libvirt XML format

This section describes the XML format used to represent domains, there are variations on the format based on the kind of domains run and the options used to launch them. For hypervisor specific details consult the driver docs

Element and attribute overview

The root element required for all virtual machines is named domain. It has two attributes, the type specifies the hypervisor used for running the domain. The allowed values are driver specific, but include "xen", "kvm", "qemu", "lxc" and "kqemu". The second attribute is id which is a unique integer identifier for the running guest machine. Inactive machines have no id value.

General metadata

      <domain type='xen' id='3'>
        <name>fv0</name>
        <uuid>4dea22b31d52d8f32516782e98ab3fa0</uuid>
        ...

name

The content of the name element providesa short name for the virtual machine. This name shouldconsist only of alpha-numeric characters and is requiredto be unique within the scope of a single host. It isoften used to form the filename for storing the persistentconfiguration file. Since 0.0.1

uuid

The content of the uuid element providesa globally unique identifier for the virtual machine.The format must be RFC 4122 compliant, eg 3e3fce45-4f53-4fa7-bb32-11f34168b82b.If omitted when defining/creating a new machine, a randomUUID is generated. Since 0.0.1

Operating system booting

There are a number of different ways to boot virtual machines each with their own pros and cons.

BIOS bootloader

Booting via the BIOS is available for hypervisors supporting full virtualization. In this case the BIOS has a boot order priority (floppy, harddisk, cdrom, network) determining where to obtain/find the boot image.

        ...
        <os>
          <type>hvm</type>
          <loader>/usr/lib/xen/boot/hvmloader</loader>
          <boot dev='hd'/>
        </os>
        ...

type

The content of the type element specifies thetype of operating system to be booted in the virtual machine.hvm indicates that the OS is one designed to runon bare metal, so requires full virtualization. linux(badly named!) refers to an OS that supports the Xen 3 hypervisorguest ABI. There are also two optional attributes, archspecifying the CPU architecture to virtualization, and machinereferring to the machine type. The Capabilities XMLprovides details on allowed values for these. Since 0.0.1

loader

The optional loader tag refers to a firmware blobused to assist the domain creation process. At this time, it isonly needed by Xen fully virtualized domains. Since 0.1.0

boot

The dev attribute takes one of the values "fd", "hd","cdrom" or "network" and is used to specify the next boot deviceto consider. The boot element can be repeated multipletimes to setup a priority list of boot devices to try in turn.Since 0.1.3

Host bootloader

Hypervisors employing paravirtualization do not usually emulate a BIOS, and instead the host is responsible to kicking off the operating system boot. This may use a pseudo-bootloader in the host to provide an interface to choose a kernel for the guest. An example is pygrub with Xen.

        ...
<bootloader>/usr/bin/pygrub</bootloader>
<bootloader_args>--append single</bootloader_args>
        ...

bootloader

The content of the bootloader element providesa fully qualified path to the bootloader executable in thehost OS. This bootloader will be run to choose which kernelto boot. The required output of the bootloader is dependenton the hypervisor in use. Since 0.1.0

bootloader_args

The optional bootloader_args element allowscommand line arguments to be passed to the bootloader.Since 0.2.3

Direct kernel boot

When installing a new guest OS it is often useful to boot directly from a kernel and initrd stored in the host OS, allowing command line arguments to be passed directly to the installer. This capability is usually available for both para and full virtualized guests.

        ...
<os>
          <type>hvm</type>
          <loader>/usr/lib/xen/boot/hvmloader</loader>
          <kernel>/root/f8-i386-vmlinuz</kernel>
          <initrd>/root/f8-i386-initrd</initrd>
          <cmdline>console=ttyS0 ks=http://example.com/f8-i386/os/</cmdline>
</os>
...

type

This element has the same semantics as described earlier in theBIOS boot section

loader

This element has the same semantics as described earlier in theBIOS boot section

kernel

The contents of this element specify the fully-qualified pathto the kernel image in the host OS.

initrd

The contents of this element specify the fully-qualified pathto the (optional) ramdisk image in the host OS.

cmdline

The contents of this element specify arguments to be passed tothe kernel (or installer) at boottime. This is often used tospecify an alternate primary console (eg serial port), or theinstallation media source / kickstart file

Basic resources

        ...
<memory>524288</memory>
<currentMemory>524288</currentMemory>
<vcpu>1</vcpu>
...

memory

The maximum allocation of memory for the guest at boot time.The units for this value are kilobytes (i.e. blocks of 1024 bytes)

currentMemory

The actual allocation of memory for the guest. This valuebe less than the maximum allocation, to allow for ballooningup the guests memory on the fly. If this is omitted, it defaultsto the same value as the memory element

vcpu

The content of this element defines the number of virtualCPUs allocated for the guest OS.

