# Network XML format

- Element and attribute overview
  - General metadata
  - Connectivity
  - Addressing
- Example configuration
  - NAT based network
  - Routed network config
  - Isolated network config
  - Isolated IPv6 network config
  - Using an existing host bridge
  - Using a macvtap "direct" connection
  - Network config with no gateway addresses

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 configurable attributes (although since 0.10.0 there is one optional read-only attribute - when examining the live configuration of a network, the attribute `connections`, if present, specifies the number of guest interfaces currently connected via this network). The network XML format is available since 0.3.0

### General metadata

The first elements provide basic metadata about the virtual network.

          <network ipv6='yes' trustGuestRxFilters='no'>
            <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
`ipv6='yes'`
  The new, optional parameter `ipv6='yes'` enables a network definition with no IPv6 gateway addresses specified to have guest-to-guest communications. For further information, see the example below for the example with no gateway addresses. Since 1.0.1
`trustGuestRxFilters='yes'`
  The optional parameter `trustGuestRxFilters` can be used to set that attribute of the same name for each domain interface connected to this network (since 1.2.10). See the Network interfaces section of the domain XML documentation for more details. Note that an explicit setting of this attribute in a portgroup or the individual domain interface will override the setting in the network.

### 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" macTableManager="libvirt"/>
            <domain name="example.com" localOnly="no"/>
            <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. When defining a new network with a `<forward>` mode of "nat" or "route" (or an isolated network with no `<forward>` element), libvirt will automatically generate a unique name for the bridge device if none is given, and this name will be permanently stored in the network configuration so that that the same name will be used every time the network is started. For these types of networks (nat, routed, and isolated), a bridge name beginning with the prefix "virbr" is recommended (and that is what is auto-generated), but not enforced. 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

  The `macTableManager` attribute of the bridge element is used to tell libvirt how the bridge's MAC address table (used to determine the correct egress port for packets based on destination MAC address) will be managed. In the default `kernel` setting, the kernel automatically adds and removes entries, typically using learning, flooding, and promiscuous mode on the bridge's ports in order to determine the proper egress port for packets. When `macTableManager` is set to `libvirt`, libvirt disables kernel management of the MAC table (in the case of the Linux host bridge, this means enabling vlan_filtering on the bridge, and disabling learning and unicast_filter for all bridge ports), and explicitly adds/removes entries to the table according to the MAC addresses in the domain interface configurations. Allowing libvirt to manage the MAC table can improve performance - with a Linux host bridge, for example, turning off learning and unicast_flood on ports has its own performance advantage, and can also lead to an additional boost by permitting the kernel to automatically turn off promiscuous mode on some ports of the bridge (in particular, the port attaching the bridge to the physical network). However, it can also cause some networking setups to stop working (e.g. vlan tagging, multicast, guest-initiated changes to MAC address) and is not supported by older kernels. Since 1.2.11, requires kernel 3.17 or newer

`domain`
  The `name` attribute on the `domain` element defines the DNS domain of the DHCP server. This element is optional, and is only used for those networks with a `<forward>` mode of "nat" or "route" (or an isolated network with no `<forward>` element). Since 0.4.5

  If the optional `localOnly` attribute on the `domain` element is "yes", then DNS requests under this domain will only be resolved by the virtual network's own DNS server - they will not be forwarded to the host's upstream DNS server. If `localOnly` is "no", and by default, unresolved requests will be forwarded. Since 1.2.12

`forward`
  Inclusion of the `forward` element indicates that the virtual network is to be connected to the physical LAN.Since 0.3.0. The `mode` attribute determines the method of forwarding. If there is no `forward` element, the network will be isolated from any other network (unless a guest connected to that network is acting as a router, of course). The following are valid settings for `mode` (if there is a `forward` element but mode is not specified, `mode='nat'` is assumed):
  `nat`
    All traffic between guests connected to this network and the physical network will be forwarded to the physical network via the host's IP routing stack, after the guest's IP address is translated to appear as the host machine's public IP address (a.k.a. Network Address Translation, or "NAT"). This allows multiple guests, all having access to the physical network, on a host that is only allowed a single public IP address. If a network has any IPv6 addresses defined, the IPv6 traffic will be forwarded using plain routing, since IPv6 has no concept of NAT. Firewall rules will allow outbound connections 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. Inbound connections from other networks are all prohibited; all connections between guests on the same network, and to/from the host to the guests, are unrestricted and not NATed.Since 0.4.2

