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. The network XML format is available since 0.3.0

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. Since 0.3.0
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. Since 0.3.0

Connectivity

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

        ...
        <bridge name="virbr0" stp="on" delay="5"/>
        <domain name="example"/>
        <forward mode="nat" dev="eth0"/>
        ...

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. Attribute stp specifies if Spanning Tree Protocol is 'on' or 'off' (default is 'on'). Attribute delay sets the bridge's forward delay value in seconds (default is 0). Since 0.3.0
domain: The name attribute on the domain element defines the DNS domain of the DHCP server. This element is optional. Since 0.4.5
forward: Inclusion of the forward element indicates that the virtual network is to be connected to the physical LAN. the mode attribute determines the method of forwarding; possible selections are 'nat' and 'route'. If mode is not specified, NAT forwarding will be used for connectivity. If a network has any IPv6 addresses defined, even if mode is given as 'nat', the IPv6 traffic will be forwarded using routing, since IPv6 has no concept of NAT. 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 mode 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. Since 0.3.0; 'mode' attribute since 0.4.2

The address attribute defines a MAC (hardware) address formatted as 6 groups of 2-digit hexadecimal numbers, the groups separated by colons (eg, "52:54:00:1C:DA:2F"). This MAC address is assigned to the bridge device when it is created. Generally it is best to not specify a MAC address when creating a network - in this case, if a defined MAC address is needed for proper operation, libvirt will automatically generate a random MAC address and save it in the config. Allowing libvirt to generate the MAC address will assure that it is compatible with the idiosyncrasies of the platform where libvirt is running. Since 0.8.8

ip

The address attribute defines an IPv4 address in dotted-decimal format, or an IPv6 address in standard colon-separated hexadecimal format, that will be configured on the bridge device associated with the virtual network. To the guests this address will be their default route. For IPv4 addresses, the netmask attribute defines the significant bits of the network address, again specified in dotted-decimal format. For IPv6 addresses, and as an alternate method for IPv4 addresses, you can specify the significant bits of the network address with the prefix attribute, which is an integer (for example, netmask='255.255.255.0' could also be given as prefix='24'. The family attribute is used to specify the type of address - 'ipv4' or 'ipv6'; if no family is given, 'ipv4' is assumed. A network can have more than one of each family of address defined, but only a single address can have a dhcp or tftp element. Since 0.3.0; IPv6, multiple addresses on a single network, family, and prefix since 0.8.7

tftp

Immediately within the ip element there is an optional tftp element. The presence of this element and of its attribute root enables TFTP services. The attribute specifies the path to the root directory served via TFTP. tftp is not supported for IPv6 addresses, can only be specified on a single IPv4 address per network. Since 0.7.1

dns

The dns element of a network contains configuration information for the virtual network's DNS server. Since 0.9.3 Currently supported elements are:

txt: A dns element can have 0 or more txt elements. Each txt element defines a DNS TXT record and has two attributes, both required: a name that can be queried via dns, and a value that will be returned when that name is queried. names cannot contain embedded spaces or commas. value is a single string that can contain multiple values separated by commas. Since 0.9.3

dhcp

Also 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. The dhcp element is not supported for IPv6, and is only supported on a single IP address per network for IPv4. Since 0.3.0

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. Since 0.3.0

host

Within the dhcp element there may be zero or more host elements; these specify hosts which will be given names and predefined IP addresses by the built-in DHCP server. Any such element must specify the MAC address of the host to be assigned a given name (via the mac attribute), the IP to be assigned to that host (via the ip attribute), and the name to be given that host by the DHCP server (via the name attribute). Since 0.4.5

bootp

The optional bootp element specifies BOOTP options to be provided by the DHCP server. Two attributes are supported: file is mandatory and gives the file to be used for the boot image; server is optional and gives the address of the TFTP server from which the boot image will be fetched. server defaults to the same host that runs the DHCP server, as is the case when the tftp element is used. The BOOTP options currently have to be the same for all address ranges and statically assigned addresses.Since 0.7.1 (server since 0.7.3).

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 mode="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>
        <ip family="ipv6" address="2001:8794:ca2:2::1" prefix="64" />
      </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 mode="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>
        <ip family="ipv6" address="2001:8794:ca2:2::1" prefix="64" />
      </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>
        <ip family="ipv6" address="2001:8794:ca2:3::1" prefix="64" />
      </network>