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Re: [libvirt] The config of IP address for 'macvtap' network



On 05/15/2012 12:04 PM, Lei Li wrote:
On 05/15/2012 11:12 PM, Laine Stump wrote:
On 05/15/2012 05:33 AM, Michal Privoznik wrote:
On 15.05.2012 10:57, Lei Li wrote:
Hi guys,

I know the macvtap network is supported by libvirt as forward mode
'passthrough', I wonder is there anyway to configure the IP address
for its interface?

For example, If I create a network as below:

<network>
  <name>vdsm-testnet</name>
  <uuid>31f6b3b3-e959-0dd1-ad3a-bf95db660415</uuid>
  <forward dev='eth0.8' mode='passthrough'>
    <interface dev='eth0.8'/>
  </forward>
</network>

For now, I have to set the IP address by 'ifconfig eth0.8 XXX.XXX.XXX.XXX'
after defining the network.
If only the guest will use the interface, that isn't necessary. As a
matter of fact, once an interface has been assigned to a guest using
macvtap passthrough mode, the host *can't* use the interface (it can
continue to use it in the other macvtap modes, however), so it's
completely pointless to configure an IP address for that interface on
the host.


How can I set IP address for this VLAN device 'eth0.8' by libvirt, I mean
does libvirt support to assign IP address in this mode now? If does,
what is the
xml format for it?
A network device being used for a guest macvtap connection only needs an
IP address set on the host if the host will also be using that interface
- the guest does not magically acquire/use the IP address that has been
set on the host, it needs its own IP address, configured on the guest in
the same fashion you would configure any other guest interface (keep in
mind that even in macvtap 'bridge' mode, the host and guest cannot
communicate with each other via a macvtap interface). Otherwise, it's
enough for the interface to be defined on the host (it may also be
necessary for it to be "up" if it's a vlan device - I haven't tried
macvtap with vlans). If you are using RHEL or Fedora, you can do any/all
of that configuration with the virsh iface-define command. For example,
here is the xml file that would define an "eth0.8" interface on a host
(This is *not* the <interface> element of a domain configuration):


<interface type="vlan" name="eth0.8">
  <start mode="none"/>
  <protocol family="ipv4">
    <ip address="192.168.43.1"/>
  </protocol>
  <vlan tag="8">
    <interface name="eth0"/>
  </vlan>
</interface>

To get this defined in the system, you would use:

   virsh iface-define eth0.8.xml

But again, it seems doubtful this is what you really want.

It'd appreciate a lot if anybody could read my post and give me some
suggestions!


I suppose adding:

 <ip address='192.168.123.1' netmask='255.255.255.0'/>

into <network/> should work, doesn't it?
No.

The IP address used by the host for that interface (if any) should be
configured in the normal manner of the host system. The IP address used
by any guests using this interface / network based on this interface
should be configured in the guest just as you would any other interface
(if the guest is configured to get its IP address from DHCP, then you
would need to have a DHCP server running *on a different host* connected
to the .8 vlan.

(BTW, note that in passthrough mode, only a single guest can connect to
each physical device, so having a <network> defined for passthrough mode
is mostly only useful if you have a pool of devices available. The way
you have it defined above, only a single guest would be able to use that
network.)
Hi Laine,

Thank you for your detailed reply!
The vdsm support bridgless network by libvirt through passthrough mode, it just
define a network by the xml format:

<network>
  <name>vdsm-XXX</name>
  <uuid>...</uuid>
  <forward dev=... mode='passthrough>
    <interface dev=.../>
  </forward>
</network>

I want to let IP address config for the host network enabled. It doesn't have to 
be connected by a VLAN device, a simpler example, If I create a bridgeless 
network by passthrough mode, its interface is eth0. What is the xml format for 
the host to let the IP address configuration enabled?

1) There is no place in the <network> XML to configure the host's IP address for an interface that is being used by that network for a macvtap-type connection. If you want the host to have an IP address configured for that interface, you would do it via the normal host interface configuration method (for example, by editing the file /etc/sysconfig/network-scripts/ifcfg-eth0.8 on a RHEL/Fedora/Suse system, or by using the "virsh iface-define" command as I pointed out in my previous message).

2) Since you are using macvtap "passthrough" mode, there is no point in configuring an IP address on the host for this interface anyway - as soon as the guest is started, that interface will be setup for use exclusively by the guest, and any attempts by the host to use it will fail.

You can easily perform a test to verify this (as I did before my earlier mail):

a) configure an interface on the host with an IP address, and verify that it is working by starting up a "ping" to another machine on the network. Leave this ping running.

b) setup a guest to use that interface in "passthrough" mode.

c) start the guest.

d) notice that the pings that you had left running in step (a) are now failing.

If you want to allow the host to continue using the interface while the guest is using it (and coincidentally also to allow multiple guests to use the same interface at the same time), use "mode='bridge'" instead of "mode='passthrough'".

(I think it's a bit misguided to define a network with mode='passthrough' and only a single device, since only a single guest can use a device in passthrough mode - normally when passthrough mode is used, a pool of several interfaces is defined in the network (usually virtual functions of an SRIOV-capable network card)).

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