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 standard ouput
 259             is available in the <varname>RESULT</varname> 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 <varname>PROGRAM</varname> call.
 269             This key can be used in the same or in any later rule after a
 270             <varname>PROGRAM</varname> call.</para>
 271           </listitem>
 272         </varlistentry>
 273       </variablelist>
 274
 275       <para>Most of the fields support shell glob pattern matching. The following
 276       pattern characters are supported:</para>
 277       <variablelist>
 278         <varlistentry>
 279           <term><literal>*</literal></term>
 280           <listitem>
 281             <para>Matches zero or more characters.</para>
 282           </listitem>
 283         </varlistentry>
 284         <varlistentry>
 285           <term><literal>?</literal></term>
 286           <listitem>
 287             <para>Matches any single character.</para>
 288           </listitem>
 289         </varlistentry>
 290         <varlistentry>
 291           <term><literal>[]</literal></term>
 292           <listitem>
 293             <para>Matches any single character specified within the brackets. For
 294             example, the pattern string <literal>tty[SR]</literal>
 295             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 296             Ranges are also supported via the <literal>-</literal> character.
 297             For example, to match on the range of all digits, the pattern
 298             <literal>[0-9]</literal> could be used. If the first character
 299             following the <literal>[</literal> is a <literal>!</literal>,
 300             any characters not enclosed are matched.</para>
 301           </listitem>
 302         </varlistentry>
 303       </variablelist>
 304
 305       <para>The following keys can get values assigned:</para>
 306       <variablelist class='udev-directives'>
 307         <varlistentry>
 308           <term><varname>NAME</varname></term>
 309           <listitem>
 310             <para>The name to use for a network interface. The name of a device node
 311             cannot be changed by udev, only additional symlinks can be created.</para>
 312           </listitem>
 313         </varlistentry>
 314
 315         <varlistentry>
 316           <term><varname>SYMLINK</varname></term>
 317           <listitem>
 318             <para>The name of a symlink targeting the node. Every matching rule adds
 319             this value to the list of symlinks to be created.</para>
 320             <para>The set of characters to name a symlink is limited. Allowed
 321             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 322             sequences, and <literal>\x00</literal> hex encoding. All other
 323             characters are replaced by a <literal>_</literal> character.</para>
 324             <para>Multiple symlinks may be specified by separating the names by the
 325             space character. In case multiple devices claim the same name, the link
 326             always points to the device with the highest link_priority. If the current
 327             device goes away, the links are re-evaluated and the device with the
 328             next highest link_priority becomes the owner of the link. If no
 329             link_priority is specified, the order of the devices (and which one of
 330             them owns the link) is undefined.</para>
 331             <para>Symlink names must never conflict with the kernel's default device
 332             node names, as that would result in unpredictable behavior.
 333             </para>
 334           </listitem>
 335         </varlistentry>
 336
 337         <varlistentry>
 338           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 339           <listitem>
 340             <para>The permissions for the device node. Every specified value overrides
 341             the compiled-in default value.</para>
 342           </listitem>
 343         </varlistentry>
 344
 345         <varlistentry>
 346           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 347           <listitem>
 348             <para>Applies the specified Linux Security Module label to the device node.</para>
 349           </listitem>
 350         </varlistentry>
 351
 352         <varlistentry>
 353           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 354           <listitem>
 355             <para>The value that should be written to a sysfs attribute of the
 356             event device.</para>
 357           </listitem>
 358         </varlistentry>
 359
 360         <varlistentry>
 361           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 362           <listitem>
 363             <para>Set a device property value. Property names with a leading <literal>.</literal>
 364             are neither stored in the database nor exported to events or
 365             external tools (run by, for example, the <varname>PROGRAM</varname>
 366             match key).</para>
 367           </listitem>
 368         </varlistentry>
 369
 370         <varlistentry>
 371           <term><varname>TAG</varname></term>
 372           <listitem>
 373             <para>Attach a tag to a device. This is used to filter events for users
 374             of libudev's monitor functionality, or to enumerate a group of tagged
 375             devices. The implementation can only work efficiently if only a few
 376             tags are attached to a device. It is only meant to be used in
 377             contexts with specific device filter requirements, and not as a
 378             general-purpose flag. Excessive use might result in inefficient event
 379             handling.</para>
 380           </listitem>
 381         </varlistentry>
 382
 383         <varlistentry>
 384           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 385           <listitem>
 386             <para>Add a program to the list of programs to be executed after
 387             processing all the rules for a specific event, depending on
 388             <literal>type</literal>:</para>
 389             <variablelist>
 390               <varlistentry>
 391                 <term><literal>program</literal></term>
 392                 <listitem>
 393                   <para>Execute an external program specified as the assigned
 394                   value. If no absolute path is given, the program is expected
 395                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 396                   absolute path must be specified.</para>
 397                   <para>This is the default if no <replaceable>type</replaceable>
 398                   is specified.</para>
 399                 </listitem>
 400               </varlistentry>
 401               <varlistentry>
 402                 <term><literal>builtin</literal></term>
 403                 <listitem>
 404                   <para>As <varname>program</varname>, but use one of the
 405                   built-in programs rather than an external one.</para>
 406                 </listitem>
 407               </varlistentry>
 408             </variablelist>
 409             <para>The program name and following arguments are separated by spaces.