    Since 1.0.3 it is possible to specify a public IPv4 address and port range to be used for the NAT by using the `<nat>` subelement. Note that all addresses from the range are used, not just those that are in use on the host. The address range is set with the `<address>` subelements and `start` and `stop` attributes:

        ...
          <forward mode='nat'>
            <nat>
              <address start='1.2.3.4' end='1.2.3.10'/>
            </nat>
          </forward>
        ...

    A single IPv4 address can be set by setting `start` and `end` attributes to the same value.

    The port range to be used for the `<nat>` can be set via the subelement `<port>`:

        ...
          <forward mode='nat'>
            <nat>
              <port start='500' end='1000'/>
            </nat>
          </forward>
        ...

  `route`
    Guest network traffic will be forwarded to the physical network via the host's IP routing stack, but without having NAT applied. Again, if the `dev` attribute is set, firewall rules will restrict forwarding to the named device only. This presumes that the local LAN router has suitable routing table entries to return traffic to this host. All incoming and outgoing sessions to guest on these networks are unrestricted. (To restrict incoming traffic to a guest on a routed network, you can configure nwfilter rules on the guest's interfaces.) Since 0.4.2
  `bridge`
    This network describes either 1) an existing host bridge that was configured outside of libvirt (if a `<bridge name='xyz'/>` element has been specified, Since 0.9.4), 2) an existing Open vSwitch bridge that was configured outside of libvirt (if both a `<bridge name='xyz'/>` element and a `<virtualport type='openvswitch'/>` have been specified Since 0.10.0) 3) an interface or group of interfaces to be used for a "direct" connection via macvtap using macvtap's "bridge" mode (if the forward element has one or more `<interface>` subelements, Since 0.9.4) (see Direct attachment to physical interface for descriptions of the various macvtap modes). libvirt doesn't attempt to manage the bridge interface at all, thus the `<bridge>` element's `stp` and `delay` attributes are not allowed; no iptables rules, IP addresses, or DHCP/DNS services are added; at the IP level, the guest interface appears to be directly connected to the physical interface.Since 0.9.4
  `private`
    This network uses a macvtap "direct" connection in "private" mode to connect each guest to the network. The physical interface to be used will be picked from among those listed in `<interface>` subelements of the `<forward>` element; when using 802.1Qbh mode (as indicated by the `<virtualport>` type attribute - note that this requires an 802.1Qbh-capable hardware switch), each physical interface can only be in use by a single guest interface at a time; in modes other than 802.1Qbh, multiple guest interfaces can share each physical interface (libvirt will attempt to balance usage between all available interfaces).Since 0.9.4
  `vepa`
    This network uses a macvtap "direct" connection in "vepa" mode to connect each guest to the network (this requires that the physical interfaces used be connected to a vepa-capable hardware switch. The physical interface to be used will be picked from among those listed in `<interface>` subelements of the `<forward>` element; multiple guest interfaces can share each physical interface (libvirt will attempt to balance usage between all available interfaces).Since 0.9.4
  `passthrough`
    This network uses a macvtap "direct" connection in "passthrough" mode to connect each guest to the network (note that this is not the same thing as "PCI passthrough"). The physical interface to be used will be picked from among those listed in `<interface>` subelements of the `<forward>` element. Each physical interface can only be in use by a single guest interface at a time, so libvirt will keep track of which interfaces are currently in use, and only assign unused interfaces (if there are no available physical interfaces when a domain interface is being attached, an error will be logged, and the operation causing the attach will fail (usually either a domain start, or a hotplug interface attach to a domain).Since 0.9.4
  `hostdev`
    This network facilitates PCI Passthrough of a network device. A network device is chosen from the interface pool and directly assigned to the guest using generic device passthrough, after first optionally setting the device's MAC address and vlan tag to the configured value, and optionally associating the device with an 802.1Qbh capable switch using a `<virtualport>` element. Note that - due to limitations in standard single-port PCI ethernet card driver design - only SR-IOV (Single Root I/O Virtualization) virtual function (VF) devices can be assigned in this manner; to assign a standard single-port PCI or PCIe ethernet card to a guest, use the traditional `<hostdev>` device definition. Since 0.10.0

