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
fv0 4dea22b31d52d8f32516782e98ab3fa0 ...
name
The content of the
name 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 0.0.1
uuid
The content of the
uuid 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 0.0.1
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.
... hvm /usr/lib/xen/boot/hvmloader
...
type
The content of the
type 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 referring to the machine type. The Capabilities XML provides details on allowed values for these.
Since 0.0.1
loader
The optional
loader tag refers to a firmware blob used to assist the domain creation process. At this time, it is only 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 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
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.
... /usr/bin/pygrub --append single ...
bootloader
The content of the
bootloader element provides a fully qualified 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 dependent on the hypervisor in use.
Since 0.1.0
bootloader_args
The optional
bootloader_args element allows command 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.
This element has the same semantics as described earlier in the BIOS boot section
loader
This element has the same semantics as described earlier in the BIOS boot section
kernel
The contents of this element specify the fully-qualified path to the kernel image in the host OS.
initrd
The contents of this element specify the fully-qualified path to the (optional) ramdisk image in the host OS.
cmdline
The contents of this element specify arguments to be passed to the kernel (or installer) at boottime. This is often used to specify an alternate primary console (eg serial port), or the installation media source / kickstart file
Basic resources
... 524288 524288 1 ...
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 value be less than the maximum allocation, to allow for ballooning up the guests memory on the fly. If this is omitted, it defaults to the same value as the
memory element
vcpu
The content of this element defines the number of virtual CPUs 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.
... destroy restart restart ...
on_poweroff
The content of this element specifies the action to take when the guest requests a poweroff.
on_reboot
The content of this element specifies the action to take when the guest requests a reboot.
on_crash
The content of this element specifies the action to take when the guest crashes.
Each of these states allow for the same four possible actions.
destroy
The domain will be terminated completely and all resources released
restart
The domain will be terminated, and then restarted with the same configuration
preserve
The domain will be terminated, and its resource preserved to allow analysis.
rename-restart
The domain will be terminated, and then restarted with a new name
Hypervisor features
Hypervisors may allow certain CPU / machine features to be toggled on/off.
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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 guests to address more than 4 GB of memory.
acpi
ACPI is useful for power management, for example, with KVM 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
The
sync attribute takes either "utc" or "localtime" to specify how the guest clock is initialized in 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
... /usr/lib/xen/bin/qemu-dm ...
emulator
The contents of the
emulator 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.
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.
...
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disk
The
disk 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 0.1.4
source
If the disk
type 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 0.0.3
target
The
target 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 guaranteed 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" or "usb". 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 0.4.3; "usb" attribute value since after 0.4.4
driver
If the hypervisor supports multiple backend drivers, then the optional
driver 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
USB devices
USB devices attached to the host can be passed through to the guest using the hostdev element. since after 0.4.4
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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
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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 element.
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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 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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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.
... ...
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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 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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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.
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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.
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Setting the NIC model
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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:
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.
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input
The
input element has one mandatory 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".
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
The
graphics 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 attribute specifies the TCP port number (with -1 as legacy syntax indicating that it should be auto-allocated). The
autoport attribute is the new preferred syntax for indicating autoallocation of the TCP port to use. The
listen attribute is an IP address for the server to listen on. The
password attribute provides a VNC password in 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.
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parallel
Represents a parallel port
serial
Represents a serial port
console
Represents the primary console. This can be the paravirtualized console with Xen guests, or duplicates the primary serial port for fully virtualized guests without a paravirtualized console.
source
The attributes available for the
source element vary according to the
type attribute on the parent tag. Allowed variations will be described below
target
The port number of the character device is specified via the
port attribute, numbered starting from 1. There is usually only one console device, and 0, 1 or 2 serial devices or parallel devices.
Domain logfile
This disables all input on the character device, and sends output into the virtual machine's logfile
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Device logfile
A file is opened and all data sent to the character device is written to the file.
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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"
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Null device
Connects the character device to the void. No data is ever provided to the input. All data written is discarded.
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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.
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NB special case if , then the TTY path is also duplicated as an attribute tty='/dev/pts/3' on the top level tag. This provides compat with existing syntax for 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.
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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.
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UDP network console
The character device acts as a UDP netconsole service, sending and receiving packets. This is a lossy service.
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UNIX domain socket client/server
The character device acts as a UNIX domain socket server, accepting connections from local clients.
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Example configs
Example configurations for each driver are provide on the driver specific pages listed below
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