[CentOS-virt] DHCP for Xen VMs

Mon Mar 24 19:01:23 UTC 2008
Kai Schaetzl <maillists at conactive.com>

Ross S. W. Walker wrote on Mon, 24 Mar 2008 09:57:06 -0400:

> You might want to check iptables in the dom0.

That was off, anyway. No firewalls in the way. I now know why I had firewalling on my 
guests. These installs were not done with kickstart and there's no way to disable the 
firewall in anaconda anymore. And when the VM gets started I never see the firstboot screen 
as I usually attach to the console much later. But the firewall apparently wasn't the 

> I believe iptables gets set on eth0 before it 'pivots' to peth0 and I doubt it flushes it 
and retargets it to the new bridged eth0 afterwards, so the dhcp traffic may appear as 
unsolicited and dropped on dom0 as it enters the bridge.

Thanks for pointing my attention to peth0, I've found 
http://wiki.xensource.com/xenwiki/XenNetworking this way.

> If that turns out to be the case you will need to have iptables stopped right before the 
creation of peth0 and restarted afterwards (and maybe file a bug report too).

I restarted the network on the host and this did the trick, now this and all other VMs I 
convert from virbr to xenbr and DHCP get their IP number. I assume that during all the 
tests the necessary vif interface might have gotten lost or so.
Interesting observation: virt-manager doesn't want to run VMs that configure a bridge, but 
have no MAC address. No problem to start them with xm. As it seems one can also disable 
libvirtd if you use xenbr. I disabled it and haven't seen a problem so far.


Kai Schätzl, Berlin, Germany
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