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    <h1>Hooks for specific system management</h1>

    <ul id="toc"></ul>

    <h2><a name="intro">Custom event scripts</a></h2>
    <p>Beginning with libvirt 0.8.0, specific events on a host system will
       trigger custom scripts.</p>
    <p>These custom <b>hook</b> scripts are executed when any of the following
       actions occur:</p>
    <ul>
      <li>The libvirt daemon starts, stops, or reloads its
          configuration
          (<span class="since">since 0.8.0</span>)<br/><br/></li>
      <li>A QEMU guest is started or stopped
         (<span class="since">since 0.8.0</span>)<br/><br/></li>
         <li>An LXC guest is started or stopped
         (<span class="since">since 0.8.0</span>)<br/><br/></li>
      <li>A network is started or stopped or an interface is
          plugged/unplugged to/from the network
          (<span class="since">since 1.2.2</span>)<br/><br/></li>
    </ul>

    <h2><a name="location">Script location</a></h2>
    <p>The libvirt hook scripts are located in the directory
       <code>$SYSCONFDIR/libvirt/hooks/</code>.</p>
    <ul>
      <li>In Linux distributions such as Fedora and RHEL, this is
          <code>/etc/libvirt/hooks/</code>.  Other Linux distributions may do
          this differently.</li>
      <li>If your installation of libvirt has instead been compiled from
          source, it is likely to be
          <code>/usr/local/etc/libvirt/hooks/</code>.</li>
    </ul>
    <p>To use hook scripts, you will need to create this <code>hooks</code>
       directory manually, place the desired hook scripts inside, then make
       them executable.</p>
    <br/>

    <h2><a name="names">Script names</a></h2>
    <p>At present, there are three hook scripts that can be called:</p>
    <ul>
      <li><code>/etc/libvirt/hooks/daemon</code><br/><br/>
          Executed when the libvirt daemon is started, stopped, or reloads
          its configuration<br/><br/></li>
      <li><code>/etc/libvirt/hooks/qemu</code><br/><br/>
          Executed when a QEMU guest is started, stopped, or migrated<br/><br/></li>
      <li><code>/etc/libvirt/hooks/lxc</code><br /><br/>
          Executed when an LXC guest is started or stopped</li>
      <li><code>/etc/libvirt/hooks/network</code><br/><br/>
          Executed when a network is started or stopped or an
          interface is plugged/unplugged to/from the network</li>
    </ul>
    <br/>

    <h2><a name="structure">Script structure</a></h2>
    <p>The hook scripts are executed using standard Linux process creation
       functions.  Therefore, they must begin with the declaration of the
       command interpreter to use.</p>
    <p>For example:</p>
    <pre>#!/bin/bash</pre>
    <p>or:</p>
    <pre>#!/usr/bin/python</pre>
    <p>Other command interpreters are equally valid, as is any executable
       binary, so you are welcome to use your favourite languages.</p>
    <br/>

    <h2><a name="arguments">Script arguments</a></h2>
    <p>The hook scripts are called with specific command line arguments,
       depending upon the script, and the operation being performed.</p>
    <p>The guest hook scripts, qemu and lxc, are also given the <b>full</b>
       XML description for the domain on their stdin. This includes items
       such the UUID of the domain and its storage information, and is
       intended to provide all the libvirt information the script needs.</p>
    <p>For all cases, stdin of the network hook script is provided with the
       full XML description of the network status in the following form:</p>

<pre>&lt;hookData&gt;
  &lt;network&gt;
     &lt;name&gt;$network_name&lt;/name&gt;
     &lt;uuid&gt;afca425a-2c3a-420c-b2fb-dd7b4950d722&lt;/uuid&gt;
     ...
  &lt;/network&gt;
&lt;/hookData&gt;</pre>

    <p>In the case of an interface
       being plugged/unplugged to/from the network, the network XML will be
       followed with the full XML description of the domain containing the
       interface that is being plugged/unplugged:</p>

