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

Andrew Lunn andrew@lunn.ch
Wed Mar 30 14:27:00 GMT 2011

On Tue, Mar 29, 2011 at 10:33:09PM +0000, Grant Edwards wrote:
> Somebody I work with has spent the last three days trying to send UDP
> multicast packets using eCos and the FreeBSD network stack.
> After quite a bit of trial-and-error we've discovered that you can't
> send UDP multicast packets without configuring a valid gateway
> address.
> That seems broken to me.
> What are you supposed to do if you're on a strictly local network that
> doesn't _have_ a gateway?  [That is the case for many of our products.]

Hi Grant

In the Linux world you have to do something like:

ip route add dev eth0

before multicast works. The kernel has no idea which interface to send
the multicast packet out, unless you add a route like this. Maybe in
the BSD world, instead of defining the interface directly, you give
the local IP address on the interface?

Maybe this helps:


In the example routing table is:
224              link#1             UC          0        0


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