[Gllug] Virtual disk allocation advice requested

Bruce Richardson itsbruce at workshy.org
Mon Jun 30 12:43:29 UTC 2008

On Mon, Jun 30, 2008 at 11:54:56AM +0100, David wrote:
> > I would never run NFS or anything like that from a dom0; it's a waste of
> > the resources used by dom0 and a huge security risk.  If dom0 is
> > compromised then the attacker gains access to all the domUs.  Running
> > network services from dom0 just makes this much more likely.
> Accordingly I thought to use the NFS shared area almost only as a
> transfer mechanism, and still use for example, sFTP etc to transfer
> stuff between VMs/domains, ie web-dev to acc-test (sub)domains. Given
> that the NFS storage will not be intrinsic to the operation of any VM
> does it still provide such an attack vector?

Yes.  If you are running any network service from dom0 then that service
is a potential vulnerability; if the service is successfully compromised
then the attacker has access to dom0 and dom0 has *full* access to all
the domUs.  Don't do this if you can do it any other way.  For example,
it is perfectly possible to ensure that one domU comes up before any
others (you simply have to add to or modify the standard Xen start-up
scripts) and run the NFS server from there.

> > For security, I prefer to have the domUs bridging across one physical
> > interface (or bonded pair) and dom0 accessible via a separate one (on a
> > different subnet and network segment if at all possible.)
> Have you now moved away from 'disk' to talking about virtual network
> interfacing? Yes, I thought it might actually be easier to allocate each
> DomU its own MAC and IPaddr. 

That's what I do.  But while each Xen domain has its own virtual
interface, to communicate with the outside world the virtual interfaces
have to be associated with a physical interface on the box.  For
security, I have dom0 on one physical interface and all the domUs on
another; if those physical interfaces are connected to different
switches/networks, this means that there is no danger of traffic to the
dom0 being sniffed/intercepted from one of the domUs.  It also has the
extra benefit that admin access to dom0 is not affected by heavy network
traffic on the connection used by the domUs.


Hierophant: someone who remembers, when you are on the way down,
everything you did to them on the way up.
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