man/udev.xml

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   6 <refentry id="udev">
   7   <refentryinfo>
   8     <title>udev</title>
   9     <productname>systemd</productname>
  10     <authorgroup>
  11       <author>
  12         <contrib>Developer</contrib>
  13         <firstname>Greg</firstname>
  14         <surname>Kroah-Hartmann</surname>
  15         <email>greg@kroah.com</email>
  16       </author>
  17       <author>
  18         <contrib>Developer</contrib>
  19         <firstname>Kay</firstname>
  20         <surname>Sievers</surname>
  21         <email>kay@vrfy.org</email>
  22       </author>
  23     </authorgroup>
  24   </refentryinfo>
  25
  26   <refmeta>
  27     <refentrytitle>udev</refentrytitle>
  28     <manvolnum>7</manvolnum>
  29   </refmeta>
  30
  31   <refnamediv>
  32     <refname>udev</refname>
  33     <refpurpose>Dynamic device management</refpurpose>
  34   </refnamediv>
  35
  36   <refsect1><title>Description</title>
  37     <para>udev supplies the system software with device events, manages permissions
  38     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  39     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  40     device names based on the order of discovery. Meaningful symlinks or network device
  41     names provide a way to reliably identify devices based on their properties or
  42     current configuration.</para>
  43
  44     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  45     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  46     the kernel whenever a device is added or removed from the system, or it changes its
  47     state. When udev receives a device event, it matches its configured set of rules
  48     against various device attributes to identify the device. Rules that match may
  49     provide additional device information to be stored in the udev database or
  50     to be used to create meaningful symlink names.</para>
  51
  52     <para>All device information udev processes is stored in the udev database and
  53     sent out to possible event subscribers. Access to all stored data and the event
  54     sources is provided by the library libudev.</para>
  55   </refsect1>
  56
  57   <refsect1><title>Rules Files</title>
  58       <para>The udev rules are read from the files located in the
  59       system rules directory <filename>/usr/lib/udev/rules.d</filename>,
  60       the volatile runtime directory <filename>/run/udev/rules.d</filename>
  61       and the local administration directory <filename>/etc/udev/rules.d</filename>.
  62       All rules files are collectively sorted and processed in lexical order,
  63       regardless of the directories in which they live. However, files with
  64       identical filenames replace each other. Files in <filename>/etc</filename>
  65       have the highest priority, files in <filename>/run</filename> take precedence
  66       over files with the same name in <filename>/usr/lib</filename>. This can be
  67       used to override a system-supplied rules file with a local file if needed;
  68       a symlink in <filename>/etc</filename> with the same name as a rules file in
  69       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
  70       disables the rules file entirely. Rule files must have the extension
  71       <filename>.rules</filename>; other extensions are ignored.</para>
  72
  73       <para>Every line in the rules file contains at least one key-value pair.
  74       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  75       There are two kinds of keys: match and assignment.
  76       If all match keys match against their values, the rule gets applied and the
  77       assignment keys get the specified values assigned.</para>
  78
  79       <para>A matching rule may rename a network interface, add symlinks
  80       pointing to the device node, or run a specified program as part of
  81       the event handling.</para>
  82
  83       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  84       Each key has a distinct operation, depending on the used operator. Valid
  85       operators are:</para>
  86       <variablelist>
  87         <varlistentry>
  88           <term><literal>==</literal></term>
  89           <listitem>
  90             <para>Compare for equality.</para>
  91           </listitem>
  92         </varlistentry>
  93
  94         <varlistentry>
  95           <term><literal>!=</literal></term>
  96           <listitem>
  97             <para>Compare for inequality.</para>
  98           </listitem>
  99         </varlistentry>
 100
 101         <varlistentry>
 102           <term><literal>=</literal></term>
 103           <listitem>
 104             <para>Assign a value to a key. Keys that represent a list are reset
 105             and only this single value is assigned.</para>
 106           </listitem>
 107         </varlistentry>
 108
 109         <varlistentry>
 110           <term><literal>+=</literal></term>
 111           <listitem>
 112             <para>Add the value to a key that holds a list of entries.</para>
 113           </listitem>
 114         </varlistentry>
 115
 116         <varlistentry>
 117           <term><literal>:=</literal></term>
 118           <listitem>
 119             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 120           </listitem>
 121         </varlistentry>
 122       </variablelist>
 123
 124       <para>The following key names can be used to match against device properties.
