docs/hooks.html.in

   1 <?xml version="1.0" encoding="UTF-8"?>
   2 <!DOCTYPE html>
   3 <html xmlns="http://www.w3.org/1999/xhtml">
   4   <body>
   5     <h1>Hooks for specific system management</h1>
   6
   7     <ul id="toc"></ul>
   8
   9     <h2><a id="intro">Custom event scripts</a></h2>
  10     <p>Beginning with libvirt 0.8.0, specific events on a host system will
  11        trigger custom scripts.</p>
  12     <p>These custom <b>hook</b> scripts are executed when any of the following
  13        actions occur:</p>
  14     <ul>
  15       <li>The libvirt daemon starts, stops, or reloads its
  16           configuration
  17           (<span class="since">since 0.8.0</span>)<br/><br/></li>
  18       <li>A QEMU guest is started or stopped
  19          (<span class="since">since 0.8.0</span>)<br/><br/></li>
  20       <li>An LXC guest is started or stopped
  21          (<span class="since">since 0.8.0</span>)<br/><br/></li>
  22       <li>A libxl-handled Xen guest is started or stopped
  23          (<span class="since">since 2.1.0</span>)<br/><br/></li>
  24       <li>A network is started or stopped or an interface is
  25           plugged/unplugged to/from the network
  26           (<span class="since">since 1.2.2</span>)<br/><br/></li>
  27     </ul>
  28
  29     <h2><a id="location">Script location</a></h2>
  30     <p>The libvirt hook scripts are located in the directory
  31        <code>$SYSCONFDIR/libvirt/hooks/</code>.</p>
  32     <ul>
  33       <li>In Linux distributions such as Fedora and RHEL, this is
  34           <code>/etc/libvirt/hooks/</code>.  Other Linux distributions may do
  35           this differently.</li>
  36       <li>If your installation of libvirt has instead been compiled from
  37           source, it is likely to be
  38           <code>/usr/local/etc/libvirt/hooks/</code>.</li>
  39     </ul>
  40     <p>To use hook scripts, you will need to create this <code>hooks</code>
  41        directory manually, place the desired hook scripts inside, then make
  42        them executable.</p>
  43     <br/>
  44
  45     <h2><a id="names">Script names</a></h2>
  46     <p>At present, there are five hook scripts that can be called:</p>
  47     <ul>
  48       <li><code>/etc/libvirt/hooks/daemon</code><br/><br/>
  49           Executed when the libvirt daemon is started, stopped, or reloads
  50           its configuration<br/><br/></li>
  51       <li><code>/etc/libvirt/hooks/qemu</code><br/><br/>
  52           Executed when a QEMU guest is started, stopped, or migrated<br/><br/></li>
  53       <li><code>/etc/libvirt/hooks/lxc</code><br /><br/>
  54           Executed when an LXC guest is started or stopped</li>
  55       <li><code>/etc/libvirt/hooks/libxl</code><br/><br/>
  56           Executed when a libxl-handled Xen guest is started, stopped, or
  57           migrated<br/><br/></li>
  58       <li><code>/etc/libvirt/hooks/network</code><br/><br/>
  59           Executed when a network is started or stopped or an
  60           interface is plugged/unplugged to/from the network</li>
  61     </ul>
  62     <br/>
  63
  64     <h2><a id="structure">Script structure</a></h2>
  65     <p>The hook scripts are executed using standard Linux process creation
  66        functions.  Therefore, they must begin with the declaration of the
  67        command interpreter to use.</p>
  68     <p>For example:</p>
  69     <pre>#!/bin/bash</pre>
  70     <p>or:</p>
  71     <pre>#!/usr/bin/python</pre>
  72     <p>Other command interpreters are equally valid, as is any executable
  73        binary, so you are welcome to use your favourite languages.</p>
  74     <br/>
  75
  76     <h2><a id="arguments">Script arguments</a></h2>
  77     <p>The hook scripts are called with specific command line arguments,
  78        depending upon the script, and the operation being performed.</p>
  79     <p>The guest hook scripts, qemu and lxc, are also given the <b>full</b>
  80        XML description for the domain on their stdin. This includes items
  81        such the UUID of the domain and its storage information, and is
  82        intended to provide all the libvirt information the script needs.</p>
  83     <p>For all cases, stdin of the network hook script is provided with the
  84        full XML description of the network status in the following form:</p>
  85
  86 <pre>&lt;hookData&gt;
  87   &lt;network&gt;
  88      &lt;name&gt;$network_name&lt;/name&gt;
  89      &lt;uuid&gt;afca425a-2c3a-420c-b2fb-dd7b4950d722&lt;/uuid&gt;
  90      ...
