Date: Mon, 14 Dec 2009 15:21:15 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: Johannes Berg <johannes@sipsolutions.net>
cc: linux-wireless@vger.kernel.org, Felix Fietkau <nbd@openwrt.org>
Subject: Re: Ethernet bridging with wireless
In-Reply-To: <1260809315.2442.408.camel@johannes.local>
Message-ID: <Pine.LNX.4.44L0.0912141519420.2690-100000@iolanthe.rowland.org>
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Sender: linux-wireless-owner@vger.kernel.org

On Mon, 14 Dec 2009, Johannes Berg wrote:

> On Mon, 2009-12-14 at 11:35 -0500, Alan Stern wrote:
> > I just tried to set up ethernet bridging between the wireless interface
> > and the wired ethernet interface on my laptop machine.  It didn't work.  
> > As far as I could tell from the tcpdump output, the wireless stack
> > doesn't want to transmit packets with a foreign link-layer source
> > address.
> > 
> > Is there any way around this restriction?  Or must I resort to IP 
> > forwarding instead?
> 
> There's no standard way around this restriction, the on-air packets are
> required to be transmitted with the correct TA (transmitter address) [1]
> which also must be the SA (sender address) in the regular frame format,
> since the DA (destination address) and RA (receiver address) must be
> present and the format only has three addresses. In recent kernels (what
> will be 2.6.33), we have added code to disallow such configurations
> since they cannot work.

Okay, it sounds like the upshot is that (short of some rather unusual
and/or heroic measures) it can't be done.

Thanks for the information.

Alan Stern