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Re: [libvirt] [PATCH] lxc: support <interface type='ethernet'>

On 05/25/2016 11:13 AM, Vasiliy Tolstov wrote:
2016-05-25 18:00 GMT+03:00 Laine Stump <laine laine org>:
So $VM_GW is on the same subnet as $VM_IP/24 ? Is that also the same subnet
as $HOST_IP? Or is that on a completely different network?

(This is all very useful, because it's pointing out that the config of the
two ends definitely aren't mirror images, so we really do need independent
settings for both).

VM_GW on the same subnet as PEER , but on host side i apply /32
address, but inside vm /24.

peer only used on host machine to determine on which interface kernel
needs to send traffic

Well, you *could* do the same thing with a /30 subnet for each tap (so that
your routing daemon would get a bunch of /30 routes pointing to your host
for all of them), but that would use up a lot more address space.

(I'm not sure what the guest does with

What if you instead set the host to:

          $HOST_IP peer $VM_IP/32

and set the guest to:

          $VM_IP peer $HOST_IP/32
          route add default $HOST_IP


I don't use this scheme... If i set $VM_IP peer $HOST_IP/32 on host
machine i think kernel recieves packet and drop it =)

No - you would set that on the *guest*, not the host. I setup an LXC
container in this manner and everything seemed to work properly.

May be, but in case of qemu i use simple dhcp server and on vm simple
dhcp client. It can't assign peer addresses

Ah, right. I haven't looked, but doubt there is a dhcp option to specify a peer address.

may be this is works. I
don't try.
In my setup guest vm does not know host side of network, so it does
not need know about peer... Also i can live migrate guest without
(Changes only host side of network)

Anyway, it's important to know if you set the IP config on host and guest
exact mirrors of each other. It seems like the answer is "no", though, so
i'm going to make a patch that allows what I was talking about last week:

         <interface type='ethernet'>
              <ip address='HOST_IP' family='ipv4' peer='VM_IP'
              <ip address='VM_GW' family='ipv4' prefix='32'/>
            <ip address='VM_IP' family='ipv4' peer='HOST_IP' prefix='24'/>
            <route family='ipv4' address='' gateway='HOST_IP'/>

On qemu only the address info inside <source> would be used, since we
have control over the guest's network config. On LXC, we can set both.

Does that sound usable?
Yes, but don't cleanup please  <ip address='VM_IP' family='ipv4'
peer='HOST_IP' prefix='24'/> in qemu case, because on somedays we can
launch dnsmasq or orhet software that can provide dhcp and use this

An interesting point - even if libvirt itself doesn't launch the dnsmasq, it
could be done by a more intelligent script (one that received the full
interface XML on stdin, as I discussed in my last mail), so we should
probably just ignore it rather than complaining that it's unsupported...

Thanks for the info!
Yes, i handmade my own dhcp server on golang what listens libvirt
hooks and serves needed addresses for needed tap devices.

Yeah, I had considered that if we were to support such a thing in libvirt, it would be best to do it by having a single dnsmasq instance for all tap-only guest interfaces.

So i have only one daemon that have stateless config (it get it via
libvirt xml). So i don't need database or something else.

How does it gather than config? With a qemu hook script?

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