  91   &lt;/network&gt;
  92 &lt;/hookData&gt;</pre>
  93
  94     <p>In the case of an network port being created / deleted, the network
  95        XML will be followed with the full XML description of the port:</p>
  96
  97 <pre>&lt;hookData&gt;
  98   &lt;network&gt;
  99      &lt;name&gt;$network_name&lt;/name&gt;
 100      &lt;uuid&gt;afca425a-2c3a-420c-b2fb-dd7b4950d722&lt;/uuid&gt;
 101      ...
 102   &lt;/network&gt;
 103   &lt;networkport&gt;
 104     &lt;uuid&gt;5d744f21-ba4a-4d6e-bdb2-30a35ff3207d&lt;/uuid&gt;
 105     ...
 106     &lt;plug type='direct' dev='ens3' mode='vepa'/&gt;
 107   &lt;/networkport&gt;
 108 &lt;/hookData&gt;</pre>
 109
 110     <p>Please note that this approach is different from other cases such as
 111        <code>daemon</code>, <code>qemu</code> or <code>lxc</code> hook scripts,
 112        because two XMLs may be passed here, while in the other cases only a single
 113        XML is passed.</p>
 114
 115     <p>The command line arguments take this approach:</p>
 116     <ol>
 117       <li>The first argument is the name of the <b>object</b> involved in the
 118           operation, or '-' if there is none.<br/><br/>
 119           For example, the name of a guest being started.<br/><br/></li>
 120       <li>The second argument is the name of the <b>operation</b> being
 121           performed.<br/><br/>
 122           For example, "start" if a guest is being started.<br/><br/></li>
 123       <li>The third argument is a <b>sub-operation</b> indication, or '-' if there
 124           is none.<br/><br/></li>
 125       <li>The last argument is an <b>extra argument</b> string, or '-' if there is
 126           none.</li>
 127     </ol>
 128
 129     <h4><a id="arguments_specifics">Specifics</a></h4>
 130     <p>This translates to the following specifics for each hook script:</p>
 131
 132     <h5><a id="daemon">/etc/libvirt/hooks/daemon</a></h5>
 133     <ul>
 134       <li>When the libvirt daemon is started, this script is called as:<br/>
 135           <pre>/etc/libvirt/hooks/daemon - start - start</pre></li>
 136       <li>When the libvirt daemon is shut down, this script is called as:<br/>
 137           <pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre></li>
 138       <li>When the libvirt daemon receives the SIGHUP signal, it reloads its
 139           configuration and triggers the hook script as:<br/>
 140           <pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre></li>
 141     </ul>
 142     <p>Please note that when the libvirt daemon is restarted, the <i>daemon</i>
 143        hook script is called once with the "shutdown" operation, and then once
 144        with the "start" operation.  There is no specific operation to indicate
 145        a "restart" is occurring.</p>
 146
 147     <h5><a id="qemu">/etc/libvirt/hooks/qemu</a></h5>
 148     <ul>
 149       <li>Before a QEMU guest is started, the qemu hook script is
 150         called in three locations; if any location fails, the guest
 151         is not started.  The first location, <span class="since">since
 152         0.9.0</span>, is before libvirt performs any resource
 153         labeling, and the hook can allocate resources not managed by
 154         libvirt such as DRBD or missing bridges.  This is called as:<br/>
 155         <pre>/etc/libvirt/hooks/qemu guest_name prepare begin -</pre>
 156         The second location, available <span class="since">Since
 157         0.8.0</span>, occurs after libvirt has finished labeling
 158         all resources, but has not yet started the guest, called as:<br/>
 159         <pre>/etc/libvirt/hooks/qemu guest_name start begin -</pre>
 160         The third location, <span class="since">0.9.13</span>,
 161         occurs after the QEMU process has successfully started up:<br/>
 162         <pre>/etc/libvirt/hooks/qemu guest_name started begin -</pre>
 163       </li>
 164       <li>When a QEMU guest is stopped, the qemu hook script is called
 165         in two locations, to match the startup.