 125       Some of the keys also match against properties of the parent devices in sysfs,
 126       not only the device that has generated the event. If multiple keys that match
 127       a parent device are specified in a single rule, all these keys must match at
 128       one and the same parent device.</para>
 129       <variablelist class='udev-directives'>
 130         <varlistentry>
 131           <term><varname>ACTION</varname></term>
 132           <listitem>
 133             <para>Match the name of the event action.</para>
 134           </listitem>
 135         </varlistentry>
 136
 137         <varlistentry>
 138           <term><varname>DEVPATH</varname></term>
 139           <listitem>
 140             <para>Match the devpath of the event device.</para>
 141           </listitem>
 142         </varlistentry>
 143
 144         <varlistentry>
 145           <term><varname>KERNEL</varname></term>
 146           <listitem>
 147             <para>Match the name of the event device.</para>
 148           </listitem>
 149         </varlistentry>
 150
 151         <varlistentry>
 152           <term><varname>NAME</varname></term>
 153           <listitem>
 154             <para>Match the name of a network interface. It can be used once the
 155             NAME key has been set in one of the preceding rules.</para>
 156           </listitem>
 157         </varlistentry>
 158
 159         <varlistentry>
 160           <term><varname>SYMLINK</varname></term>
 161           <listitem>
 162             <para>Match the name of a symlink targeting the node. It can
 163             be used once a SYMLINK key has been set in one of the preceding
 164             rules. There may be multiple symlinks; only one needs to match.
 165             </para>
 166           </listitem>
 167         </varlistentry>
 168
 169         <varlistentry>
 170           <term><varname>SUBSYSTEM</varname></term>
 171           <listitem>
 172             <para>Match the subsystem of the event device.</para>
 173           </listitem>
 174         </varlistentry>
 175         <varlistentry>
 176           <term><varname>DRIVER</varname></term>
 177           <listitem>
 178             <para>Match the driver name of the event device. Only set this key for devices
 179             which are bound to a driver at the time the event is generated.</para>
 180           </listitem>
 181         </varlistentry>
 182         <varlistentry>
 183           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 184           <listitem>
 185             <para>Match sysfs attribute values of the event device. Trailing
 186             whitespace in the attribute values is ignored unless the specified match
 187             value itself contains trailing whitespace.
 188             </para>
 189           </listitem>
 190         </varlistentry>
 191
 192         <varlistentry>
 193           <term><varname>KERNELS</varname></term>
 194           <listitem>
 195             <para>Search the devpath upwards for a matching device name.</para>
 196           </listitem>
 197         </varlistentry>
 198
 199         <varlistentry>
 200           <term><varname>SUBSYSTEMS</varname></term>
 201           <listitem>
 202             <para>Search the devpath upwards for a matching device subsystem name.</para>
 203           </listitem>
 204         </varlistentry>
 205
 206         <varlistentry>
 207           <term><varname>DRIVERS</varname></term>
 208           <listitem>
 209             <para>Search the devpath upwards for a matching device driver name.</para>
 210           </listitem>
 211         </varlistentry>
 212
 213         <varlistentry>
 214           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 215           <listitem>
 216             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 217             If multiple <varname>ATTRS</varname> matches are specified, all of them
 218             must match on the same device. Trailing whitespace in the attribute values is ignored
 219             unless the specified match value itself contains trailing whitespace.</para>
 220           </listitem>
 221         </varlistentry>
 222
 223         <varlistentry>
 224           <term><varname>TAGS</varname></term>
 225           <listitem>
 226             <para>Search the devpath upwards for a device with matching tag.</para>
 227           </listitem>
 228         </varlistentry>
 229
 230         <varlistentry>
 231           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 232           <listitem>
 233             <para>Match against a device property value.</para>
 234           </listitem>
 235         </varlistentry>
 236
 237         <varlistentry>
 238           <term><varname>TAG</varname></term>
 239           <listitem>
 240             <para>Match against a device tag.</para>
 241           </listitem>
 242         </varlistentry>
 243
 244         <varlistentry>
 245           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 246           <listitem>
 247             <para>Test the existence of a file. An octal mode mask can be specified
 248             if needed.</para>
 249           </listitem>
 250         </varlistentry>
 251
 252         <varlistentry>
 253           <term><varname>PROGRAM</varname></term>
 254           <listitem>
 255             <para>Execute a program to determine whether there
 256             is a match; the key is true if the program returns
 257             successfully. The device properties are made available to the
 258             executed program in the environment. The program's stdout
 259             is available in the RESULT key.</para>
 260             <para>This can only be used for very short-running foreground tasks. For details
 261             see <varname>RUN</varname>.</para>
 262           </listitem>
 263         </varlistentry>
 264
 265         <varlistentry>
 266           <term><varname>RESULT</varname></term>
 267           <listitem>
 268             <para>Match the returned string of the last PROGRAM call. This key can
 269             be used in the same or in any later rule after a PROGRAM call.</para>
 270           </listitem>
 271         </varlistentry>
 272       </variablelist>
 273
 274       <para>Most of the fields support shell glob pattern matching. The following
 275       pattern characters are supported:</para>
 276       <variablelist>
 277         <varlistentry>
 278           <term><literal>*</literal></term>
