[Libvir] PATCH: Add docs on the network XML format

<p> This page provides an introduction to the network XML format. For background information on the concepts referred to here, consult the <a href="archnetwork.html">network driver architecture</a> page. </p> <h2>Element and attribute overview</h2> <p> The root element required for all virtual networks is named <code>network</code> and has no attributes. </p> <h3>General metadata</h3> <p> The first elements provide basic metadata about the virtual network. </p> <pre> &lt;network&gt; &lt;name&gt;default&lt;/name&gt; &lt;uuid&gt;3e3fce45-4f53-4fa7-bb32-11f34168b82b&lt;/uuid&gt; ...</pre> <dl> <dt><code>name</code></dt> <dd>The content of the <code>name</code> 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.</dd> <dt><code>uuid</code></dt> <dd>The content of the <code>uuid</code> element provides a globally unique identifier for the virtual network. The format must be RFC 4122 compliant, eg <code>3e3fce45-4f53-4fa7-bb32-11f34168b82b</code>. If omitted when defining/creating a new network, a random UUID is generated.</dd> </dl> <h3>Connectivity</h3> <p> The next set of elements control how a virtual network is provided connectivity to the physical LAN (if at all). </p> <pre> ... &lt;bridge name="virbr0" /&gt; &lt;forward type="nat"/&gt; ...</pre> <dl> <dt><code>bridge</code></dt> <dd>The <code>name</code> attribute on the <code>bridge</code> 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 <code>vir</code>, but the name <code>virbr0</code> is reserved for the "default" virtual network. This element should always be provided when defining a new network </dd> <dt><code>forward</code></dt> <dd>Inclusion of the <code>forward</code> 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 <code>dev</code> attribute is set, the firewall rules will restrict forwarding to the named device only. If the <code>type</code> attribute is set to <code>route</code> 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.</dd> </dl> <h3>Addressing</h3> <p> The final set of elements define the IPv4 address range available, and optionally enable DHCP sevices. </p> <pre> ... &lt;ip address="192.168.122.1" netmask="255.255.255.0"&gt; &lt;dhcp&gt; &lt;range start="192.168.122.2" end="192.168.122.254" /&gt; &lt;/dhcp&gt; &lt;/ip&gt; &lt;/network&gt;</pre> <dl> <dt><code>ip</code></dt> <dd>The <code>address</code> 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 <code>netmask</code> attribute defines the significant bits of the network address, again specified in dotted-decimal format. </dd> <dt><code>dhcp</code></dt> <dd>Immediately within the <code>ip</code> element there is an optional <code>dhcp</code> element. The presence of this element enables DHCP services on the virtual network. It will further contain one or more <code>range</code> elements. </dd> <dt><code>range</code></dt> <dd>The <code>start</code> and <code>end</code> attributes on the <code>range</code> 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 <code>ip</code> element. </dd> </dl>

<p> 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. </p>

<p> 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 <code>eth1</code> host network device. </p>

<p> 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 <code>forward</code> element in the XML description. </p>