    To force use of a particular type of device assignment, a <forward type='hostdev'> interface can have an optional `driver` sub-element with a `name` attribute set to either "vfio" (VFIO is a new method of device assignment that is compatible with UEFI Secure Boot) or "kvm" (the legacy device assignment handled directly by the KVM kernel module) Since 1.0.5 (QEMU and KVM only, requires kernel 3.6 or newer). When specified, device assignment will fail if the requested method of device assignment isn't available on the host. When not specified, the default is "vfio" on systems where the VFIO driver is available and loaded, and "kvm" on older systems, or those where the VFIO driver hasn't been loaded Since 1.1.3 (prior to that the default was always "kvm").

    Note that this "intelligent passthrough" of network devices is very similar to the functionality of a standard `<hostdev>` device, the difference being that this method allows specifying a MAC address, vlan tag, and `<virtualport>` for the passed-through device. If these capabilities are not required, if you have a standard single-port PCI, PCIe, or USB network card that doesn't support SR-IOV (and hence would anyway lose the configured MAC address during reset after being assigned to the guest domain), or if you are using a version of libvirt older than 0.10.0, you should use a standard `<hostdev>` device definition in the domain's configuration to assign the device to the guest instead of defining an `<interface type='network'>` pointing to a network with `<forward mode='hostdev'/>`.

  As mentioned above, a `<forward>` element can have multiple `<interface>` subelements, each one giving the name of a physical interface that can be used for this network Since 0.9.4:

      ...
        <forward mode='passthrough'>
          <interface dev='eth10'/>
          <interface dev='eth11'/>
          <interface dev='eth12'/>
          <interface dev='eth13'/>
          <interface dev='eth14'/>
        </forward>
      ...

  since 0.10.0, `<interface>` also has an optional read-only attribute - when examining the live configuration of a network, the attribute `connections`, if present, specifies the number of guest interfaces currently connected via this physical interface.

  Additionally, since 0.9.10, libvirt allows a shorthand for specifying all virtual interfaces associated with a single physical function, by using the `<pf>` subelement to call out the corresponding physical interface associated with multiple virtual interfaces:

      ...
        <forward mode='passthrough'>
          <pf dev='eth0'/>
        </forward>
      ...

  When a guest interface is being constructed, libvirt will pick an interface from this list to use for the connection. In modes where physical interfaces can be shared by multiple guest interfaces, libvirt will choose the interface that currently has the least number of connections. For those modes that do not allow sharing of the physical device (in particular, 'passthrough' mode, and 'private' mode when using 802.1Qbh), libvirt will choose an unused physical interface or, if it can't find an unused interface, fail the operation.

  since 0.10.0 When using forward mode 'hostdev', the interface pool is specified with a list of `<address>` elements, each of which has `<type>` (must always be `'pci'`), `<domain>`, `<bus>`, `<slot>`and `<function>` attributes.

      ...
        <forward mode='hostdev' managed='yes'>
          <driver name='vfio'/>
          <address type='pci' domain='0' bus='4' slot='0' function='1'/>
          <address type='pci' domain='0' bus='4' slot='0' function='2'/>
          <address type='pci' domain='0' bus='4' slot='0' function='3'/>
        </forward>
      ...

  Alternatively the interface pool can also be defined using a single physical function `<pf>` subelement to call out the corresponding physical interface associated with multiple virtual interfaces (similar to passthrough mode):

      ...
        <forward mode='hostdev' managed='yes'>
          <pf dev='eth0'/>
        </forward>
      ...