Lifecycle control

It is sometimes necessary to override the default actions taken when a guest OS triggers a lifecycle operation. The following collections of elements allow the actions to be specified. A common use case is to force a reboot to be treated as a poweroff when doing the initial OS installation. This allows the VM to be re-configured for the first post-install bootup.

        ...
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
...

on_poweroff

The content of this element specifies the action to take whenthe guest requests a poweroff.

on_reboot

The content of this element specifies the action to take whenthe guest requests a reboot.

on_crash

The content of this element specifies the action to take whenthe guest crashes.

Each of these states allow for the same four possible actions.

destroy

The domain will be terminated completely and all resourcesreleased

restart

The domain will be terminated, and then restarted withthe same configuration

preserve

The domain will be terminated, and its resource preservedto allow analysis.

rename-restart

The domain will be terminated, and then restarted witha new name

Hypervisor features

Hypervisors may allow certain CPU / machine features to be toggled on/off.

        ...
<features>
  <pae/>
  <acpi/>
  <apic/>
</features>
...

All features are listed within the features element, omitting a togglable feature tag turns it off. The available features can be found by asking for the capabilities XML, but a common set for fully virtualized domains are:

pae

Physical address extension mode allows 32-bit gueststo address more than 4 GB of memory.

acpi

ACPI is useful for power management, for example, withKVM guests it is required for graceful shutdown to work.

Time keeping

The guest clock is typically initialized from the host clock. Most operating systems expect the hardware clock to be kept in UTC, and this is the default. Windows, however, expects it to be in so called 'localtime'.

        ...
        <clock sync="localtime"/>
...

clock

The sync attribute takes either "utc" or"localtime" to specify how the guest clock is initializedin relation to the host OS.

Devices

The final set of XML elements are all used to describe devices provided to the guest domain. All devices occur as children of the main devices element. Since 0.1.3

        ...
        <devices>
  <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
          ...

emulator

The contents of the emulator element specifythe fully qualified path to the device model emulator binary.The capabilities XML specifiesthe recommended default emulator to use for each particulardomain type / architecture combination.

Hard drives, floppy disks, CDROMs

Any device that looks like a disk, be it a floppy, harddisk, cdrom, or paravirtualized driver is specified via the disk element.

          ...
  <disk type='file'>
    <driver name="tap" type="aio">
    <source file='/var/lib/xen/images/fv0'/>
    <target dev='hda' bus='ide'/>
  </disk>
  ...

disk

The disk element is the main container for describingdisks. The type attribute is either "file" or "block"and refers to the underlying source for the disk. The optionaldevice attribute indicates how the disk is to be exposedto the guest OS. Possible values for this attribute are "floppy", "disk"and "cdrom", defaulting to "disk".Since 0.0.3; "device" attribute since 0.1.4

source

If the disk type is "file", then the file attributespecifies the fully-qualified path to the file holding the disk. If the disktype is "block", then the dev attribute specifiesthe path to the host device to serve as the disk. Since 0.0.3

target

The target element controls the bus / device under which thedisk is exposed to the guest OS. The dev attribute indicatesthe "logical" device name. The actual device name specified is not guaranteed to map tothe device name in the guest OS. Treat it as a device ordering hint.The optional bus attribute specifies the type of disk deviceto emulate; possible values are driver specific, with typical values being"ide", "scsi", "virtio", "xen" or "usb". If omitted, the bus type isinferred from the style of the device name. eg, a device named 'sda'will typically be exported using a SCSI bus.Since 0.0.3; bus attribute since 0.4.3; "usb" attribute value since after 0.4.4

driver

If the hypervisor supports multiple backend drivers, then the optionaldriver element allows them to be selected. The nameattribute is the primary backend driver name, while the optional typeattribute provides the sub-type. Since 0.1.8

USB devices

USB devices attached to the host can be passed through to the guest using the hostdev element. since after 0.4.4

          ...
  <hostdev mode='subsystem' type='usb'>
    <source>
      <vendor id='0x1234'/>
      <product id='0xbeef'/>
    </source>
  </disk>
  ...

hostdev

The hostdev element is the main container for describing host devices. For usb device passthrough mode is always "subsystem" and type is "usb".

source

The source element describes the device as seen from the host. The USB device can either be addressed by vendor / product id using the vendor and product elements or by the device's address on the hosts using the address element.

vendor, product

The vendor and product elements each have an id attribute that specifies the USB vendor and product id. The ids can be given in decimal, hexadecimal (starting with 0x) or octal (starting with 0) form.

address

The address element has a bus and device attribute to specify the USB bus and device number the device appears at on the host. The values of these attributes can be given in decimal, hexadecimal (starting with 0x) or octal (starting with 0) form.