 410             Single quotes can be used to specify arguments with spaces.</para>
 411             <para>This can only be used for very short-running foreground tasks. Running an
 412             event process for a long period of time may block all further events for
 413             this or a dependent device.</para>
 414             <para>Starting daemons or other long-running processes is not appropriate
 415             for udev; the forked processes, detached or not, will be unconditionally
 416             killed after the event handling has finished.</para>
 417           </listitem>
 418         </varlistentry>
 419
 420         <varlistentry>
 421           <term><varname>LABEL</varname></term>
 422           <listitem>
 423             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 424           </listitem>
 425         </varlistentry>
 426
 427         <varlistentry>
 428           <term><varname>GOTO</varname></term>
 429           <listitem>
 430             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 431           </listitem>
 432         </varlistentry>
 433
 434         <varlistentry>
 435           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 436           <listitem>
 437             <para>Import a set of variables as device properties,
 438             depending on <literal>type</literal>:</para>
 439             <variablelist>
 440               <varlistentry>
 441                 <term><literal>program</literal></term>
 442                 <listitem>
 443                   <para>Execute an external program specified as the assigned value and
 444                   import its output, which must be in environment key
 445                   format. Path specification, command/argument separation,
 446                   and quoting work like in <varname>RUN</varname>.</para>
 447                 </listitem>
 448               </varlistentry>
 449               <varlistentry>
 450                 <term><literal>builtin</literal></term>
 451                 <listitem>
 452                   <para>Similar to <literal>program</literal>, but use one of the
 453                   built-in programs rather than an external one.</para>
 454                 </listitem>
 455               </varlistentry>
 456              <varlistentry>
 457                 <term><literal>file</literal></term>
 458                 <listitem>
 459                   <para>Import a text file specified as the assigned value, the content
 460                   of which must be in environment key format.</para>
 461                 </listitem>
 462               </varlistentry>
 463               <varlistentry>
 464                 <term><literal>db</literal></term>
 465                 <listitem>
 466                   <para>Import a single property specified as the assigned value from the
 467                   current device database. This works only if the database is already populated
 468                   by an earlier event.</para>
 469                 </listitem>
 470               </varlistentry>
 471               <varlistentry>
 472                 <term><literal>cmdline</literal></term>
 473                 <listitem>
 474                   <para>Import a single property from the kernel command line. For simple flags
 475                   the value of the property is set to <literal>1</literal>.</para>
 476                 </listitem>
 477               </varlistentry>
 478               <varlistentry>
 479                 <term><literal>parent</literal></term>
 480                 <listitem>
 481                   <para>Import the stored keys from the parent device by reading
 482                   the database entry of the parent device. The value assigned to
 483                   <option>IMPORT{parent}</option> is used as a filter of key names
 484                   to import (with the same shell glob pattern matching used for
 485                   comparisons).</para>
 486                 </listitem>
 487               </varlistentry>
 488             </variablelist>
 489             <para>This can only be used for very short-running foreground tasks. For details
 490             see <option>RUN</option>.</para>
 491           </listitem>
 492         </varlistentry>
 493
 494         <varlistentry>
 495           <term><varname>WAIT_FOR</varname></term>
 496           <listitem>
 497             <para>Wait for a file to become available or until a timeout of
 498             10 seconds expires. The path is relative to the sysfs device;
 499             if no path is specified, this waits for an attribute to appear.</para>
 500           </listitem>
 501         </varlistentry>
 502
 503         <varlistentry>
 504           <term><varname>OPTIONS</varname></term>
 505           <listitem>
 506             <para>Rule and device options:</para>
 507             <variablelist class='udev-directives'>
 508               <varlistentry>
 509                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 510                 <listitem>
 511                   <para>Specify the priority of the created symlinks. Devices with higher
 512                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 513                 </listitem>
 514               </varlistentry>
 515               <varlistentry>
 516                 <term><option>event_timeout=</option></term>
 517                 <listitem>
 518                   <para>Number of seconds an event waits for operations to finish before
 519                   giving up and terminating itself.</para>
 520                 </listitem>
 521               </varlistentry>
 522               <varlistentry>
 523                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 524                 <listitem>
 525                   <para>Usually control and other possibly unsafe characters are replaced
 526                   in strings used for device naming. The mode of replacement can be specified
 527                   with this option.</para>
 528                 </listitem>
 529               </varlistentry>
 530               <varlistentry>
 531                 <term><option>static_node=</option></term>
 532                 <listitem>
 533                   <para>Apply the permissions specified in this rule to the
 534                   static device node with the specified name. Also, for every
 535                   tag specified in this rule, create a symlink
 536                   in the directory
 537                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 538                   pointing at the static device node with the specified name.
