a way.


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