[libvirt-users] Do not attempt to add physical NIC to virbr0

Sagar Shedge sagar.shedge92 at gmail.com
Mon Nov 17 05:56:47 UTC 2014

Hi Laine,

Ok. Got the point that how it will affect my network and NATed
Thanks a lot for this description.


On Sat, Nov 15, 2014 at 9:11 AM, Laine Stump <laine at laine.org> wrote:

> On 11/12/2014 08:21 AM, Sagar Shedge wrote:
> > Hi,
> >
> > I got this NOTE on most of the link. But I am not getting reason for
> this.
> > Why someone should not add physical NIC to virbr0.
> Well, for a start, if you do that then the dhcp server that is running
> on virbr0 will be exposed to the physical network and begin answering
> DHCP requests from devices out in the real world. And then you will have
> some people *very* angry with you (conversely, any DHCP server listening
> on the physical network will also be responding to DHCP requests from
> your guests).
> Beyond that, why would you even want to do that? The entire point of the
> NATed network is to isolate the guests from the physical network. That
> is done by forcing all traffic to pass through the host's IP routing
> stack in order to get beyond the host, and if you have a physical device
> attached to the bridge, the host's IP stack can be bypassed - if a guest
> gets configured with an IP address that is on the physical network, all
> of its traffic will go directly via the attached physdev without ever
> going through the host's IP stack, or being NATed by iptables.
> If you really want your guests directly visible on the physical network,
> separately create a host bridge in the host's network config using the
> directions that are available in many places (including the libvirt
> wiki), and connect the guest interfaces to that bridge, rather than to
> libvirt's default network.
> > I tried to add my eth1 to virbr0 and it get added.
> Just because something can be done with no immediate error does not mean
> that it should be done, nor that it is not going to cause a lot of other
> problems that aren't immediately visible.
> > So whether it affects to some functionality of NAT network?
> See above.

Sagar Dilip Shedge,

With Regards.
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