 539                   Static device node creation is performed by systemd-tmpfiles
 540                   before systemd-udevd is started. The static nodes might not
 541                   have a corresponding kernel device; they are used to trigger
 542                   automatic kernel module loading when they are accessed.</para>
 543                 </listitem>
 544               </varlistentry>
 545               <varlistentry>
 546                 <term><option>watch</option></term>
 547                 <listitem>
 548                   <para>Watch the device node with inotify; when the node is
 549                   closed after being opened for writing, a change uevent is
 550                   synthesized.</para>
 551                 </listitem>
 552               </varlistentry>
 553               <varlistentry>
 554                 <term><option>nowatch</option></term>
 555                 <listitem>
 556                   <para>Disable the watching of a device node with inotify.</para>
 557                 </listitem>
 558               </varlistentry>
 559             </variablelist>
 560           </listitem>
 561         </varlistentry>
 562       </variablelist>
 563
 564       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 565       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 566       <varname>GROUP</varname>, <varname>MODE</varname>, and
 567       <varname>RUN</varname> fields support simple string substitutions.
 568       The <varname>RUN</varname> substitutions are performed after all rules
 569       have been processed, right before the program is executed, allowing for
 570       the use of device properties set by earlier matching rules. For all other
 571       fields, substitutions are performed while the individual rule is being
 572       processed. The available substitutions are:</para>
 573       <variablelist class='udev-directives'>
 574         <varlistentry>
 575           <term><option>$kernel</option>, <option>%k</option></term>
 576           <listitem>
 577             <para>The kernel name for this device.</para>
 578           </listitem>
 579         </varlistentry>
 580
 581         <varlistentry>
 582           <term><option>$number</option>, <option>%n</option></term>
 583           <listitem>
 584             <para>The kernel number for this device. For example,
 585               <literal>sda3</literal> has kernel number <literal>3</literal>.
 586             </para>
 587           </listitem>
 588         </varlistentry>
 589
 590         <varlistentry>
 591           <term><option>$devpath</option>, <option>%p</option></term>
 592           <listitem>
 593             <para>The devpath of the device.</para>
 594           </listitem>
 595         </varlistentry>
 596
 597         <varlistentry>
 598           <term><option>$id</option>, <option>%b</option></term>
 599           <listitem>
 600             <para>The name of the device matched while searching the devpath
 601               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 602               <option>DRIVERS</option>, and <option>ATTRS</option>.
 603             </para>
 604           </listitem>
 605         </varlistentry>
 606
 607         <varlistentry>
 608           <term><option>$driver</option></term>
 609           <listitem>
 610             <para>The driver name of the device matched while searching the
 611               devpath upwards for <option>SUBSYSTEMS</option>,
 612               <option>KERNELS</option>, <option>DRIVERS</option>, and
 613               <option>ATTRS</option>.
 614             </para>
 615           </listitem>
 616         </varlistentry>
 617
 618         <varlistentry>
 619           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 620           <listitem>
 621             <para>The value of a sysfs attribute found at the device where
 622               all keys of the rule have matched. If the matching device does not
 623               have such an attribute, and a previous <option>KERNELS</option>,
 624               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 625               <option>ATTRS</option> test selected a parent device, then the
 626               attribute from that parent device is used.
