Re: [Qemu-devel] Question about tun/tap networking

[Phil Rasch]

Thanks Jim,

I dont really require everything to be on the same subnet. I just hate
having to figure out how to setup the nameservers, gateways, IP
numbers etc on each virtual machine I create. It would have been great
to just tell the virtual machine to "go get the relevant info from the
DHCP server". Maybe the alternative isnt really a big deal, but like I
said, I am a beginner at configuring networks.

Phil

On Wed, Jun 23, 2004 at 01:03:25PM -0400, Jim C. Brown wrote:
> On Wed, Jun 23, 2004 at 07:54:07AM -0600, Phil Rasch wrote:
> > I have been running a mixture of linux and windows machines, all real 
> > machines 
> > on a LAN connected to the outside world through a router, which is acting 
> > as 
> > my DHCP server, DNS server, and gateway machine. 
> > 
> > The LAN subnet is 192.168.0.*
> > 
> > Now I want to connect up the virtual machine running under QEMU. I want the 
> > virtual machine to be on the same subnet as the real machines, and to use 
> > DHCP from the router to get all the relevant info.
> 
> That is tricky. You will need to look into bridging.
> 
> > 
> > Jim suggested the following commands for somebody else on the mailing list, 
> > but I think that was headed towards devising a subnet that two virtual 
> > machines could use to talk to each other.
> > 
> > [/space/qemu/qemu-0.5.4/ $] su root
> > [/space/qemu/qemu-0.5.4/ #] vde_switch -daemon -tap tap0
> > [/space/qemu/qemu-0.5.4/ #] ifconfig tap0 192.168.1.254
> > [/space/qemu/qemu-0.5.4/ #] chmod 777 /tmp/vde.ctl
> > [/space/qemu/qemu-0.5.4/ #] exit
> > [/space/qemu/qemu-0.5.4/ $] vdeq qemu -hda disk1.img
> 
> That is correct. You now have 2 subnets, 198.168.0.* for the real network
> and 192.168.1.* for qemu/VDE.
> 
> > 
> > The thing I dont understand is whether the above command is setting up 
> > seperate subnet for the virtual machines, or whether I can also set them up 
> > on the same subnet as the real machines, and let them see the DHCP server,
> > etc, and if so, then how to do it.  So far, I have never needed to learn the
> > subtleties of ifconfig. Am I going to have to do so now? Do you have 
> > anymore 
> > advice on using VDE and QEMU together for my purpose?
> 
> Like I said, you will need to use bridging. After you have loaded vde_switch,
> skip the ifconfig step, and combine eth0 and tap0 into br0. Then you will need
> to set up br0 properly. If you are going to use bridging however, I recommend
> you use tuntap instead of VDE. (You don't really need VDE if you are going to
> to use bridging.) If you decide to go with tuntap and bridging, you should 
> look
> in the qemu archives for assistance. There are several messages that appear
> to have useful information on this topic, such as this one which tells you
> step-by-step:
> 
> http://lists.gnu.org/archive/html/qemu-devel/2004-05/msg00032.html
> 
> (you'll need to do a 'brctl addif br0 eth0' and a 'ifconfig br0 192.168.0.XXX'
> for the machine that will be running qemu (almost certainly in one of the 
> startup
> scripts, as eth0 must not have an ip yet) but otherwise the instructions are
> exactly the same.)
> 
> I am curious though, why do you require that the virtual machines be on the 
> same
> subnet as the real ones?
> 
> > 
> > Thanks
> > 
> > Phil
> 
> -- 
> Infinite complexity begets infinite beauty.
> Infinite precision begets infinite perfection.

-- 
Phil Rasch, Climate Modeling Section, National Center for Atmospheric Research
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