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
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.
<domain type='xen' id='3'>
<name>fv0</name>
<uuid>4dea22b31d52d8f32516782e98ab3fa0</uuid>
...
namename element provides
a short name for the virtual machine. This name should
consist only of alpha-numeric characters and is required
to be unique within the scope of a single host. It is
often used to form the filename for storing the persistent
configuration file. Since 0.0.1uuiduuid element provides
a 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 random
UUID is generated. Since 0.0.1There are a number of different ways to boot virtual machines each with their own pros and cons.
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>
...
typetype element specifies the
type of operating system to be booted in the virtual machine.
hvm indicates that the OS is one designed to run
on bare metal, so requires full virtualization. linux
(badly named!) refers to an OS that supports the Xen 3 hypervisor
guest ABI. There are also two optional attributes, arch
specifying the CPU architecture to virtualization, and machine
refering to the machine type. The Capabilities XML
provides details on allowed values for these. Since 0.0.1loaderloader tag refers to a firmware blob
used to assist the domain creation process. At this time, it is
only needed by Xen fullyvirtualized domains. Since 0.1.0bootdev attribute takes one of the values "fd", "hd",
"cdrom" or "network" and is used to specify the next boot device
to consider. The boot element can be repeated multiple
times to setup a priority list of boot devices to try in turn.
Since 0.1.3
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 pseduo-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>
...
bootloaderbootloader element provides
a fullyqualified path to the bootloader executable in the
host OS. This bootloader will be run to choose which kernel
to boot. The required output of the bootloader is dependant
on the hypervisor in use. Since 0.1.0bootloader_argsbootloader_args element allows
command line arguments to be passed to the bootloader.
Since 0.2.3
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.
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<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>
...
typeloaderkernelinitrdcmdline
...
<memory>524288</memory>
<currentMemory>524288</currentMemory>
<vcpu>1</vcpu>
...
memorycurrentMemorymemory elementvcpuIt is sometimes neccessary 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_poweroffon_rebooton_crashEach of these states allow for the same four possible actions.
destroyrestartpreserverename-restartHypervisors may allow certain CPU / machine features to be toggled on/off.
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<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:
paeacpiThe 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"/>
...
clocksync attribute takes either "utc" or
"localtime" to specify how the guest clock is initialized
in relation to the host OS.
The final set of XML elements are all used to descibe 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>
...
emulatoremulator element specify
the fully qualified path to the device model emulator binary.
The capabilities XML specifies
the recommended default emulator to use for each particular
domain type / architecture combination.
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>
...
diskdisk element is the main container for describing
disks. The type attribute is either "file" or "block"
and refers to the underlying source for the disk. The optional
device attribute indicates how the disk is to be exposed
to the guest OS. Possible values for this attribute are "floppy", "disk"
and "cdrom", defaulting to "disk".
Since 0.0.3; "device" attribute since 0.1.4sourcetype is "file", then the file attribute
specifies the fully-qualified path to the file holding the disk. If the disk
type is "block", then the dev attribute specifies
the path to the host device to serve as the disk. Since 0.0.3targettarget element controls the bus / device under which the
disk is exposed to the guest OS. The dev attribute indicates
the "logical" device name. The actual device name specified is not guarenteed to map to
the device name in the guest OS. Treat it as a device ordering hint.
The optional bus attribute specifies the type of disk device
to emulate; possible values are driver specific, with typical values being
"ide", "scsi", "virtio", "xen". If omitted, the bus type is inferred 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 0.4.3driverdriver element allows them to be selected. The name
attribute is the primary backend driver name, while the optional type
attribute provides the sub-type. Since 0.1.8
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<interface type='bridge'>
<source bridge='xenbr0'/>
<mac address='00:16:3e:5d:c7:9e'/>
<script path='vif-bridge'/>
</interface>
...
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>
...
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.
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<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>
...
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.
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<interface type='user'/>
...
<interface type='user'>
<mac address="11:22:33:44:55:66"/>
</interface>
...
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.
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<interface type='ethernet'/>
...
<interface type='ethernet'>
<target dev='vnet7'/>
<script path='/etc/qemu-ifup-mynet'/>
</interface>
...
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.
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<interface type='mcast'>
<source address='230.0.0.1' port='5558'/>
</interface>
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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.
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<interface type='server'>
<source address='192.168.0.1' port='5558'/>
</interface>
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<interface type='client'>
<source address='192.168.0.1' port='5558'/>
</interface>
...
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.
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<input type='mouse' bus='usb'/>
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inputinput element has one madatory attribute, the type
whose value can be either 'mouse' or 'tablet'. The latter provides absolute
cursor movement, while the former uses relative movement. The optional
bus attribute can be used to refine the exact device type.
It takes values "xen" (paravirtualized), "ps2" and "usb".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.
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<graphics type='vnc' port='5904'/>
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graphicsgraphics element has a mandatory type
attribute which takes the value "sdl" or "vnc". The former displays
a window on the host desktop, while the latter activates a VNC server.
If the latter is used the port attributes specifies the
TCP port number (with -1 indicating that it should be auto-allocated).
The listen attribute is an IP address for the server to
listen on. The password attribute provides a VNC password
in clear text.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>
parallelserialconsolesourcesource element
vary according to the type attribute on the parent
tag. Allowed variations will be described belowtargetport attribute, numbered starting from 1. There is
usually only one console device, and 0, 1 or 2 serial devices
or parallel devices.
This disables all input on the character device, and sends output into the virtual machine's logfile
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<console type='stdio'>
<target port='1'>
</console>
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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>
...
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"
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<serial type='vc'>
<target port="1"/>
</serial>
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Connects the character device to the void. No data is ever provided to the input. All data written is discarded.
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<serial type='null'>
<target port="1"/>
</serial>
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A Pseudo TTY is allocated using /dev/ptmx. A suitable client such as 'virsh console' can connect to interact with the serial port locally.
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<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='/dv/pts/3' on the top level <console> tag. This provides compat with existing syntax for <console> tags.
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 - dont connect a serial port to a parallel port.
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<serial type="dev">
<source path="/dev/ttyS0"/>
<target port="1"/>
</serial>
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The character device acts as a TCP client connecting to a remote server, or as a server waiting for a client connection.
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<serial type="tcp">
<source mode="connect" host="0.0.0.0" service="2445"/>
<wiremode type="telnet"/>
<target port="1"/>
</serial>
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The character device acts as a UDP netconsole service, sending and receiving packets. This is a lossy service.
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<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>
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The character device acts as a UNIX domain socket server, accepting connections from local clients.
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<serial type="unix">
<source mode="bind" path="/tmp/foo"/>
<target port="1"/>
</serial>
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Example configurations for each driver are provide on the driver specific pages listed below