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

[nhand42]

Quoting Arne Bernin <address@hidden>:

> 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.