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

["Mariusz Janiak" <mariuszjaniak@wp.pl>]

Dnia Środa, 2 Marca 2016 14:25 Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org> napisał(a) 
> On Wed, Mar 02, 2016 at 02:07:33PM +0100, Gilles Chanteperdrix wrote:
> > 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.
> 
> Note that doing this at ethernet level is better, as it will also
> work for packets sent with raw sockets, and also note that to test
> if a MAC address is broadcast, you only need to test the most
> significant byte least significant bit. If you do it that way, it
> also gets the trick working with multicast addresses.

Ok, I will do that that way, but unfortunately RTnet does no support multicast yet. 

Regards,
Mariusz

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