##### Quality of service

    ...
      <forward mode='nat' dev='eth0'/>
      <bandwidth>
        <inbound average='1000' peak='5000' burst='5120'/>
        <outbound average='128' peak='256' burst='256'/>
      </bandwidth>
    ...

The `<bandwidth>` element allows setting quality of service for a particular network (since 0.9.4). Setting `bandwidth` for a network is supported only for networks with a `<forward>` mode of `route`, `nat`, or no mode at all (i.e. an "isolated" network). Setting `bandwidth` is not supported for forward modes of `bridge`, `passthrough`, `private`, or `hostdev`. Attempts to do this will lead to a failure to define the network or to create a transient network.

The `<bandwidth>` element can only be a subelement of a domain's `<interface>`, a subelement of a `<network>`, or a subelement of a `<portgroup>` in a `<network>`.

As a subelement of a domain's `<interface>`, the bandwidth only applies to that one interface of the domain. As a subelement of a `<network>`, the bandwidth is a total aggregate bandwidth to/from all guest interfaces attached to that network, not to each guest interface individually. If a domain's `<interface>` has `<bandwidth>` element values higher than the aggregate for the entire network, then the aggregate bandwidth for the `<network>` takes precedence. This is because the two choke points are independent of each other where the domain's `<interface>` bandwidth control is applied on the interface's tap device, while the `<network>` bandwidth control is applied on the interface part of the bridge device created for that network.

As a subelement of a `<portgroup>` in a `<network>`, if a domain's `<interface>` has a `portgroup` attribute in its `<source>` element and if the `<interface>` itself has no `<bandwidth>` element, then the `<bandwidth>` element of the portgroup will be applied individually to each guest interface defined to be a member of that portgroup. Any `<bandwidth>` element in the domain's `<interface>` definition will override the setting in the portgroup (since 1.0.1).

Incoming and outgoing traffic can be shaped independently. The `bandwidth` element can have at most one `inbound` and at most one `outbound` child element. Leaving either of these children elements out results in no QoS applied for that traffic direction. So, when you want to shape only incoming traffic, use `inbound` only, and vice versa. Each of these elements have one mandatory attribute - `average` (or `floor` as described below). The attributes are as follows, where accepted values for each attribute is an integer number.

`average`
  Specifies the desired average bit rate for the interface being shaped (in kilobytes/second).
`peak`
  Optional attribute which specifies the maximum rate at which the bridge can send data (in kilobytes/second). Note the limitation of implementation: this attribute in the `outbound` element is ignored (as Linux ingress filters don't know it yet).
`burst`
  Optional attribute which specifies the amount of kilobytes that can be transmitted in a single burst at `peak` speed.
`floor`
  Optional attribute available only for the `inbound` element. This attribute guarantees minimal throughput for shaped interfaces. This, however, requires that all traffic goes through one point where QoS decisions can take place, hence why this attribute works only for virtual networks for now (that is `<interface type='network'/>` with a forward type of route, nat, or no forward at all). Moreover, the virtual network the interface is connected to is required to have at least inbound QoS set (`average` at least). If using the `floor` attribute users don't need to specify `average`. However, `peak` and `burst` attributes still require `average`. Currently, the Linux kernel doesn't allow ingress qdiscs to have any classes therefore `floor` can be applied only on `inbound` and not `outbound`.

Attributes `average`, `peak`, and `burst` are available since 0.9.4, while the `floor` attribute is available since 1.0.1.

