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

["Jim C. Brown" <jbrown106@phreaker.net>]

I recommend using vde by Renzo Davoli instead of the default tuntap.
I am able to network 2 instances of windows 98 (in separate qemu's) and
minix 2.0.4 (in a hand-modified bochs). It worked without a hitch, and all
the machines are on the same subnet.

You still need forwarding to let the virtual machines see the lan or the
internet, however. The plus with vde is that you don't need to set up routing
for all these different tunN devices. (I use the Shorewall firewall, and using
tun was a pain...vde means I just have to set up routing and masquerading
for tap0, instead of for tun0, tun1, tun2, ...)

Available here btw: http://sourceforge.net/projects/vde/

On Mon, May 03, 2004 at 11:35:32AM +1000, nhand42@tpg.com.au wrote:
> Quoting Arne Bernin <arne@alamut.de>:
> 
> > Hi,
> > 
> > i am trying to set up two virtual machines with qemu 0.5.4. I have them
> > running, both with 2 network cards. I am able to ping all 4 interfaces
> > from the host they are running from but am not able to ping between both
> > virtual machines...
> > If i do a tcpdump on the corrsponding tun interfaces while trying to
> > ping i see a lot of arp who-has 172.16.0.254 tell 172.16.0.1 messages
> > (172.16.0.1 is the one machine, the other is 172.16.0.254). 
> > Is there something i have to do to get the arp responses right ??
> > Or am i totally wrong ?
> > 
> 
> You've stuck both virtual machines on the same subnet, so they think they're on the 
> same physical LAN. That's why they're doing ARPs and expecting replies. But you're 
> actually running two QEMU instances on two seperate TUN interfaces and they *aren't* 
> bridged. So the ARP on tun0 never gets seen on tun1 (and vice-versa). 
> 
> Set the second TUN interface to a different subnet (eg, 172.16.1.x) and enable ipv4 
> forwarding. Linux will then forward packets between tun0 and tun1.
> 
> You could also muck about with proxyarp or bridging but honestly, I wouldn't bother 
> with that. Routing is a lot easier to understand.
> 
> 
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