[CentOS] Connecting ethX devices directly to a KVM/QEMU guest OS (SOLVED)

Chuck Munro chuckm at seafoam.net
Fri Jan 20 17:18:39 UTC 2012

On 01/20/2012 09:00 AM, Tait Clarridge wrote:
>> >  But a new problem arises ... one of the m0n0wall instances needs to use
>> >  DHCP to get a dynamic IP from my ISP (the others are static, so they're
>> >  ok).  How do I get the bridge to proxy the DHCP request and forward the
>> >  response to the VM guest?  I brought up a test instance of m0n0wall but
>> >  when it asks for an IP on its WAN side, the result is  so it
>> >  would appear the request never makes it to the ISP.
>> >
>> >  As a sanity check, I configured the WAN-side bridge to use DHCP and that
>> >  worked, but it's the m0n0wall VM guest that needs to make the request,
>> >  not the CentOS host.
>> >
>> >  Suggestions anyone?  In the mean time, I'll keep looking for a solution.
>> >
>> >  Thanks,
>> >  Chuck
> Hmmm. Is the bridge up and you can see the interface attached to it from
> running "brctl show" and "ifconfig"? Could you attach that output?
> If yes, then I would do a packet capture on the CentOS host on the eth
> interface attached to the bridge to see if the request makes it out.
> I'm running XEN at home and I have a very similar setup where I pass a
> VLAN from my switch that a provider modem is connected to into a bridge
> and it gets DHCP no problem (as long as the bridge and dot1q
> subinterfaces show up in ifconfig).
> The bridge should automatically send all the traffic out regardless of
> protocol, I doubt ebtables would be actively blocking anything as well
> and same goes for the ISP locking down with MAC addresses.

I'm embarrassed to say the problem was solved in the most simplistic 
possible way ... reboot the cable modem.  I notice that when the host 
boots up, all NIC ports are placed in promiscuous mode, which means that 
DHCP broadcasts should be free to travel across the bridge, so it had to 
be something about a mismatch in the cable modem's arp cache.

Thanks for your suggestions.  It's full steam ahead now, and I'm pleased 
to say the performance of the four m0n0wall VM instances has been very good.


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