##### Setting VLAN tag (on supported network types only)

    <network>
      <name>ovs-net</name>
      <forward mode='bridge'/>
      <bridge name='ovsbr0'/>
      <virtualport type='openvswitch'>
        <parameters interfaceid='09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f'/>
      </virtualport>
      <vlan trunk='yes'>
        <tag id='42' nativeMode='untagged'/>
        <tag id='47'/>
      </vlan>
      <portgroup name='dontpanic'>
        <vlan>
          <tag id='42'/>
        </vlan>
      </portgroup>
    </network>

If (and only if) the network type supports vlan tagging transparent to the guest, an optional `<vlan>` element can specify one or more vlan tags to apply to the traffic of all guests using this network Since 0.10.0. (openvswitch and type='hostdev' SR-IOV networks do support transparent vlan tagging of guest traffic; everything else, including standard linux bridges and libvirt's own virtual networks, do not support it. 802.1Qbh (vn-link) and 802.1Qbg (VEPA) switches provide their own way (outside of libvirt) to tag guest traffic onto specific vlans.) As expected, the `tag` attribute specifies which vlan tag to use. If a network has more than one `<vlan>` element defined, it is assumed that the user wants to do VLAN trunking using all the specified tags. In the case that vlan trunking with a single tag is desired, the optional attribute `trunk='yes'` can be added to the vlan element.

For network connections using openvswitch it is possible to configure the 'native-tagged' and 'native-untagged' vlan modes Since 1.1.0. This uses the optional `nativeMode` attribute on the `<tag>` element: `nativeMode` may be set to 'tagged' or 'untagged'. The id attribute of the element sets the native vlan.

`<vlan>` elements can also be specified in a `<portgroup>` element, as well as directly in a domain's `<interface>` element. In the case that a vlan tag is specified in multiple locations, the setting in `<interface>` takes precedence, followed by the setting in the `<portgroup>` selected by the interface config. The `<vlan>` in `<network>` will be selected only if none is given in `<portgroup>` or `<interface>`.

##### Portgroups

    ...
      <forward mode='private'/>
        <interface dev="eth20"/>
        <interface dev="eth21"/>
        <interface dev="eth22"/>
        <interface dev="eth23"/>
        <interface dev="eth24"/>
      </forward>
      <portgroup name='engineering' default='yes'>
        <virtualport type='802.1Qbh'>
          <parameters profileid='test'/>
        </virtualport>
        <bandwidth>
          <inbound average='1000' peak='5000' burst='5120'/>
          <outbound average='1000' peak='5000' burst='5120'/>
        </bandwidth>
      </portgroup>
      <portgroup name='sales' trustGuestRxFilters='no'>
        <virtualport type='802.1Qbh'>
          <parameters profileid='salestest'/>
        </virtualport>
        <bandwidth>
          <inbound average='500' peak='2000' burst='2560'/>
          <outbound average='128' peak='256' burst='256'/>
        </bandwidth>
      </portgroup>
    ...

Since 0.9.4 A portgroup provides a method of easily putting guest connections to the network into different classes, with each class potentially having a different level/type of service. Since 0.9.4 Each network can have multiple portgroup elements (and one of those can optionally be designated as the 'default' portgroup for the network), and each portgroup has a name, as well as various attributes and subelements associated with it. The currently supported subelements are `<bandwidth>` (described here) and `<virtualport>` (documented here). If a domain interface definition specifies a portgroup (by adding a `portgroup` attribute to the `<source>` subelement), that portgroup's info will be merged into the interface's configuration. If no portgroup is given in the interface definition, and one of the network's portgroups has `default='yes'`, that default portgroup will be used. If no portgroup is given in the interface definition, and there is no default portgroup, then none will be used. Any `<bandwidth>` specified directly in the domain XML will take precedence over any setting in the chosen portgroup. if a `<virtualport>` is specified in the portgroup (and/or directly in the network definition), the multiple virtualports will be merged, and any parameter that is specified in more than one virtualport, and is not identical, will be considered an error, and will prevent the interface from starting.

portgroups also support the optional parameter `trustGuestRxFilters` which can be used to set that attribute of the same name for each domain interface using this portgroup (since 1.2.10). See the Network interfaces section of the domain XML documentation for more details. Note that an explicit setting of this attribute in the portgroup overrides the network-wide setting, and an explicit setting in the individual domain interface will override the setting in the portgroup.

##### Static Routes

Static route definitions are used to provide routing information to the virtualization host for networks which are not directly reachable from the virtualization host, but *are* reachable from a guest domain that is itself reachable from the host since 1.0.6.

