Re: [Xenomai] Odp: Re: RTnet -- receive broadcast frame at the local machine

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On Tue, Mar 01, 2016 at 11:24:55PM +0100, Mariusz Janiak wrote:
> > From what I understand from this post, it is not easy to fix, as it
> > requires duplicating packets. So, patch welcome.
> 
> Ok, we went back to the original question, could someone point the
> place that has to be modified. I have played a little bit with the
> RTnet stack recently, but this task require far better
> understanding of the stack. Maybe someone more advanced can give
> some guidelines.

It is not that complicated. The udp send function (rt_udp_sendmsg)
is in udp.c, it calls rt_ip_route_output (found in route.c) to find
the output route, than rt_ip_build_xmit (found in ip_output.c) to
build the packet and send in on the device. rt_ip_build_xmit calls
rt_eth_header to build the ethernet header (found in eth.c), then
rtdev_xmit (found in dev.c) to send the packet.

So you can do this either at the IP level in rt_ip_build_xmit, or at
the ethernet level in rtdev_xmit. What you need to do is that when
the destination is broadcast (there should be a macro to test that
an IP address is broadcast if you want to do it at IP level, and the
mac address for broadcast is just ff:ff:ff:ff:ff:ff if you want to
do it at ethernet level) you need to duplicate the packet, and
either trigger a receive on the same device, or send the packet on
the loopback device. If you go the ethernet route, you will have to
use a per-device callback to test if the destination is broadcast,
as rtdev_xmit is supposed to not be specific to ethernet, like is
done with the hard_header and start_xmit callbacks.

Hope this helps.
Regards.

-- 
					    Gilles.
https://click-hack.org