Re: [Qemu-devel] accidental mistyping of command line kills networking

[Anthony Liguori <anthony@codemonkey.ws>]

Beth Kon wrote:
> Beth Kon wrote:
>> I accidentally entered a command line as follows:
>>
>> /usr/bin/qemu-kvm -drive 
>> file=/scratch/images/beth/windows/win2k3_32_R2.dat.10G.img,if=ide -m 
>> 2048 -boot cd -net nic,model=rtl8139 -net tap,script=/etc/qemu-ifup 
>> -vnc :12 -usbdevice tablet -monitor stdio -net nic,model=e1000 -net 
>> tap,script=/etc/qemu-ifup
>>
>> and the machine's networking broke, requiring a network restart to 
>> get it back in order. The second -net tap,script=/etc/qemu-ifup 
>> causes the problem.
>>
>> /var/log/messages shows
>> tap0: received packet with  own address as source address
>>
>> I don't have time at the moment to look into what's going wrong.  
>> Just wanted to make people aware.
>>
>> Beth Kon
> A clarification... this command line is ok. But as it happens, windows 
> datacenter does not have a driver for rtl8139. So somehow, this 
> driverless adapter in windows is effectively causing an "extra" tap 
> device to be specified on the qemu command line, wreaking havoc with 
> the networking on the host.

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.

Regards,

Anthony Liguori