[CentOS-virt] [CentOS] KVM and DHCP

Thu Mar 13 20:11:45 UTC 2014
James B. Byrne <byrnejb at harte-lyne.ca>

On Thu, March 13, 2014 15:49, Reindl Harald wrote:
> Am 13.03.2014 20:43, schrieb James B. Byrne:
>> CentOS-6.5
>> We have a KVM guest running MS-WinV7pro.  This guest is joined to an Active
>> Directory Domain.  That domain provides DHCP to the members.  The KVM guest
>> does not obtain its IP from the domain but from the local host's qemu
>> hypervisor instead.
>> Is there anyway to get around this and have the guest MS-Win OS get its DHCP
>> from the same place as the rest of the domain members?
> smells like you are using NAT for your guest instead
> bridge it to the host network, the guest does not
> know from where it gets DHCP infos and there should
> not be more than one dhcpd on a network segment, anything
> else leads to race-conditions - the faster dhcpd wins
> http://www.dedoimedo.com/computers/kvm-bridged.html
> maybe the article is outdated, try google as i did
> https://www.google.at/search?q=kvm+nat+versus+bridging

Yes, that was it.  Apparently if one creates a kvm guest via the virt-manager
gui application then NAT is automatically selected as the default.  It may be
that the other guests were originally hand-crafted via virsh and the br0
interface was specified for them, but I doubt that.  I have been using
virt-manager since day one on this project and I am getting a queasy feeling
that virt-manager's behaviour has obscurely changed in significant ways since
I first began using it. This is not the first problem that I have had creating
new guests with recent versions of virt-manager which were not in evidence
with earlier use.

In any case, to fix this via virt-manager's __virtual hardware details__
interface one must select the __NIC__ and change the settings from <Virtual
network default 'NAT'> to <Specify shared device name> and then input the
bridge interface name <br0> in the text box that subsequently appears.  One
must also change the Device model: from <Hypervisor default> to <virtio>.

Thanks for the pointer.

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