<pre>&lt;hookData&gt;
  &lt;network&gt;
     &lt;name&gt;$network_name&lt;/name&gt;
     &lt;uuid&gt;afca425a-2c3a-420c-b2fb-dd7b4950d722&lt;/uuid&gt;
     ...
  &lt;/network&gt;
  &lt;domain type='$domain_type' id='$domain_id'&gt;
     &lt;name&gt;$domain_name&lt;/name&gt;
     &lt;uuid&gt;afca425a-2c3a-420c-b2fb-dd7b4950d722&lt;/uuid&gt;
     ...
  &lt;/domain&gt;
&lt;/hookData&gt;</pre>

    <p>Please note that this approach is different from other cases such as
       <code>daemon</code>, <code>qemu</code> or <code>lxc</code> hook scripts,
       because two XMLs may be passed here, while in the other cases only a single
       XML is passed.</p>

    <p>The command line arguments take this approach:</p>
    <ol>
      <li>The first argument is the name of the <b>object</b> involved in the
          operation, or '-' if there is none.<br/><br/>
          For example, the name of a guest being started.<br/><br/></li>
      <li>The second argument is the name of the <b>operation</b> being
          performed.<br/><br/>
          For example, "start" if a guest is being started.<br/><br/></li>
      <li>The third argument is a <b>sub-operation</b> indication, or '-' if there
          is none.<br/><br/></li>
      <li>The last argument is an <b>extra argument</b> string, or '-' if there is
          none.</li>
    </ol>

    <h4><a name="arguments_specifics">Specifics</a></h4>
    <p>This translates to the following specifics for each hook script:</p>

    <h5><a name="daemon">/etc/libvirt/hooks/daemon</a></h5>
    <ul>
      <li>When the libvirt daemon is started, this script is called as:<br/>
          <pre>/etc/libvirt/hooks/daemon - start - start</pre></li>
      <li>When the libvirt daemon is shut down, this script is called as:<br/>
          <pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre></li>
      <li>When the libvirt daemon receives the SIGHUP signal, it reloads its
          configuration and triggers the hook script as:<br/>
          <pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre></li>
    </ul>
    <p>Please note that when the libvirt daemon is restarted, the <i>daemon</i>
       hook script is called once with the "shutdown" operation, and then once
       with the "start" operation.  There is no specific operation to indicate
       a "restart" is occurring.</p>

    <h5><a name="qemu">/etc/libvirt/hooks/qemu</a></h5>
    <ul>
      <li>Before a QEMU guest is started, the qemu hook script is
        called in three locations; if any location fails, the guest
        is not started.  The first location, <span class="since">since
        0.9.0</span>, is before libvirt performs any resource
        labeling, and the hook can allocate resources not managed by
        libvirt such as DRBD or missing bridges.  This is called as:<br/>
        <pre>/etc/libvirt/hooks/qemu guest_name prepare begin -</pre>
        The second location, available <span class="since">Since
        0.8.0</span>, occurs after libvirt has finished labeling
        all resources, but has not yet started the guest, called as:<br/>
        <pre>/etc/libvirt/hooks/qemu guest_name start begin -</pre>
        The third location, <span class="since">0.9.13</span>,
        occurs after the QEMU process has successfully started up:<br/>
        <pre>/etc/libvirt/hooks/qemu guest_name started begin -</pre>
      </li>
      <li>When a QEMU guest is stopped, the qemu hook script is called
        in two locations, to match the startup.
        First, <span class="since">since 0.8.0</span>, the hook is
        called before libvirt restores any labels:<br/>
        <pre>/etc/libvirt/hooks/qemu guest_name stopped end -</pre>
        Then, after libvirt has released all resources, the hook is
        called again, <span class="since">since 0.9.0</span>, to allow
        any additional resource cleanup:<br/>
        <pre>/etc/libvirt/hooks/qemu guest_name release end -</pre></li>
      <li><span class="since">Since 0.9.11</span>, the qemu hook script
        is also called at the beginning of incoming migration. It is called
        as: <pre>/etc/libvirt/hooks/qemu guest_name migrate begin -</pre>
        with domain XML sent to standard input of the script. In this case,
        the script acts as a filter and is supposed to modify the domain
        XML and print it out on its standard output. Empty output is
        identical to copying the input XML without changing it. In case the
        script returns failure or the output XML is not valid, incoming
        migration will be canceled. This hook may be used, e.g., to change
        location of disk images for incoming domains.</li>
      <li><span class="since">Since 0.9.13</span>, the qemu hook script
        is also called when the libvirtd daemon restarts and reconnects
        to previously running QEMU processes. If the script fails, the
        existing QEMU process will be killed off. It is called as:
        <pre>/etc/libvirt/hooks/qemu guest_name reconnect begin -</pre>
      </li>
      <li><span class="since">Since 0.9.13</span>, the qemu hook script
        is also called when the QEMU driver is told to attach to an
        externally launched QEMU process. It is called as:
        <pre>/etc/libvirt/hooks/qemu guest_name attach begin -</pre>
      </li>
    </ul>