 279           <listitem>
 280             <para>Matches zero or more characters.</para>
 281           </listitem>
 282         </varlistentry>
 283         <varlistentry>
 284           <term><literal>?</literal></term>
 285           <listitem>
 286             <para>Matches any single character.</para>
 287           </listitem>
 288         </varlistentry>
 289         <varlistentry>
 290           <term><literal>[]</literal></term>
 291           <listitem>
 292             <para>Matches any single character specified within the brackets. For
 293             example, the pattern string <literal>tty[SR]</literal>
 294             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 295             Ranges are also supported via the <literal>-</literal> character.
 296             For example, to match on the range of all digits, the pattern [0-9] could
 297             be used. If the first character following the <literal>[</literal> is a
 298             <literal>!</literal>, any characters not enclosed are matched.</para>
 299           </listitem>
 300         </varlistentry>
 301       </variablelist>
 302
 303       <para>The following keys can get values assigned:</para>
 304       <variablelist class='udev-directives'>
 305         <varlistentry>
 306           <term><varname>NAME</varname></term>
 307           <listitem>
 308             <para>The name to use for a network interface. The name of a device node
 309             cannot be changed by udev, only additional symlinks can be created.</para>
 310           </listitem>
 311         </varlistentry>
 312
 313         <varlistentry>
 314           <term><varname>SYMLINK</varname></term>
 315           <listitem>
 316             <para>The name of a symlink targeting the node. Every matching rule adds
 317             this value to the list of symlinks to be created.</para>
 318             <para>The set of characters to name a symlink is limited. Allowed
 319             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 320             sequences, and <literal>\x00</literal> hex encoding. All other
 321             characters are replaced by a <literal>_</literal> character.</para>
 322             <para>Multiple symlinks may be specified by separating the names by the
 323             space character. In case multiple devices claim the same name, the link
 324             always points to the device with the highest link_priority. If the current
 325             device goes away, the links are re-evaluated and the device with the
 326             next highest link_priority becomes the owner of the link. If no
 327             link_priority is specified, the order of the devices (and which one of
 328             them owns the link) is undefined.</para>
 329             <para>Symlink names must never conflict with the kernel's default device
 330             node names, as that would result in unpredictable behavior.
 331             </para>
 332           </listitem>
 333         </varlistentry>
 334
 335         <varlistentry>
 336           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 337           <listitem>
 338             <para>The permissions for the device node. Every specified value overrides
 339             the compiled-in default value.</para>
 340           </listitem>
 341         </varlistentry>
 342
 343         <varlistentry>
 344           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 345           <listitem>
 346             <para>Applies the specified Linux Security Module label to the device node.</para>
 347           </listitem>
 348         </varlistentry>
 349
 350         <varlistentry>
 351           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 352           <listitem>
 353             <para>The value that should be written to a sysfs attribute of the
 354             event device.</para>
 355           </listitem>
 356         </varlistentry>
 357
 358         <varlistentry>
 359           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 360           <listitem>
 361             <para>Set a device property value. Property names with a leading <literal>.</literal>
 362             are neither stored in the database nor exported to events or
 363             external tools (run by, say, the PROGRAM match key).</para>
 364           </listitem>
 365         </varlistentry>
 366
 367         <varlistentry>
 368           <term><varname>TAG</varname></term>
 369           <listitem>
 370             <para>Attach a tag to a device. This is used to filter events for users
 371             of libudev's monitor functionality, or to enumerate a group of tagged
 372             devices. The implementation can only work efficiently if only a few
 373             tags are attached to a device. It is only meant to be used in
 374             contexts with specific device filter requirements, and not as a
 375             general-purpose flag. Excessive use might result in inefficient event
 376             handling.</para>
 377           </listitem>
 378         </varlistentry>
 379
 380         <varlistentry>
 381           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 382           <listitem>
 383             <para>Add a program to the list of programs to be executed after processing all the
 384             rules for a specific event, depending on <literal>type</literal>:</para>
 385             <variablelist>
 386               <varlistentry>
 387                 <term><literal>program</literal></term>
 388                 <listitem>
 389                   <para>Execute an external program specified as the assigned
 390                   value. If no absolute path is given, the program is expected to live in
 391                   /usr/lib/udev, otherwise the absolute path must be specified.</para>
 392                   <para>This is the default if no <replaceable>type</replaceable> is
 393                   specified.</para>
 394                 </listitem>
 395               </varlistentry>
 396               <varlistentry>
 397                 <term><literal>builtin</literal></term>
 398                 <listitem>
 399                   <para>As <varname>program</varname>, but use one of the built-in programs rather
 400                   than an external one.</para>
 401                 </listitem>
 402               </varlistentry>
 403             </variablelist>
 404             <para>The program name and following arguments are separated by spaces.