 166         First, <span class="since">since 0.8.0</span>, the hook is
 167         called before libvirt restores any labels:<br/>
 168         <pre>/etc/libvirt/hooks/qemu guest_name stopped end -</pre>
 169         Then, after libvirt has released all resources, the hook is
 170         called again, <span class="since">since 0.9.0</span>, to allow
 171         any additional resource cleanup:<br/>
 172         <pre>/etc/libvirt/hooks/qemu guest_name release end -</pre></li>
 173       <li><span class="since">Since 0.9.11</span>, the qemu hook script
 174         is also called at the beginning of incoming migration. It is called
 175         as: <pre>/etc/libvirt/hooks/qemu guest_name migrate begin -</pre>
 176         with domain XML sent to standard input of the script. In this case,
 177         the script acts as a filter and is supposed to modify the domain
 178         XML and print it out on its standard output. Empty output is
 179         identical to copying the input XML without changing it. In case the
 180         script returns failure or the output XML is not valid, incoming
 181         migration will be canceled. This hook may be used, e.g., to change
 182         location of disk images for incoming domains.</li>
 183       <li><span class="since">Since 1.2.9</span>, the qemu hook script is
 184         also called when restoring a saved image either via the API or
 185         automatically when restoring a managed save machine. It is called
 186         as: <pre>/etc/libvirt/hooks/qemu guest_name restore begin -</pre>
 187         with domain XML sent to standard input of the script. In this case,
 188         the script acts as a filter and is supposed to modify the domain
 189         XML and print it out on its standard output. Empty output is
 190         identical to copying the input XML without changing it. In case the
 191         script returns failure or the output XML is not valid, restore of the
 192         image will be aborted. This hook may be used, e.g., to change
 193         location of disk images for restored domains.</li>
 194       <li><span class="since">Since 0.9.13</span>, the qemu hook script
 195         is also called when the libvirtd daemon restarts and reconnects
 196         to previously running QEMU processes. If the script fails, the
 197         existing QEMU process will be killed off. It is called as:
 198         <pre>/etc/libvirt/hooks/qemu guest_name reconnect begin -</pre>
 199       </li>
 200       <li><span class="since">Since 0.9.13</span>, the qemu hook script
 201         is also called when the QEMU driver is told to attach to an
 202         externally launched QEMU process. It is called as:
 203         <pre>/etc/libvirt/hooks/qemu guest_name attach begin -</pre>
 204       </li>
 205     </ul>
 206
 207     <h5><a id="lxc">/etc/libvirt/hooks/lxc</a></h5>
 208     <ul>
 209       <li>Before a LXC guest is started, the lxc hook script is
 210         called in three locations; if any location fails, the guest
 211         is not started.  The first location, <span class="since">since
 212         0.9.13</span>, is before libvirt performs any resource
 213         labeling, and the hook can allocate resources not managed by
 214         libvirt such as DRBD or missing bridges.  This is called as:<br/>
 215         <pre>/etc/libvirt/hooks/lxc guest_name prepare begin -</pre>
 216         The second location, available <span class="since">Since
 217         0.8.0</span>, occurs after libvirt has finished labeling
 218         all resources, but has not yet started the guest, called as:<br/>
 219         <pre>/etc/libvirt/hooks/lxc guest_name start begin -</pre>
 220         The third location, <span class="since">0.9.13</span>,
 221         occurs after the LXC process has successfully started up:<br/>
 222         <pre>/etc/libvirt/hooks/lxc guest_name started begin -</pre>
 223       </li>
 224       <li>When a LXC guest is stopped, the lxc hook script is called
 225         in two locations, to match the startup.
