[libvirt-users] connecting host and guest vm using a dummy nic

[Laine Stump]

On 05/02/2018 12:05 PM, daggs wrote:
> Greetings Laine,
> 
>> Sent: Wednesday, May 02, 2018 at 5:56 PM
>> From: "Laine Stump" <laine at redhat.com>
>> To: libvirt-users at redhat.com
>> Cc: daggs <daggs at gmx.com>
>> Subject: Re: [libvirt-users] connecting host and guest vm using a dummy nic
>>
>> On 05/01/2018 12:12 PM, daggs wrote:
>>>
>>> I want to see if I understood you correctly, I add:
>>> <interface type='ethernet'>
>>>    <model type='e1000'/>
>>> </interface>
>>> to the vm's xml, this will result in a new nic in the vm which and in the host which I can use by the host to request dhcp from the router within the vm?
>>>
>>
>>
>> Yes. As long as the emulated device in the guest is listening for DHCP
>> on that interface.
>>
>> Also, note that (as long as your libvirt is at least version 2.1.0) if
>> you don't want to bother doing dhcp on the host and guest, you can
>> assign just a static IP address for the host side of that interface in
>> the libvirt XML like this:
>>
>>
>>   <interface type='ethernet'>
>>     <source>
>>        <ip address='192.168.254.2'/>
>>     </source>
>>     ...
>>
>> You can even specify routes to be added automatically to the host when
>> that guest is started, e.g.:
>>
>>
>>   <interface type='ethernet'>
>>     <source>
>>        <ip address='192.168.254.2'/>
>>        <route family='ipv4' address='0.0.0.0' prefix='0'
>>               gateway='192.168.254.1' metric='1'/>
>>     </source>
>>     ...
>>
>> (this assumes that on the guest side, the OS has configured the emulated
>> ethernet device to have address 192.168.254.1). This can be useful if
>> you want the host to use one path for traffic when the guest isn't
>> running, but switch when the guest *is* running. (In this case, if you
>> can set the default route on the host to have a metric of 2 or higher,
>> then the default route that goes through the guest would be preferred
>> when present).
>>
>> (NB: if you read the documentation for configuring interfaces at
>> https://libvirt.org/formatdomain.html#ipconfig you'll see references on
>> how to configure the IP address of the *guest* in libvirt's xml. Note
>> that this *does not work* for qemu, but only for lxc (and possibly xen,
>> I don't recall right now). This is a limitation of the way that qemu/kvm
>> works (the fact that all guest-side IP info is configured in the guest
>> OS, and libvirt has no reasonable to get to it).)
>>
>>
> 
> thanks alot!
> like you've said, I've defined the interface in the xml, booted the device and updated /etc/network/interfaces to alloc dhcp to vnet0.
> that resulted in the proper ip and I can see the router inside the vm.

BTW, if you'll be running other guests on your host, and may start them
in different orders, you should probably add "<target dev='vmrouter'/>
(or whatever name you like, 15 chars or less and *not* starting with
"vnet") to the <interface> config. IF you do that, you can be assured
that the name of the device on the host will always be consistently
named "vmrouter" (otherwise it just gets the lowest numbered available
"vnetX" device name each time it's started).


> 
> one thing bothers me, ethtool vnet0 says the speed is 10Mb/s, asking google suggested several solutions.
> I've tried changing the model to virtio but the speed stays the same, the vm router is running kernel 4.4.92, could that be the issue?

The reported speed should be inconsequential, since there is no wire and
no physical switch involved. It will transfer data as fast as the code
can be executed.

Also, if virtio works, then *definitely* use that instead of e1000 - the
performance will be much better and overhead much lower.