As shown in this example, it is possible to define a virtual network interface with no IPv4 or IPv6 addresses. Such networks are useful to provide host connectivity to networks which are only reachable via a guest. A guest with connectivity both to the guest-only network and to another network that is directly reachable from the host can act as a gateway between the networks. A static route added to the "host-visible" network definition provides the routing information so that IP packets can be sent from the virtualization host to guests on the hidden network.

Here is a fragment of a definition which shows the static route specification as well as the IPv4 and IPv6 definitions for network addresses which are referred to in the `gateway` gateway address specifications. Note that the third static route specification includes the `metric` attribute specification with a value of 2. This particular route would *not* be preferred if there was another existing rout on the system with the same address and prefix but with a lower value for the metric. If there is a route in the host system configuration that should be overridden by a route in a virtual network whenever the virtual network is running, the configuration for the system-defined route should be modified to have a higher metric, and the route on the virtual network given a lower metric (for example, the default metric of "1").

          ...
            <ip address="192.168.122.1" netmask="255.255.255.0">
              <dhcp>
                <range start="192.168.122.128" end="192.168.122.254" />
              </dhcp>
            </ip>
            <route address="192.168.222.0" prefix="24" gateway="192.168.122.2" />
            <ip family="ipv6" address="2001:db8:ca2:2::1" prefix="64" />
            <route family="ipv6" address="2001:db8:ca2:3::" prefix="64" gateway="2001:db8:ca2:2::2"/>
            <route family="ipv6" address="2001:db9:4:1::" prefix="64" gateway="2001:db8:ca2:2::3" metric='2'>
            </route>
          ...

### Addressing

The final set of elements define the addresses (IPv4 and/or IPv6, as well as MAC) to be assigned to the bridge device associated with the virtual network, and optionally enable DHCP services. These elements are only valid for isolated networks (no `forward` element specified), and for those with a forward mode of 'route' or 'nat'.

            ...
            <mac address='00:16:3E:5D:C7:9E'/>
            <domain name="example.com"/>
            <dns>
              <txt name="example" value="example value" />
              <forwarder addr="8.8.8.8"/>
              <forwarder addr="8.8.4.4"/>
              <srv service='name' protocol='tcp' domain='test-domain-name' target='.' port='1024' priority='10' weight='10'/>
              <host ip='192.168.122.2'>
                <hostname>myhost</hostname>
                <hostname>myhostalias</hostname>
              </host>
            </dns>
            <ip address="192.168.122.1" netmask="255.255.255.0">
              <dhcp>
                <range start="192.168.122.100" end="192.168.122.254" />
                <host mac="00:16:3e:77:e2:ed" name="foo.example.com" ip="192.168.122.10" />
                <host mac="00:16:3e:3e:a9:1a" name="bar.example.com" ip="192.168.122.11" />
              </dhcp>
            </ip>
            <ip family="ipv6" address="2001:db8:ca2:2::1" prefix="64" />
            <route family="ipv6" address="2001:db9:ca1:1::" prefix="64" gateway="2001:db8:ca2:2::2" />
          </network>

`mac`
  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
`dns`
  The dns element of a network contains configuration information for the virtual network's DNS server Since 0.9.3.

  The dns element can have an optional `forwardPlainNames` attribute Since 1.1.2. If `forwardPlainNames` is "no", then DNS resolution requests for names that are not qualified with a domain (i.e. names with no "." character) will not be forwarded to the host's upstream DNS server - they will only be resolved if they are known locally within the virtual network's own DNS server. If `forwardPlainNames` is "yes", unqualified names will be forwarded to the upstream DNS server if they can't be resolved by the virtual network's own DNS server.