Network interfaces

          ...
  <interface type='bridge'>
    <source bridge='xenbr0'/>
    <mac address='00:16:3e:5d:c7:9e'/>
    <script path='vif-bridge'/>
  </interface>
  ...

Virtual network

This is the recommended config for general guest connectivity on hosts with dynamic / wireless networking configs

Provides a virtual network using a bridge device in the host. Depending on the virtual network configuration, the network may be totally isolated, NAT'ing to an explicit network device, or NAT'ing to the default route. DHCP and DNS are provided on the virtual network in all cases and the IP range can be determined by examining the virtual network config with 'virsh net-dumpxml [networkname]'. There is one virtual network called 'default' setup out of the box which does NAT'ing to the default route and has an IP range of 192.168.22.0/255.255.255.0. Each guest will have an associated tun device created with a name of vnetN, which can also be overridden with the <target> element.

      ...
      <interface type='network'>
        <source network='default'/>
      </interface>
      ...
      <interface type='network'>
        <source network='default'/>
        <target dev='vnet7'/>
        <mac address="11:22:33:44:55:66"/>
      </interface>
      ...

Bridge to to LAN

This is the recommended config for general guest connectivity on hosts with static wired networking configs

Provides a bridge from the VM directly onto the LAN. This assumes there is a bridge device on the host which has one or more of the hosts physical NICs enslaved. The guest VM will have an associated tun device created with a name of vnetN, which can also be overridden with the <target> element. The tun device will be enslaved to the bridge. The IP range / network configuration is whatever is used on the LAN. This provides the guest VM full incoming & outgoing net access just like a physical machine.

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

      <interface type='bridge'>
        <source bridge='br0'/>
        <target dev='vnet7'/>
        <mac address="11:22:33:44:55:66"/>
      </interface>
      ...

Userspace SLIRP stack

Provides a virtual LAN with NAT to the outside world. The virtual network has DHCP & DNS services and will give the guest VM addresses starting from 10.0.2.15. The default router will be 10.0.2.2 and the DNS server will be 10.0.2.3. This networking is the only option for unprivileged users who need their VMs to have outgoing access.

      ...
      <interface type='user'/>
      ...
      <interface type='user'>
        <mac address="11:22:33:44:55:66"/>
      </interface>
      ...

Generic ethernet connection

Provides a means for the administrator to execute an arbitrary script to connect the guest's network to the LAN. The guest will have a tun device created with a name of vnetN, which can also be overridden with the <target> element. After creating the tun device a shell script will be run which is expected to do whatever host network integration is required. By default this script is called /etc/qemu-ifup but can be overridden.

      ...
      <interface type='ethernet'/>
      ...
      <interface type='ethernet'>
        <target dev='vnet7'/>
        <script path='/etc/qemu-ifup-mynet'/>
      </interface>
      ...

Multicast tunnel

A multicast group is setup to represent a virtual network. Any VMs whose network devices are in the same multicast group can talk to each other even across hosts. This mode is also available to unprivileged users. There is no default DNS or DHCP support and no outgoing network access. To provide outgoing network access, one of the VMs should have a 2nd NIC which is connected to one of the first 4 network types and do the appropriate routing. The multicast protocol is compatible with that used by user mode linux guests too. The source address used must be from the multicast address block.

      ...
      <interface type='mcast'>
        <source address='230.0.0.1' port='5558'/>
      </interface>
      ...

TCP tunnel

A TCP client/server architecture provides a virtual network. One VM provides the server end of the network, all other VMS are configured as clients. All network traffic is routed between the VMs via the server. This mode is also available to unprivileged users. There is no default DNS or DHCP support and no outgoing network access. To provide outgoing network access, one of the VMs should have a 2nd NIC which is connected to one of the first 4 network types and do the appropriate routing.

      ...
      <interface type='server'>
        <source address='192.168.0.1' port='5558'/>
      </interface>
      ...
      <interface type='client'>
      <source address='192.168.0.1' port='5558'/>
      </interface>
      ...

Setting the NIC model

      ...
      <interface type='network'>
        <source network='default'/>
        <target dev='vnet1'/>
        <model type='ne2k_pci'/>
      </interface>
      ...

For hypervisors which support this, you can set the model of emulated network interface card.

The values for type aren't defined specifically by libvirt, but by what the underlying hypervisor supports (if any). For QEMU and KVM you can get a list of supported models with these commands:

qemu -net nic,model=? /dev/null
qemu-kvm -net nic,model=? /dev/null

Typical values for QEMU and KVM include: ne2k_isa i82551 i82557b i82559er ne2k_pci pcnet rtl8139 e1000 virtio

Input devices

Input devices allow interaction with the graphical framebuffer in the guest virtual machine. When enabling the framebuffer, an input device is automatically provided. It may be possible to add additional devices explicitly, for example, to provide a graphics tablet for absolute cursor movement.

