Network XML format

This page provides an introduction to the network XML format. For background information on the concepts referred to here, consult the network driver architecture page.

Element and attribute overview

The root element required for all virtual networks is named network and has no attributes.

General metadata

The first elements provide basic metadata about the virtual network.

      <network>
        <name>default</name>
        <uuid>3e3fce45-4f53-4fa7-bb32-11f34168b82b</uuid>
        ...

name: The content of the name element provides a short name for the virtual network. This name should consist only of alpha-numeric characters and is required to be unique within the scope of a single host. It is used to form the filename for storing the persistent configuration file.
uuid: The content of the uuid element provides a globally unique identifier for the virtual network. The format must be RFC 4122 compliant, eg 3e3fce45-4f53-4fa7-bb32-11f34168b82b. If omitted when defining/creating a new network, a random UUID is generated.

Connectivity

The next set of elements control how a virtual network is provided connectivity to the physical LAN (if at all).

        ...
        <bridge name="virbr0" />
	<forward type="nat"/>
	...

bridge: The name attribute on the bridge element defines the name of a bridge device which will be used to construct the virtual network. The virtual machines will be connected to this bridge device allowing them to talk to each other. The bridge device may also be connected to the LAN. It is recommended that bridge device names started with the prefix vir, but the name virbr0 is reserved for the "default" virtual network. This element should always be provided when defining a new network
forward: Inclusion of the forward element indicates that the virtual network is to be connected to the physical LAN. If no attributes are set, NAT forwarding will be used for connectivity. Firewall rules will allow forwarding to any other network device whether ethernet, wireless, dialup, or VPN. If the dev attribute is set, the firewall rules will restrict forwarding to the named device only. If the type attribute is set to route then the traffic will not have NAT applied. This presumes that the local LAN router has suitable routing table entries to return traffic to this host.

Addressing

The final set of elements define the IPv4 address range available, and optionally enable DHCP sevices.

        ...
	<ip address="192.168.122.1" netmask="255.255.255.0">
	  <dhcp>
	    <range start="192.168.122.2" end="192.168.122.254" />
	  </dhcp>
	</ip>
      </network>

ip: The address attribute defines an IPv4 address in dotted-decimal format, that will be configured on the bridge device associated with the virtual network. To the guests this address will be their default route. The netmask attribute defines the significant bits of the network address, again specified in dotted-decimal format.
dhcp: Immediately within the ip element there is an optional dhcp element. The presence of this element enables DHCP services on the virtual network. It will further contain one or more range elements.
range: The start and end attributes on the range element specify the boundaries of a pool of IPv4 addresses to be provided to DHCP clients. These two addresses must lie within the scope of the network defined on the parent ip element.

Example configuration

NAT based network

This example is the so called "default" virtual network. It is provided and enabled out-of-the-box for all libvirt installations. This is a configuration that allows guest OS to get outbound connectivity regardless of whether the host uses ethernet, wireless, dialup, or VPN networking without requiring any specific admin configuration. In the absence of host networking, it at least allows guests to talk directly to each other.

      <network>
	<name>default</name>
	<bridge name="virbr0" />
	<forward type="nat"/>
	<ip address="192.168.122.1" netmask="255.255.255.0">
	  <dhcp>
	    <range start="192.168.122.2" end="192.168.122.254" />
	  </dhcp>
	</ip>
      </network>

Routed network config

This is a variant on the default network which routes traffic from the virtual network to the LAN without applying any NAT. It requires that the IP address range be pre-configured in the routing tables of the router on the host network. This example further specifies that guest traffic may only go out via the eth1 host network device.

      <network>
	<name>local</name>
	<bridge name="virbr1" />
	<forward type="route" dev="eth1"/>
	<ip address="192.168.122.1" netmask="255.255.255.0">
	  <dhcp>
	    <range start="192.168.122.2" end="192.168.122.254" />
	  </dhcp>
	</ip>
      </network>

Isolated network config

This variant provides a completely isolated private network for guests. The guests can talk to each other, and the host OS, but cannot reach any other machines on the LAN, due to the omission of the forward element in the XML description.

      <network>
	<name>private</name>
	<bridge name="virbr2" />
	<ip address="192.168.152.1" netmask="255.255.255.0">
	  <dhcp>
	    <range start="192.168.152.2" end="192.168.152.254" />
	  </dhcp>
	</ip>
      </network>