 405             Single quotes can be used to specify arguments with spaces.</para>
 406             <para>This can only be used for very short-running foreground tasks. Running an
 407             event process for a long period of time may block all further events for
 408             this or a dependent device.</para>
 409             <para>Starting daemons or other long running processes is not appropriate
 410             for udev; the forked processes, detached or not, will be unconditionally
 411             killed after the event handling has finished.</para>
 412           </listitem>
 413         </varlistentry>
 414
 415         <varlistentry>
 416           <term><varname>LABEL</varname></term>
 417           <listitem>
 418             <para>A named label to which a GOTO may jump.</para>
 419           </listitem>
 420         </varlistentry>
 421
 422         <varlistentry>
 423           <term><varname>GOTO</varname></term>
 424           <listitem>
 425             <para>Jumps to the next LABEL with a matching name.</para>
 426           </listitem>
 427         </varlistentry>
 428
 429         <varlistentry>
 430           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 431           <listitem>
 432             <para>Import a set of variables as device properties,
 433             depending on <literal>type</literal>:</para>
 434             <variablelist>
 435               <varlistentry>
 436                 <term><literal>program</literal></term>
 437                 <listitem>
 438                   <para>Execute an external program specified as the assigned value and
 439                   import its output, which must be in environment key
 440                   format. Path specification, command/argument separation,
 441                   and quoting work like in <varname>RUN</varname>.</para>
 442                 </listitem>
 443               </varlistentry>
 444               <varlistentry>
 445                 <term><literal>builtin</literal></term>
 446                 <listitem>
 447                   <para>Similar to <literal>program</literal>, but use one of the
 448                   built-in programs rather than an external one.</para>
 449                 </listitem>
 450               </varlistentry>
 451              <varlistentry>
 452                 <term><literal>file</literal></term>
 453                 <listitem>
 454                   <para>Import a text file specified as the assigned value, the content
 455                   of which must be in environment key format.</para>
 456                 </listitem>
 457               </varlistentry>
 458               <varlistentry>
 459                 <term><literal>db</literal></term>
 460                 <listitem>
 461                   <para>Import a single property specified as the assigned value from the
 462                   current device database. This works only if the database is already populated
 463                   by an earlier event.</para>
 464                 </listitem>
 465               </varlistentry>
 466               <varlistentry>
 467                 <term><literal>cmdline</literal></term>
 468                 <listitem>
 469                   <para>Import a single property from the kernel command line. For simple flags
 470                   the value of the property is set to <literal>1</literal>.</para>
 471                 </listitem>
 472               </varlistentry>
 473               <varlistentry>
 474                 <term><literal>parent</literal></term>
 475                 <listitem>
 476                   <para>Import the stored keys from the parent device by reading
 477                   the database entry of the parent device. The value assigned to
 478                   <option>IMPORT{parent}</option> is used as a filter of key names
 479                   to import (with the same shell glob pattern matching used for
 480                   comparisons).</para>
 481                 </listitem>
 482               </varlistentry>
 483             </variablelist>
 484             <para>This can only be used for very short-running foreground tasks. For details
 485             see <option>RUN</option>.</para>
 486           </listitem>
 487         </varlistentry>
 488
 489         <varlistentry>
 490           <term><varname>WAIT_FOR</varname></term>
 491           <listitem>
 492             <para>Wait for a file to become available or until a timeout of
 493             10 seconds expires. The path is relative to the sysfs device;
 494             if no path is specified, this waits for an attribute to appear.</para>
 495           </listitem>
 496         </varlistentry>
 497
 498         <varlistentry>
 499           <term><varname>OPTIONS</varname></term>
 500           <listitem>
 501             <para>Rule and device options:</para>
 502             <variablelist class='udev-directives'>
 503               <varlistentry>
 504                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 505                 <listitem>
 506                   <para>Specify the priority of the created symlinks. Devices with higher
 507                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 508                 </listitem>
 509               </varlistentry>
 510               <varlistentry>
 511                 <term><option>event_timeout=</option></term>
 512                 <listitem>
 513                   <para>Number of seconds an event waits for operations to finish before
 514                   giving up and terminating itself.</para>
 515                 </listitem>
 516               </varlistentry>
 517               <varlistentry>
 518                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 519                 <listitem>
 520                   <para>Usually control and other possibly unsafe characters are replaced