          ...
  <input type='mouse' bus='usb'/>
  ...

input

The input element has one mandatory attribute, the typewhose value can be either 'mouse' or 'tablet'. The latter provides absolutecursor movement, while the former uses relative movement. The optionalbus attribute can be used to refine the exact device type.It takes values "xen" (paravirtualized), "ps2" and "usb".

Graphical framebuffers

A graphics device allows for graphical interaction with the guest OS. A guest will typically have either a framebuffer or a text console configured to allow interaction with the admin.

          ...
  <graphics type='vnc' port='5904'/>
  ...

graphics

The graphics element has a mandatory typeattribute which takes the value "sdl" or "vnc". The former displaysa window on the host desktop, while the latter activates a VNC server.If the latter is used the port attribute specifies the TCPport number (with -1 as legacy syntax indicating that it should beauto-allocated). The autoport attribute is the newpreferred syntax for indicating autoallocation of the TCP port to use.The listen attribute is an IP address for the server tolisten on. The password attribute provides a VNC passwordin clear text.

Consoles, serial & parallel devices

A character device provides a way to interact with the virtual machine. Paravirtualized consoles, serial ports and parallel ports are all classed as character devices and so represented using the same syntax.

        ...
        <parallel type='pty'>
  <source path='/dev/pts/2'/>
  <target port='0'/>
        </parallel>
        <serial type='pty'>
  <source path='/dev/pts/3'/>
  <target port='0'/>
        </serial>
        <console type='pty'>
  <source path='/dev/pts/4'/>
  <target port='0'/>
        </console>
        </devices>
      </domain>

parallel

Represents a parallel port

serial

Represents a serial port

console

Represents the primary console. This can be the paravirtualizedconsole with Xen guests, or duplicates the primary serial portfor fully virtualized guests without a paravirtualized console.

source

The attributes available for the source elementvary according to the type attribute on the parenttag. Allowed variations will be described below

target

The port number of the character device is specified via theport attribute, numbered starting from 1. There isusually only one console device, and 0, 1 or 2 serial devicesor parallel devices.

Domain logfile

This disables all input on the character device, and sends output into the virtual machine's logfile

      ...
      <console type='stdio'>
        <target port='1'>
      </console>
      ...

Device logfile

A file is opened and all data sent to the character device is written to the file.

      ...
      <serial type="file">
        <source path="/var/log/vm/vm-serial.log"/>
        <target port="1"/>
      </serial>
      ...

Virtual console

Connects the character device to the graphical framebuffer in a virtual console. This is typically accessed via a special hotkey sequence such as "ctrl+alt+3"

      ...
      <serial type='vc'>
        <target port="1"/>
      </serial>
      ...

Null device

Connects the character device to the void. No data is ever provided to the input. All data written is discarded.

      ...
      <serial type='null'>
        <target port="1"/>
      </serial>
      ...

Pseudo TTY

A Pseudo TTY is allocated using /dev/ptmx. A suitable client such as 'virsh console' can connect to interact with the serial port locally.

      ...
      <serial type="pty">
        <source path="/dev/pts/3"/>
        <target port="1"/>
      </serial>
      ...

NB special case if <console type='pty'>, then the TTY path is also duplicated as an attribute tty='/dev/pts/3' on the top level <console> tag. This provides compat with existing syntax for <console> tags.

Host device proxy

The character device is passed through to the underlying physical character device. The device types must match, eg the emulated serial port should only be connected to a host serial port - don't connect a serial port to a parallel port.

      ...
      <serial type="dev">
        <source path="/dev/ttyS0"/>
        <target port="1"/>
      </serial>
      ...

TCP client/server

The character device acts as a TCP client connecting to a remote server, or as a server waiting for a client connection.

      ...
      <serial type="tcp">
        <source mode="connect" host="0.0.0.0" service="2445"/>
        <wiremode type="telnet"/>
        <target port="1"/>
      </serial>
      ...

UDP network console

The character device acts as a UDP netconsole service, sending and receiving packets. This is a lossy service.

      ...
      <serial type="udp">
        <source mode="bind" host="0.0.0.0" service="2445"/>
        <source mode="connect" host="0.0.0.0" service="2445"/>
        <target port="1"/>
      </serial>
      ...

UNIX domain socket client/server

The character device acts as a UNIX domain socket server, accepting connections from local clients.

      ...
      <serial type="unix">
        <source mode="bind" path="/tmp/foo"/>
        <target port="1"/>
      </serial>
      ...

Example configs

Example configurations for each driver are provide on the driver specific pages listed below

• Xen examples
• QEMU/KVM examples