    <h5><a name="lxc">/etc/libvirt/hooks/lxc</a></h5>
    <ul>
      <li>Before a LXC guest is started, the lxc hook script is
        called in three locations; if any location fails, the guest
        is not started.  The first location, <span class="since">since
        0.9.13</span>, is before libvirt performs any resource
        labeling, and the hook can allocate resources not managed by
        libvirt such as DRBD or missing bridges.  This is called as:<br/>
        <pre>/etc/libvirt/hooks/lxc guest_name prepare begin -</pre>
        The second location, available <span class="since">Since
        0.8.0</span>, occurs after libvirt has finished labeling
        all resources, but has not yet started the guest, called as:<br/>
        <pre>/etc/libvirt/hooks/lxc guest_name start begin -</pre>
        The third location, <span class="since">0.9.13</span>,
        occurs after the LXC process has successfully started up:<br/>
        <pre>/etc/libvirt/hooks/lxc guest_name started begin -</pre>
      </li>
      <li>When a LXC guest is stopped, the lxc hook script is called
        in two locations, to match the startup.
        First, <span class="since">since 0.8.0</span>, the hook is
        called before libvirt restores any labels:<br/>
        <pre>/etc/libvirt/hooks/lxc guest_name stopped end -</pre>
        Then, after libvirt has released all resources, the hook is
        called again, <span class="since">since 0.9.0</span>, to allow
        any additional resource cleanup:<br/>
        <pre>/etc/libvirt/hooks/lxc guest_name release end -</pre></li>
      <li><span class="since">Since 0.9.13</span>, the lxc hook script
        is also called when the libvirtd daemon restarts and reconnects
        to previously running LXC processes. If the script fails, the
        existing LXC process will be killed off. It is called as:
        <pre>/etc/libvirt/hooks/lxc guest_name reconnect begin -</pre>
      </li>
    </ul>

    <h5><a name="network">/etc/libvirt/hooks/network</a></h5>
    <ul>
      <li><span class="since">Since 1.2.2</span>, before a network is started,
        this script is called as:<br/>
          <pre>/etc/libvirt/hooks/network network_name start begin -</pre></li>
      <li>After the network is started, up &and; running, the script is
        called as:<br/>
          <pre>/etc/libvirt/hooks/network network_name started begin -</pre></li>
      <li>When a network is shut down, this script is called as:<br/>
          <pre>/etc/libvirt/hooks/network network_name stopped end -</pre></li>
      <li>Later, when network is started and there's an interface from a
        domain to be plugged into the network, the hook script is called as:<br/>
          <pre>/etc/libvirt/hooks/network network_name plugged begin -</pre>
        Please note, that in this case, the script is passed both network and
        domain XMLs on its stdin.</li>
      <li>When the domain from previous case is shutting down, the interface
        is unplugged. This leads to another script invocation:<br/>
          <pre>/etc/libvirt/hooks/network network_name unplugged begin -</pre>
        And again, as in previous case, both network and domain XMLs are passed
        onto script's stdin.</li>
    </ul>