  Currently supported sub-elements of `<dns>` are:
  `forwarder`
    A `dns` element can have 0 or more `forwarder` elements. Each forwarder element defines an IP address to be used as forwarder in DNS server configuration. The addr attribute is required and defines the IP address of every forwarder. Since 1.1.3
  `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
  `host`
    The `host` element within `dns` is the definition of DNS hosts to be passed to the DNS service. The IP address is identified by the `ip` attribute and the names for that IP address are identified in the `hostname` sub-elements of the `host` element. Since 0.9.3
  `srv`
    The `dns` element can have also 0 or more `srv` record elements. Each `srv` record element defines a DNS SRV record and has 2 mandatory and 5 optional attributes. The mandatory attributes are service name and protocol (tcp, udp) and the optional attributes are target, port, priority, weight and domain as defined in DNS server SRV RFC (RFC 2782). Since 0.9.9
`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 IPv4 address will be their IPv4 default route. For IPv6, the default route is established via Router Advertisement. 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 IPv4 address can have a `dhcp` or `tftp` element. Since 0.3.0 IPv6, multiple addresses on a single network, `family`, and `prefix` are support Since 0.8.7. Similar to IPv4, one IPv6 address per network can also have a `dhcp` definition. Since 1.0.1
  `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, and can only be specified on a single IPv4 address per network. Since 0.7.1
  `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 supported for both IPv4 Since 0.3.0 and IPv6 Since 1.0.1, but only for one IP address of each type per network.
    `range`
      The `start` and `end` attributes on the `range` element specify the boundaries of a pool of addresses to be provided to DHCP clients. These two addresses must lie within the scope of the network defined on the parent `ip` element. There may be zero or more `range` elements specified. Since 0.3.0 `range` can be specified for one IPv4 address, one IPv6 address, or both. Since 1.0.1
    `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 IPv4 `host` 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 An IPv6 `host` element differs slightly from that for IPv4: there is no `mac` attribute since a MAC address has no defined meaning in IPv6. Instead, the `name` attribute is used to identify the host to be assigned the IPv6 address. For DHCPv6, the name is the plain name of the client host sent by the client to the server. Note that this method of assigning a specific IP address can also be used instead of the `mac` attribute for IPv4. Since 1.0.1
    `bootp`
      The optional `bootp` element specifies BOOTP options to be provided by the DHCP server for IPv4 only. 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:db8:ca2:2::1" prefix="64" />
          </network>

Below is a variation of the above example which adds an IPv6 dhcp range definition.

          <network>
            <name>default6</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:db8:ca2:2::1" prefix="64" >
              <dhcp>
                <range start="2001:db8:ca2:2:1::10" end="2001:db8:ca2:2:1::ff" />
              </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 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:db8:ca2:2::1" prefix="64" />
          </network>

Below is another IPv6 variation. Instead of a dhcp range being specified, this example has a couple of IPv6 host definitions. Note that most of the dhcp host definitions use an "id" (client id or DUID) since this has proven to be a more reliable way of specifying the interface and its association with an IPv6 address. The first is a DUID-LLT, the second a DUID-LL, and the third a DUID-UUID. Since 1.0.3

          <network>
            <name>local6</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:db8:ca2:2::1" prefix="64" >
              <dhcp>
                <host name="paul"   ip="2001:db8:ca2:2:3::1" />
                <host id="0:1:0:1:18:aa:62:fe:0:16:3e:44:55:66"    ip="2001:db8:ca2:2:3::2" />
                <host id="0:3:0:1:0:16:3e:11:22:33" name="ralph"  ip="2001:db8:ca2:2:3::3" />
                <host id="0:4:7e:7d:f0:7d:a8:bc:c5:d2:13:32:11:ed:16:ea:84:63" name="badbob" ip="2001:db8:ca2:2:3::4" />
              </dhcp>
            </ip>
          </network>

Below is yet another IPv6 variation. This variation has only IPv6 defined with DHCPv6 on the primary IPv6 network. A static link if defined for a second IPv6 network which will not be directly visible on the bridge interface but there will be a static route defined for this network via the specified gateway. Note that the gateway address must be directly reachable via (on the same subnet as) one of the <ip> addresses defined for this <network>. Since 1.0.6