 521                   in strings used for device naming. The mode of replacement can be specified
 522                   with this option.</para>
 523                 </listitem>
 524               </varlistentry>
 525               <varlistentry>
 526                 <term><option>static_node=</option></term>
 527                 <listitem>
 528                   <para>Apply the permissions specified in this rule to the static device node with
 529                   the specified name. Also, for every tag specified in this rule, create a symlink
 530                   in the directory
 531                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 532                   pointing at the static device node with the specified name. Static device node
 533                   creation is performed by systemd-tmpfiles before systemd-udevd is started. The
 534                   static nodes might not have a corresponding kernel device; they are used to
 535                   trigger automatic kernel module loading when they are accessed.</para>
 536                 </listitem>
 537               </varlistentry>
 538               <varlistentry>
 539                 <term><option>watch</option></term>
 540                 <listitem>
 541                   <para>Watch the device node with inotify; when the node is closed after being opened for
 542                   writing, a change uevent is synthesized.</para>
 543                 </listitem>
 544               </varlistentry>
 545               <varlistentry>
 546                 <term><option>nowatch</option></term>
 547                 <listitem>
 548                   <para>Disable the watching of a device node with inotify.</para>
 549                 </listitem>
 550               </varlistentry>
 551             </variablelist>
 552           </listitem>
 553         </varlistentry>
 554       </variablelist>
 555
 556       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>, <varname>PROGRAM</varname>,
 557       <varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname>  and  <varname>RUN</varname>
 558       fields support simple string substitutions. The <varname>RUN</varname>
 559       substitutions are performed after all rules have been processed, right before the program
 560       is executed, allowing for the use of device properties set by earlier matching
 561       rules. For all other fields, substitutions are performed while the individual rule is
 562       being processed. The available substitutions are:</para>
 563       <variablelist class='udev-directives'>
 564         <varlistentry>
 565           <term><option>$kernel</option>, <option>%k</option></term>
 566           <listitem>
 567             <para>The kernel name for this device.</para>
 568           </listitem>
 569         </varlistentry>
 570
 571         <varlistentry>
 572           <term><option>$number</option>, <option>%n</option></term>
 573           <listitem>
 574             <para>The kernel number for this device. For example,
 575             <literal>sda3</literal> has kernel number <literal>3</literal>.</para>
 576           </listitem>
 577         </varlistentry>
 578
 579         <varlistentry>
 580           <term><option>$devpath</option>, <option>%p</option></term>
 581           <listitem>
 582             <para>The devpath of the device.</para>
 583           </listitem>
 584         </varlistentry>
 585
 586         <varlistentry>
 587           <term><option>$id</option>, <option>%b</option></term>
 588           <listitem>
 589             <para>The name of the device matched while searching the devpath upwards for
 590               <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
 591             </para>
 592           </listitem>
 593         </varlistentry>
 594
 595         <varlistentry>
 596           <term><option>$driver</option></term>
 597           <listitem>
 598             <para>The driver name of the device matched while searching the devpath upwards for
 599               <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
 600             </para>
 601           </listitem>
 602         </varlistentry>
 603
 604         <varlistentry>
 605           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 606           <listitem>
 607             <para>The value of a sysfs attribute found at the device where
 608             all keys of the rule have matched. If the matching device does not have
 609             such an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or
 610             ATTRS test selected a parent device, then the attribute from that
 611             parent device is used.</para>
 612             <para>If the attribute is a symlink, the last element of the symlink target is
 613             returned as the value.</para>
 614           </listitem>
 615         </varlistentry>
 616
 617         <varlistentry>
 618           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 619           <listitem>
 620             <para>A device property value.</para>
 621           </listitem>
 622         </varlistentry>
 623
 624         <varlistentry>
 625           <term><option>$major</option>, <option>%M</option></term>
 626           <listitem>
 627             <para>The kernel major number for the device.</para>
 628           </listitem>
 629         </varlistentry>
 630
 631         <varlistentry>
 632           <term><option>$minor</option>, <option>%m</option></term>
 633           <listitem>
 634             <para>The kernel minor number for the device.</para>
 635           </listitem>
 636         </varlistentry>
 637
 638         <varlistentry>
 639           <term><option>$result</option>, <option>%c</option></term>
 640           <listitem>
 641             <para>The string returned by the external program requested with PROGRAM.