    <br/>

    <h2><a name="execution">Script execution</a></h2>
    <ul>
      <li>The "start" operation for the guest and network hook scripts,
          executes <b>prior</b> to the object (guest or network) being created.
          This allows the object start operation to be aborted if the script
          returns indicating failure.<br/><br/></li>
      <li>The "shutdown" operation for the guest and network hook scripts,
          executes <b>after</b> the object (guest or network) has stopped. If
          the hook script indicates failure in its return, the shut down of the
          object cannot be aborted because it has already been performed.
          <br/><br/></li>
      <li>Hook scripts execute in a synchronous fashion.  Libvirt waits
          for them to return before continuing the given operation.<br/><br/>
          This is most noticeable with the guest or network start operation,
          as a lengthy operation in the hook script can mean an extended wait
          for the guest or network to be available to end users.<br/><br/></li>
      <li>For a hook script to be utilised, it must have its execute bit set
          (e.g. chmod o+rx <i>qemu</i>), and must be present when the libvirt
          daemon is started.<br/><br/></li>
      <li>If a hook script is added to a host after the libvirt daemon is
          already running, it won't be used until the libvirt daemon
          next starts.</li>
    </ul>
    <br/>

    <h2><a name="qemu_migration">QEMU guest migration</a></h2>
    <p>Migration of a QEMU guest involves running hook scripts on both the
       source and destination hosts:</p>
    <ol>
      <li>At the beginning of the migration, the <i>qemu</i> hook script on
          the <b>destination</b> host is executed with the "migrate"
          operation.</li>
      <li>Before QEMU process is spawned, the two operations ("prepare" and
          "start") called for domain start are executed on
          <b>destination</b> host.</li>
      <li>If both of these hook script executions exit successfully (exit
          status 0), the migration continues.  Any other exit code indicates
          failure, and the migration is aborted.</li>
      <li>The QEMU guest is then migrated to the destination host.</li>
      <li>Unless an error occurs during the migration process, the <i>qemu</i>
          hook script on the <b>source</b> host is then executed with the
          "stopped" and "release" operations to indicate it is no longer
          running on this host. Regardless of the return codes, the
          migration is not aborted as it has already been performed.</li>
    </ol>
    <br/>

    <h2><a name="recursive">Calling libvirt functions from within a hook script</a></h2>
    <p><b>DO NOT DO THIS!</b></p>
    <p>A hook script must not call back into libvirt, as the libvirt daemon
       is already waiting for the script to exit.</p>
    <p>A deadlock is likely to occur.</p>
    <br/>

    <h2><a name="return_codes">Return codes and logging</a></h2>
    <p>If a hook script returns with an exit code of 0, the libvirt daemon
       regards this as successful and performs no logging of it.</p>
    <p>However, if a hook script returns with a non zero exit code, the libvirt
       daemon regards this as a failure, logs its return code, and
       additionally logs anything on stderr the hook script returns.</p>
    <p>For example, a hook script might use this code to indicate failure,
       and send a text string to stderr:</p>
    <pre>echo "Could not find required XYZZY" &gt;&amp;2
exit 1</pre>
    <p>The resulting entry in the libvirt log will appear as:</p>
    <pre>20:02:40.297: error : virHookCall:285 : Hook script execution failed: internal error Child process (LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
                       HOME=/root USER=root LOGNAME=root /etc/libvirt/hooks/qemu qemu prepare begin -) unexpected exit status 1: Could not find required XYZZY</pre>
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