From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1N322i-0000ZF-KH
	for qemu-devel@nongnu.org; Wed, 28 Oct 2009 02:30:00 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1N322d-0000UD-D6
	for qemu-devel@nongnu.org; Wed, 28 Oct 2009 02:29:59 -0400
Received: from [199.232.76.173] (port=41653 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1N322d-0000U2-7t
	for qemu-devel@nongnu.org; Wed, 28 Oct 2009 02:29:55 -0400
Received: from mx1.redhat.com ([209.132.183.28]:42298)
	by monty-python.gnu.org with esmtp (Exim 4.60)
	(envelope-from <avi@redhat.com>) id 1N322c-0007Bn-Qd
	for qemu-devel@nongnu.org; Wed, 28 Oct 2009 02:29:55 -0400
Message-ID: <4AE7E4D6.1000008@redhat.com>
Date: Wed, 28 Oct 2009 08:29:42 +0200
From: Avi Kivity <avi@redhat.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] accidental mistyping of command line kills networking
References: <4AE7460D.7050807@us.ibm.com> <4AE761D9.7040401@us.ibm.com>
	<4AE769A3.9080205@codemonkey.ws>
In-Reply-To: <4AE769A3.9080205@codemonkey.ws>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Beth Kon <eak@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>

On 10/27/2009 11:44 PM, Anthony Liguori wrote:
>
> It's actually not okay.  You're creating a bridge with two tap devices 
> on the bridge that happen to be connected in qemu by a vlan.  If one 
> tap device receives a packet, qemu is going to forward that packet to 
> the other tap device, which will in turn send the packet to the bridge 
> which in turn sends it to the first tap device.
>
> Resulting in an infinite networking loop.
>

It shouldn't kill networking though.  If a guest has two nics connected 
to the same host bridge (a reasonable configuration), and then connects 
these two nics through its own bridge, it creates the same sort of 
loop.  If that kills networking, we have a guest-initiated DoS (not that 
the userspace-initiated DoS was much better).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.