 642             A single part of the string, separated by a space character, may be selected
 643             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 644             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 645             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 646           </listitem>
 647         </varlistentry>
 648
 649         <varlistentry>
 650           <term><option>$parent</option>, <option>%P</option></term>
 651           <listitem>
 652             <para>The node name of the parent device.</para>
 653           </listitem>
 654         </varlistentry>
 655
 656         <varlistentry>
 657           <term><option>$name</option></term>
 658           <listitem>
 659             <para>The current name of the device. If not changed by a rule, it is the
 660             name of the kernel device.</para>
 661           </listitem>
 662         </varlistentry>
 663
 664         <varlistentry>
 665           <term><option>$links</option></term>
 666           <listitem>
 667             <para>A space-separated list of the current symlinks. The value is
 668             only set during a remove event or if an earlier rule assigned a value.</para>
 669           </listitem>
 670         </varlistentry>
 671
 672         <varlistentry>
 673           <term><option>$root</option>, <option>%r</option></term>
 674           <listitem>
 675             <para>The udev_root value.</para>
 676           </listitem>
 677         </varlistentry>
 678
 679         <varlistentry>
 680           <term><option>$sys</option>, <option>%S</option></term>
 681           <listitem>
 682             <para>The sysfs mount point.</para>
 683           </listitem>
 684         </varlistentry>
 685
 686         <varlistentry>
 687           <term><option>$devnode</option>, <option>%N</option></term>
 688           <listitem>
 689             <para>The name of the device node.</para>
 690           </listitem>
 691         </varlistentry>
 692
 693         <varlistentry>
 694           <term><option>%%</option></term>
 695           <listitem>
 696           <para>The <literal>%</literal> character itself.</para>
 697           </listitem>
 698         </varlistentry>
 699
 700         <varlistentry>
 701           <term><option>$$</option></term>
 702           <listitem>
 703           <para>The <literal>$</literal> character itself.</para>
 704           </listitem>
 705         </varlistentry>
 706       </variablelist>
 707   </refsect1>
 708
 709   <refsect1><title>Hardware Database Files</title>
 710       <para>The hwdb files are read from the files located in the
 711       system hwdb directory <filename>/usr/lib/udev/hwdb.d</filename>,
 712       the volatile runtime directory <filename>/run/udev/hwdb.d</filename>
 713       and the local administration directory <filename>/etc/udev/hwdb.d</filename>.
 714       All hwdb files are collectively sorted and processed in lexical order,
 715       regardless of the directories in which they live. However, files with
 716       identical filenames replace each other. Files in <filename>/etc</filename>
 717       have the highest priority, files in <filename>/run</filename> take precedence
 718       over files with the same name in <filename>/usr/lib</filename>. This can be
 719       used to override a system-supplied hwdb file with a local file if needed;
 720       a symlink in <filename>/etc</filename> with the same name as a hwdb file in
 721       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
 722       disables the hwdb file entirely. hwdb files must have the extension
 723       <filename>.hwdb</filename>; other extensions are ignored.</para>
 724
 725       <para>The hwdb file contains data records consisting of matches and
 726       associated key-value pairs. Every record in the hwdb starts with one or
 727       more match string, specifying a shell glob to compare the database
 728       lookup string against. Multiple match lines are specified in additional
 729       consecutive lines. Every match line is compared indivdually, they are
 730       combined by OR. Every match line must start at the first character of
 731       the line.</para>
 732
 733       <para>The match lines are followed by one or more key-value pair lines, which
 734       are recognized by a leading space character. The key name and value are separated
 735       by <literal>=</literal>. An empty line signifies the end
 736       of a record. Lines beginning with <literal>#</literal> are ignored.</para>
 737
 738       <para>The content of all hwdb files is read by
 739       <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 740       and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>.