 226         First, <span class="since">since 0.8.0</span>, the hook is
 227         called before libvirt restores any labels:<br/>
 228         <pre>/etc/libvirt/hooks/lxc guest_name stopped end -</pre>
 229         Then, after libvirt has released all resources, the hook is
 230         called again, <span class="since">since 0.9.0</span>, to allow
 231         any additional resource cleanup:<br/>
 232         <pre>/etc/libvirt/hooks/lxc guest_name release end -</pre></li>
 233       <li><span class="since">Since 0.9.13</span>, the lxc hook script
 234         is also called when the libvirtd daemon restarts and reconnects
 235         to previously running LXC processes. If the script fails, the
 236         existing LXC process will be killed off. It is called as:
 237         <pre>/etc/libvirt/hooks/lxc guest_name reconnect begin -</pre>
 238       </li>
 239     </ul>
 240
 241     <h5><a id="libxl">/etc/libvirt/hooks/libxl</a></h5>
 242     <ul>
 243       <li>Before a Xen guest is started using libxl driver, the libxl hook
 244         script is called in three locations; if any location fails, the guest
 245         is not started.  The first location, <span class="since">since
 246         2.1.0</span>, is before libvirt performs any resource
 247         labeling, and the hook can allocate resources not managed by
 248         libvirt.  This is called as:<br/>
 249         <pre>/etc/libvirt/hooks/libxl guest_name prepare begin -</pre>
 250         The second location, available <span class="since">Since
 251         2.1.0</span>, occurs after libvirt has finished labeling
 252         all resources, but has not yet started the guest, called as:<br/>
 253         <pre>/etc/libvirt/hooks/libxl guest_name start begin -</pre>
 254         The third location, <span class="since">2.1.0</span>,
 255         occurs after the domain has successfully started up:<br/>
 256         <pre>/etc/libvirt/hooks/libxl guest_name started begin -</pre>
 257       </li>
 258       <li>When a libxl-handled Xen guest is stopped, the libxl hook script
 259         is called in two locations, to match the startup.
 260         First, <span class="since">since 2.1.0</span>, the hook is
 261         called before libvirt restores any labels:<br/>
 262         <pre>/etc/libvirt/hooks/libxl guest_name stopped end -</pre>
 263         Then, after libvirt has released all resources, the hook is
 264         called again, <span class="since">since 2.1.0</span>, to allow
 265         any additional resource cleanup:<br/>
 266         <pre>/etc/libvirt/hooks/libxl guest_name release end -</pre></li>
 267       <li><span class="since">Since 2.1.0</span>, the libxl hook script
 268         is also called at the beginning of incoming migration. It is called
 269         as: <pre>/etc/libvirt/hooks/libxl guest_name migrate begin -</pre>
 270         with domain XML sent to standard input of the script. In this case,
 271         the script acts as a filter and is supposed to modify the domain
 272         XML and print it out on its standard output. Empty output is
 273         identical to copying the input XML without changing it. In case the
 274         script returns failure or the output XML is not valid, incoming
 275         migration will be canceled. This hook may be used, e.g., to change
 276         location of disk images for incoming domains.</li>
 277       <li><span class="since">Since 2.1.0</span>, the libxl hook script
 278         is also called when the libvirtd daemon restarts and reconnects
 279         to previously running Xen domains. If the script fails, the
 280         existing Xen domains will be killed off. It is called as:
 281         <pre>/etc/libvirt/hooks/libxl guest_name reconnect begin -</pre>
 282       </li>
 283     </ul>
 284
 285     <h5><a id="network">/etc/libvirt/hooks/network</a></h5>
 286     <ul>
 287       <li><span class="since">Since 1.2.2</span>, before a network is started,
 288         this script is called as:<br/>
 289           <pre>/etc/libvirt/hooks/network network_name start begin -</pre></li>
 290       <li>After the network is started, up &amp; running, the script is
 291         called as:<br/>
 292           <pre>/etc/libvirt/hooks/network network_name started begin -</pre></li>
 293       <li>When a network is shut down, this script is called as:<br/>
 294           <pre>/etc/libvirt/hooks/network network_name stopped end -</pre></li>
 295       <li>Later, when network is started and there's an interface from a
 296         domain to be plugged into the network, the hook script is called as:<br/>
 297           <pre>/etc/libvirt/hooks/network network_name port-created begin -</pre>
 298         Please note, that in this case, the script is passed both network and
 299         port XMLs on its stdin.</li>
 300       <li>When network is updated, the hook script is called as:<br/>
 301           <pre>/etc/libvirt/hooks/network network_name updated begin -</pre></li>
 302       <li>When the domain from previous case is shutting down, the interface
 303         is unplugged. This leads to another script invocation:<br/>
 304           <pre>/etc/libvirt/hooks/network network_name port-deleted begin -</pre>
 305         And again, as in previous case, both network and port XMLs are passed
 306         onto script's stdin.</li>
 307     </ul>
 308
 309     <br/>
 310
 311     <h2><a id="execution">Script execution</a></h2>
 312     <ul>
 313       <li>The "start" operation for the guest and network hook scripts,
 314           executes <b>prior</b> to the object (guest or network) being created.
