[libvirt-users] Query:: Reg: Libvirt Networking

[Laine Stump]

On 11/21/2013 11:24 PM, Nehal J Wani wrote:
> Currently, I have two networks configured via xml:
>
> virsh # net-dumpxml TestNetwork1
> <network connections='2'>
>   <name>TestNetwork1</name>
>   <uuid>a76f665a-0196-4edb-81b4-340944a6869c</uuid>
>   <forward dev='p1p1' mode='nat'>
>     <nat>
>       <port start='1024' end='65535'/>
>     </nat>
>     <interface dev='p1p1'/>
>   </forward>
>   <bridge name='virbr0' stp='on' delay='0'/>
>   <mac address='52:54:00:0b:e9:35'/>
>   <ip address='192.168.100.1' netmask='255.255.255.0'>
>     <dhcp>
>       <range start='192.168.100.128' end='192.168.100.254'/>
>     </dhcp>
>   </ip>
> </network>
>
> virsh # net-dumpxml default6
> <network connections='1'>
>   <name>default6</name>
>   <uuid>6e57d831-f09e-4b33-950d-6d51c89f9786</uuid>
>   <forward mode='nat'/>
>   <bridge name='virbr3' stp='on' delay='0'/>
>   <mac address='52:54:00:cb:c5:50'/>
>   <ip family='ipv6' address='2001:db8:ca2:2::1' prefix='64'>
>     <dhcp>
>       <range start='2001:db8:ca2:2:1::10' end='2001:db8:ca2:2:1::ff'/>
>     </dhcp>
>   </ip>
> </network>
>
> # brctl show
> bridge namebridge idSTP enabledinterfaces
> virbr08000.5254000be935yesvirbr0-nic
> vnet0
> vnet1
> virbr38000.525400cbc550yesvirbr3-nic
> vnet2
>
> My queries are:
> (i) For each interface that I attach to my guests, will a new vnetX
> (X=0,1,2,3,...) be created?

Yes. Each guest interface (except those of type='hostdev', which are
just host hardware devices passed through to the guest) has a
corresponding unique vnetX tap device or macvtapX device. The socket end
of this tap/macvtap device is connected to the qemu process, and the
netdev end is connected to a Linux host bridge, OpenvSwitch bridge (in
the case of tap), or directly to a host network device (in the case of
macvtap).


> (ii) Can two different active networks share the same bridge? Example,
> can the xml of two active networks have the same line <bridge
> name='virbr0' stp='on' delay='0'/> ?

No, not for libvirt-managed virtual networks. It *is* possible for two
libvirt "unmanaged" networks (which merely shadow already-existing host
system bridge devices without adding any extra services such as dhcp,
dns, or iptables rules) to specify the same bridge device, but I don't
think that's what you're talking about (and I also see no practical
value in doing that).

> (iii) If the answer for (ii) is false, how can I obtain the interface
> name for a given bridge name in libvirt?

What do you mean by "the interface name for a given bridge name"? A
bridge *is* a type of network device, i.e. an interface from the host's
point of view.

If you're wondering how to get a list of all vnetX interfaces that are
attached to a particular bridge via libvirt, the answer is either 1) you
don't, or 2) you would need to cycle through all active guests looking
for guest interfaces that use the network you're interested in, and grab
the interface name from that guest's <interface> xml. libvirt's networks
don't maintain a ready-made list of the devices that are attached to
that network, because it would need to be reconstructed whenever libvirt
was restarted, and up until now nobody has needed it.
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