          <network>
            <name>net7</name>
            <bridge name="virbr7" />
            <forward mode="route"/>
            <ip family="ipv6" address="2001:db8:ca2:7::1" prefix="64" >
              <dhcp>
                <range start="2001:db8:ca2:7::100" end="2001:db8:ca2::1ff" />
                <host id="0:4:7e:7d:f0:7d:a8:bc:c5:d2:13:32:11:ed:16:ea:84:63" name="lucas" ip="2001:db8:ca2:2:3::4" />
              </dhcp>
            </ip>
            <route family="ipv6" address="2001:db8:ca2:8::" prefix="64" gateway="2001:db8:ca2:7::4" >
            </route>
          </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:db8:ca2:3::1" prefix="64" />
          </network>

### Isolated IPv6 network config

This variation of an isolated network defines only IPv6. Note that most of the dhcp host definitions use an "id" (client id or DUID) since this has proven to be a more reliable way of specifying the interface and its association with an IPv6 address. The first is a DUID-LLT, the second a DUID-LL, and the third a DUID-UUID. Since 1.0.3

          <network>
            <name>sixnet</name>
            <bridge name="virbr6" />
            <ip family="ipv6" address="2001:db8:ca2:6::1" prefix="64" >
              <dhcp>
                <host name="peter"   ip="2001:db8:ca2:6:6::1" />
                <host id="0:1:0:1:18:aa:62:fe:0:16:3e:44:55:66" ip="2001:db8:ca2:6:6::2" />
                <host id="0:3:0:1:0:16:3e:11:22:33" name="dariusz" ip="2001:db8:ca2:6:6::3" />
                <host id="0:4:7e:7d:f0:7d:a8:bc:c5:d2:13:32:11:ed:16:ea:84:63" name="anita" ip="2001:db8:ca2:6:6::4" />
              </dhcp>
            </ip>
          </network>

### Using an existing host bridge

Since 0.9.4 This shows how to use a pre-existing host bridge "br0". The guests will effectively be directly connected to the physical network (i.e. their IP addresses will all be on the subnet of the physical network, and there will be no restrictions on inbound or outbound connections).

          <network>
            <name>host-bridge</name>
            <forward mode="bridge"/>
            <bridge name="br0"/>
          </network>

### Using a macvtap "direct" connection

Since 0.9.4, QEMU and KVM only, requires Linux kernel 2.6.34 or newer This shows how to use macvtap to connect to the physical network directly through one of a group of physical devices (without using a host bridge device). As with the host bridge network, the guests will effectively be directly connected to the physical network so their IP addresses will all be on the subnet of the physical network, and there will be no restrictions on inbound or outbound connections. Note that, due to a limitation in the implementation of macvtap, these connections do not allow communication directly between the host and the guests - if you require this you will either need the attached physical switch to be operating in a mirroring mode (so that all traffic coming to the switch is reflected back to the host's interface), or provide alternate means for this communication (e.g. a second interface on each guest that is connected to an isolated network). The other forward modes that use macvtap (private, vepa, and passthrough) would be used in a similar fashion.

          <network>
            <name>direct-macvtap</name>
            <forward mode="bridge">
              <interface dev="eth20"/>
              <interface dev="eth21"/>
              <interface dev="eth22"/>
              <interface dev="eth23"/>
              <interface dev="eth24"/>
            </forward>
          </network>

### Network config with no gateway addresses

A valid network definition can contain no IPv4 or IPv6 addresses. Such a definition can be used for a "very private" or "very isolated" network since it will not be possible to communicate with the virtualization host via this network. However, this virtual network interface can be used for communication between virtual guest systems. This works for IPv4 and (Since 1.0.1) IPv6. However, the new ipv6='yes' must be added for guest-to-guest IPv6 communication.

          <network ipv6='yes'>
            <name>nogw</name>
            <uuid>7a3b7497-1ec7-8aef-6d5c-38dff9109e93</uuid>
            <bridge name="virbr2" stp="on" delay="0" />
            <mac address='00:16:3E:5D:C7:9E'/>
          </network>