 741       During runtime only the binary database is used.</para>
 742   </refsect1>
 743
 744   <refsect1><title>Network Link Configuration</title>
 745       <para>Network link configuration is performed by the <literal>net_setup_link</literal>
 746       udev builtin.</para>
 747
 748       <para>The link files are read from the files located in the
 749       system network directory <filename>/usr/lib/systemd/network</filename>,
 750       the volatile runtime network directory <filename>/run/systemd/network</filename>
 751       and the local administration network directory <filename>/etc/systemd/network</filename>.
 752       Link files must have the extension <filename>.link</filename>; other extensions are ignored.
 753       All link files are collectively sorted and processed in lexical order,
 754       regardless of the directories in which they live. However, files with
 755       identical filenames replace each other. Files in <filename>/etc</filename>
 756       have the highest priority, files in <filename>/run</filename> take precedence
 757       over files with the same name in <filename>/usr/lib</filename>. This can be
 758       used to override a system-supplied link file with a local file if needed;
 759       a symlink in <filename>/etc</filename> with the same name as a link file in
 760       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
 761       disables the link file entirely.</para>
 762
 763       <para>The link file contains a <literal>[Match]</literal> section, which
 764       determines if a given link file may be applied to a given device; and a
 765       <literal>[Link]</literal> section specifying how the device should be
 766       configured. The first (in lexical order) of the link files that matches
 767       a given device is applied.</para>
 768
 769       <para>A link file is said to match a device if each of the entries in the
 770       <literal>[Match]</literal> section matches, or if the section is empty.
 771       The following keys are accepted:</para>
 772
 773       <variablelist class='network-directives'>
 774         <varlistentry>
 775           <term><varname>MACAddress</varname></term>
 776           <listitem>
 777             <para>The hardware address.
 778             </para>
 779           </listitem>
 780         </varlistentry>
 781         <varlistentry>
 782           <term><varname>Path</varname></term>
 783           <listitem>
 784             <para>The persistent path, as exposed by the udev property <literal>ID_PATH</literal>.</para>
 785           </listitem>
 786         </varlistentry>
 787         <varlistentry>
 788           <term><varname>Driver</varname></term>
 789           <listitem>
 790             <para>The driver currently bound to the device, as exposed by the udev property <literal>DRIVER</literal>.</para>
 791           </listitem>
 792         </varlistentry>
 793         <varlistentry>
 794           <term><varname>Type</varname></term>
 795           <listitem>
 796             <para>The device type, as exposed by the udev property <literal>DEVTYPE</literal>.</para>
 797           </listitem>
 798         </varlistentry>
 799       </variablelist>
 800
 801       <para>The <literal>[Link]</literal> section accepts the following keys:</para>
 802
 803       <variablelist class='network-directives'>
 804         <varlistentry>
 805           <term><varname>Description</varname></term>
 806           <listitem>
 807             <para>A description of the device.</para>
 808           </listitem>
 809         </varlistentry>
 810         <varlistentry>
 811           <term><varname>Alias</varname></term>
 812           <listitem>
 813             <para>The <literal>ifalias</literal> is set to this value.</para>
 814           </listitem>
 815         </varlistentry>
 816         <varlistentry>
 817           <term><varname>MACAddressPolicy</varname></term>
 818           <listitem>
 819             <para>The policy by which the MAC address should be set. The available policies are:</para>
 820             <variablelist>
 821               <varlistentry>
 822                 <term><literal>persistent</literal></term>
 823                 <listitem>
 824                   <para>If the hardware has a persistent MAC address, as most hardware should, and this is used by
 825                   the kernel, nothing is done. Otherwise, a new MAC address is generated which is guaranteed to be
 826                   the same on every boot for the given machine and the given device, but which is otherwise random.
 827                   </para>
 828                 </listitem>
 829               </varlistentry>
 830               <varlistentry>
 831                 <term><literal>random</literal></term>
 832                 <listitem>
 833                   <para>If the kernel is using a random MAC address, nothing is done. Otherwise, a new address is
 834                   randomly generated each time the device appears, typically at boot.</para>
 835                 </listitem>
 836               </varlistentry>
 837             </variablelist>
 838           </listitem>
 839         </varlistentry>
 840         <varlistentry>
 841           <term><varname>MACAddress</varname></term>
 842           <listitem>
 843             <para>The MAC address to use, if no <literal>MACAddressPolicy</literal> is specified.</para>
 844           </listitem>
 845         </varlistentry>
 846         <varlistentry>
 847           <term><varname>NamePolicy</varname></term>
 848           <listitem>
 849             <para>An ordered, space-separated list of policies by which the interface name should be set.