 315           This allows the object start operation to be aborted if the script
 316           returns indicating failure.<br/><br/></li>
 317       <li>The "stopped" operation for the guest and network hook scripts,
 318           executes <b>after</b> the object (guest or network) has stopped. If
 319           the hook script indicates failure in its return, the shut down of the
 320           object cannot be aborted because it has already been performed.
 321           <br/><br/></li>
 322       <li>Hook scripts execute in a synchronous fashion.  Libvirt waits
 323           for them to return before continuing the given operation.<br/><br/>
 324           This is most noticeable with the guest or network start operation,
 325           as a lengthy operation in the hook script can mean an extended wait
 326           for the guest or network to be available to end users.<br/><br/></li>
 327       <li>For a hook script to be utilised, it must have its execute bit set
 328           (e.g. chmod o+rx <i>qemu</i>), and must be present when the libvirt
 329           daemon is started.<br/><br/></li>
 330       <li>If a hook script is added to a host after the libvirt daemon is
 331           already running, it won't be used until the libvirt daemon
 332           next starts.</li>
 333     </ul>
 334     <br/>
 335
 336     <h2><a id="qemu_migration">QEMU guest migration</a></h2>
 337     <p>Migration of a QEMU guest involves running hook scripts on both the
 338        source and destination hosts:</p>
 339     <ol>
 340       <li>At the beginning of the migration, the <i>qemu</i> hook script on
 341           the <b>destination</b> host is executed with the "migrate"
 342           operation.</li>
 343       <li>Before QEMU process is spawned, the two operations ("prepare" and
 344           "start") called for domain start are executed on
 345           <b>destination</b> host.</li>
 346       <li>If both of these hook script executions exit successfully (exit
 347           status 0), the migration continues.  Any other exit code indicates
 348           failure, and the migration is aborted.</li>
 349       <li>The QEMU guest is then migrated to the destination host.</li>
 350       <li>Unless an error occurs during the migration process, the <i>qemu</i>
 351           hook script on the <b>source</b> host is then executed with the
 352           "stopped" and "release" operations to indicate it is no longer
 353           running on this host. Regardless of the return codes, the
 354           migration is not aborted as it has already been performed.</li>
 355     </ol>
 356     <br/>
 357
 358     <h2><a id="recursive">Calling libvirt functions from within a hook script</a></h2>
 359     <p><b>DO NOT DO THIS!</b></p>
 360     <p>A hook script must not call back into libvirt, as the libvirt daemon
 361        is already waiting for the script to exit.</p>
 362     <p>A deadlock is likely to occur.</p>
 363     <br/>
 364
 365     <h2><a id="return_codes">Return codes and logging</a></h2>
 366     <p>If a hook script returns with an exit code of 0, the libvirt daemon
 367        regards this as successful and performs no logging of it.</p>
 368     <p>However, if a hook script returns with a non zero exit code, the libvirt
 369        daemon regards this as a failure, logs its return code, and
 370        additionally logs anything on stderr the hook script returns.</p>
 371     <p>For example, a hook script might use this code to indicate failure,
 372        and send a text string to stderr:</p>
 373     <pre>echo "Could not find required XYZZY" &gt;&amp;2
 374 exit 1</pre>
 375     <p>The resulting entry in the libvirt log will appear as:</p>
 376     <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
 377                        HOME=/root USER=root LOGNAME=root /etc/libvirt/hooks/qemu qemu prepare begin -) unexpected exit status 1: Could not find required XYZZY</pre>
 378   </body>
 379 </html>