[libvirt] configuring network interface

Tue Feb 8 18:31:58 UTC 2011

(For future questions such as this, you may find you get a better 
response (and lower signal-noise ratio) by asking on the recently 
-created libvirt-users at redhat.com list, as libvir-list has a very large 
volume of patches and discussion about the internals of libvirt)

On 02/08/2011 12:05 PM, Marcela Castro León wrote:
> Hello
>
>
> I've a lan of virtual machine configured with bridge. I've a private 
> lan of 4 machine, but one of them have 2 lan-card with access to 
> internet.
>
> Evertything works almost fine, except that I've configured on each 
> guest the static configuration of the bridge with the ip address, like 
> this:
>
> auto lo
> iface lo inet loopback
>
> auto eth6
> iface eth6 inet static
> address 192.168.1.41
> network 192.168.1.0
> netmask 255.255.255.0
> gateway 192.168.1.10

So are you saying that you're attempting to give each guest the *same* 
IP address? There's no way that can possibly work, even if each is 
connected to a separate virtual network or bridge. Since the host's IP 
stack can "see" all of the guests' network traffic, regardless of the 
connection mode you choose, each must have a unique IP address even if 
they can't see each other.

>
> But each time I, for example, define the guest, it start with a 
> ascending number of interface: eth6,eth7, eth8, etc, Now is 
> initializing with eth12.

There must be a mixup in the explanation - I'm understanding that you 
put the same config file in each guest, but each guest, when it starts, 
shows a different ethernet interface in its ifconfig. That doesn't seem 
right, because none of the guests know about each other. Or are you 
saying something different? Where are you seeing the "eth6", "eth7", ... 
"eth12"? In the guest XML config maybe?

> So, it doesn't get the manual configuration I've done on the 
> /etc/network/interfaces.
> How can avoid this way of work of the number of ethernet interface the 
> guest start?

Why are you doing a manual/static definition on the guest at all? It's 
simpler to just leave the guest to automatically configure its eth0, and 
if you really want a particular IP address for a particular guest, 
handle that with a <host> element in the <dhcp> section of the virtual 
network the guest is connected to (or, if you're using a pure bridging 
setup, then in the static hosts config of the dhcp server on that network).

>
> A way i'm trying to solve is configuring the interface trough the 
> guest xml definition,

The guest xml interface configuration cannot be used to setup the IP 
address, route, etc config of the guest. It is only used for two things:

1) configuring what kind of hardware appears on the guest (but not what 
name the guest gives to it!)

2) configuring how that hardware is connected to the physical network 
(usually either via an existing bridge interface on the host (in order 
for the guest to appear as if its directly connected to the physical 
network), or via a "virtual network" created by libvirt (which is really 
just a bridge that's not directly connected to any physical interface, 
relying instead on the host's IP stack to route packets from the guest 
out to the physical network.)

> but the operation "define" on virsh accepted without error the xml 
> i've done, but the dumpxml doesn't show them, so, it doesn't work at all.
> I've followed the instruction that said the manual "Application 
> development guide - a guide to application development with libvirt" 
> on page 71.

That document is targeted towards software developers writing 
applications to manage virtual guests using the libvirt API, not for 
system administrators who are configuring virtual guests manually (or, 
more likely, by using software that uses the libvirt API, eg virsh or 
virt-manager). It likely contains more detail and less overview than 
what you need, which is probably what led to the confusion. I've 
included a pointer to a wiki page below that is more the appropriate 
level of information (Justin, if the docs you were working on are 
somewhere easily accessible, you can point to those instead.)

>
> The whole definition of the VM is attached, but the related to 
> interface is:
> <interface type='bridge'>
> <source bridge='br0'/>
> <target dev='vnet0'/>
> <protocol>
> <ip address="192.168.1.41" prefix="24"/>
> <route gateway="192.168.1.10"/>
> </protocol>
> <bridge stp="off" delay="0.01">
> <interface type="ethernet" name="eth12">
> <protocol>
> <ip address="192.168.1.41" prefix="24"/>
> <route gateway="192.168.1.10"/>
> </protocol>
> </interface>
> </bridge>
> </interface>

You are confusing the guest interface config (that is part of the 
guest's XML config, and described above), and host physical interface 
config (that is not in any way associated with guest config, but instead 
used to configure the host's physical network interfaces, bridges, 
bonds, and vlan interfaces; and nearly as important, is not supported on 
Ubuntu anyway).

Actual guest interface config is *much* simpler than that.

>
>
> But after define, the dumpxml only show this part:
> <interface type='bridge'>
> <mac address='52:54:00:3c:92:9a'/>
> <source bridge='br0'/>
> </interface>

That's because everything else you've put in your guest config is not 
actually a part of a valid guest interface config, and so is ignored.

>
>
> I would appreciate any help to solve the question. The problem I've is 
> I lost the access to the guest every time the libvirt change the name 
> of the interface eth1,2,3, etc.

In this case I think it may be useful to start over. You've somehow been 
tricked into misunderstanding the way that network configuration in 
libvirt works, confusing host interface config with guest interface 
config, and the resulting config bears little resemblance to what would 
actually work.

==
The first step is to read this page:

    http://wiki.libvirt.org/page/Networking

to get a basic overview.

==
Second step: you should decide which of the three methods of connecting 
to guests to the physical network you want to use:

1) NATed
2) routed
3) bridged

The simplest is (1), and that will probably work just fine as long as 1) 
you don't need incoming connections to the guests, and 2) you're not 
concerned about getting the last fraction of an ounce of performance out 
of the network.

Depending on which of those 3 you decide to use:

==
Third step: follow the instructions to either create a bridge interface 
on the host connected to a physical network interface (for (3)), or to 
create a libvirt virtual network using one of the two modes ((1) or (2) 
- actually the stock libvirt config already has a "default" network that 
is probably adequate for (1), and unless you understand IP routing very 
well, you'll likely want to avoid (2)).

Once the physical network is setup:

==
Fourth step: define your guests. In each guest, you'll have an 
<interface> element, but it will be much simpler than what you've 
previously tried. For bridged mode, the interface section would look 
like this:

<interface type='bridge'>
<source bridge='br0'/>
</interface>

Depending on the guest, you may want to specify what type of hardware is 
presented to the guest (default is rtl, which almost always works, but 
is not the best perfrorming), in particular, if the guest supports 
virtio-net, you'll want to add this into the <interface> element:

<model type='virtio'/>

For NATed or routed mode, the guest's interface definition would look 
like this:

   <interface type='network'>
      <source network='default'/>
   </interface>

(replace "default" with the name of the network you've defined, if 
you've defined your own). In this case also, you can add a <model 
type='...'/> if appropriate.

Note that I don't put anything in the <interface> element about MAC 
address, target device, or bus address. Those are all setup 
automatically by libvirt the first time you start the guest, and it will 
make sure that the guests don't interfere with each other.

==
Fifth step: only if you need a particular guest to have a particular 
fixed IP address, either 1) edit the network config file on the guest, 
2) add a static host to the libvirt virtual network definition (giving 
the MAC address provided to the guest by libvirt during the initial 
guest startup) with a <host> element added to the <dhcp> element of the 
network; details are here:

     http://www.libvirt.org/formatnetwork.html

or 3) if you're using the bridged method to connect to the physical 
network, again either configure the guest's own config file, or add a 
static host definition to the dhcp server running on your physical network.

If you still have problems don't hesitate to send another mail 
(although, as I said at the top, sending to libvirt-users at redhat.com may 
yield better results than libvir-list at redhat.com).
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20110208/827cb583/attachment-0001.htm>