 850             <literal>NamePolicy</literal> may be disabeld by specifying <literal>net.ifnames=0</literal> on the
 851             kernel commandline. Each of the policies may fail, and the first successfull one is used. The name
 852             is not set directly, but exported to udev as the property <literal>ID_NET_NAME</literal>, which is
 853             by default used by an udev rule to set <literal>NAME</literal>. The available policies are:</para>
 854             <variablelist>
 855               <varlistentry>
 856                 <term><literal>onboard</literal></term>
 857                 <listitem>
 858                   <para>The name is set based on information given by the firmware for on-board devices, as
 859                   exported by the udev property <literal>ID_NET_NAME_ONBOARD</literal>.</para>
 860                 </listitem>
 861               </varlistentry>
 862               <varlistentry>
 863                 <term><literal>slot</literal></term>
 864                 <listitem>
 865                   <para>The name is set based on information given by the firmware for hot-plug devices, as
 866                   exported by the udev property <literal>ID_NET_NAME_SLOT</literal>.</para>
 867                 </listitem>
 868               </varlistentry>
 869               <varlistentry>
 870                 <term><literal>path</literal></term>
 871                 <listitem>
 872                   <para>The name is set based on the device's physical location, as exported by the udev
 873                   property <literal>ID_NET_NAME_PATH</literal>.</para>
 874                 </listitem>
 875               </varlistentry>
 876               <varlistentry>
 877                 <term><literal>mac</literal></term>
 878                 <listitem>
 879                   <para>The name is set based on the device's persistent MAC address, as exported by the udev
 880                   property <literal>ID_NET_NAME_MAC</literal>.</para>
 881                 </listitem>
 882               </varlistentry>
 883             </variablelist>
 884           </listitem>
 885         </varlistentry>
 886         <varlistentry>
 887           <term><varname>Name</varname></term>
 888           <listitem>
 889             <para>The interface name to use in case all the policies specified in <literal>NamePolicy</literal>
 890             fail, or in case <literal>NamePolicy</literal> is missing or disabled.</para>
 891           </listitem>
 892         </varlistentry>
 893         <varlistentry>
 894           <term><varname>MTU</varname></term>
 895           <listitem>
 896             <para>The MTU to set for the device.</para>
 897           </listitem>
 898         </varlistentry>
 899         <varlistentry>
 900           <term><varname>SpeedMBytes</varname></term>
 901           <listitem>
 902             <para>The speed to set for the device.</para>
 903           </listitem>
 904         </varlistentry>
 905         <varlistentry>
 906           <term><varname>Duplex</varname></term>
 907           <listitem>
 908             <para>The duplex mode to set for the device. The accepted values are <literal>half</literal> and
 909             <literal>full</literal>.</para>
 910           </listitem>
 911         </varlistentry>
 912         <varlistentry>
 913           <term><varname>WakeOnLan</varname></term>
 914           <listitem>
 915             <para>The Wake-On-Lan policy to set for the device. The supported values are:</para>
 916             <variablelist>
 917               <varlistentry>
 918                 <term><literal>phy</literal></term>
 919                 <listitem>
 920                   <para>Wake on PHY activity.</para>
 921                 </listitem>
 922               </varlistentry>
 923               <varlistentry>
 924                 <term><literal>magic</literal></term>
 925                 <listitem>
 926                   <para>Wake on receipt of magic packet.</para>
 927                 </listitem>
 928               </varlistentry>
 929               <varlistentry>
 930                 <term><literal>off</literal></term>
 931                 <listitem>
 932                   <para>Never wake.</para>
 933                 </listitem>
 934               </varlistentry>
 935             </variablelist>
 936           </listitem>
 937         </varlistentry>
 938       </variablelist>
 939   </refsect1>
 940
 941   <refsect1>
 942     <title>See Also</title>
 943     <para><citerefentry>
 944         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 945       </citerefentry>,
 946       <citerefentry>
 947         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 948     </citerefentry></para>
 949   </refsect1>
 950 </refentry>