 627             </para>
 628             <para>If the attribute is a symlink, the last element of the
 629               symlink target is returned as the value.
 630             </para>
 631           </listitem>
 632         </varlistentry>
 633
 634         <varlistentry>
 635           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 636           <listitem>
 637             <para>A device property value.</para>
 638           </listitem>
 639         </varlistentry>
 640
 641         <varlistentry>
 642           <term><option>$major</option>, <option>%M</option></term>
 643           <listitem>
 644             <para>The kernel major number for the device.</para>
 645           </listitem>
 646         </varlistentry>
 647
 648         <varlistentry>
 649           <term><option>$minor</option>, <option>%m</option></term>
 650           <listitem>
 651             <para>The kernel minor number for the device.</para>
 652           </listitem>
 653         </varlistentry>
 654
 655         <varlistentry>
 656           <term><option>$result</option>, <option>%c</option></term>
 657           <listitem>
 658             <para>The string returned by the external program requested with
 659             <varname>PROGRAM</varname>.
 660             A single part of the string, separated by a space character, may be selected
 661             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 662             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 663             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 664           </listitem>
 665         </varlistentry>
 666
 667         <varlistentry>
 668           <term><option>$parent</option>, <option>%P</option></term>
 669           <listitem>
 670             <para>The node name of the parent device.</para>
 671           </listitem>
 672         </varlistentry>
 673
 674         <varlistentry>
 675           <term><option>$name</option></term>
 676           <listitem>
 677             <para>The current name of the device. If not changed by a rule, it is the
 678             name of the kernel device.</para>
 679           </listitem>
 680         </varlistentry>
 681
 682         <varlistentry>
 683           <term><option>$links</option></term>
 684           <listitem>
 685             <para>A space-separated list of the current symlinks. The value is
 686             only set during a remove event or if an earlier rule assigned a value.</para>
 687           </listitem>
 688         </varlistentry>
 689
 690         <varlistentry>
 691           <term><option>$root</option>, <option>%r</option></term>
 692           <listitem>
 693             <para>The udev_root value.</para>
 694           </listitem>
 695         </varlistentry>
 696
 697         <varlistentry>
 698           <term><option>$sys</option>, <option>%S</option></term>
 699           <listitem>
 700             <para>The sysfs mount point.</para>
 701           </listitem>
 702         </varlistentry>
 703
 704         <varlistentry>
 705           <term><option>$devnode</option>, <option>%N</option></term>
 706           <listitem>
 707             <para>The name of the device node.</para>
 708           </listitem>
 709         </varlistentry>
 710
 711         <varlistentry>
 712           <term><option>%%</option></term>
 713           <listitem>
 714           <para>The <literal>%</literal> character itself.</para>
 715           </listitem>
 716         </varlistentry>
 717
 718         <varlistentry>
 719           <term><option>$$</option></term>
 720           <listitem>
 721           <para>The <literal>$</literal> character itself.</para>
 722           </listitem>
 723         </varlistentry>
 724       </variablelist>
 725   </refsect1>
 726
 727   <refsect1><title>Hardware Database Files</title>
 728       <para>The hwdb files are read from the files located in the
 729       system hwdb directory <filename>/usr/lib/udev/hwdb.d</filename>,
 730       the volatile runtime directory <filename>/run/udev/hwdb.d</filename>
 731       and the local administration directory <filename>/etc/udev/hwdb.d</filename>.
 732       All hwdb files are collectively sorted and processed in lexical order,
 733       regardless of the directories in which they live. However, files with
 734       identical filenames replace each other. Files in <filename>/etc</filename>
 735       have the highest priority, files in <filename>/run</filename> take precedence
 736       over files with the same name in <filename>/usr/lib</filename>. This can be
 737       used to override a system-supplied hwdb file with a local file if needed;
 738       a symlink in <filename>/etc</filename> with the same name as a hwdb file in
 739       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
 740       disables the hwdb file entirely. hwdb files must have the extension
 741       <filename>.hwdb</filename>; other extensions are ignored.</para>
 742
 743       <para>The hwdb file contains data records consisting of matches and
 744       associated key-value pairs. Every record in the hwdb starts with one or
 745       more match string, specifying a shell glob to compare the database
 746       lookup string against. Multiple match lines are specified in additional
 747       consecutive lines. Every match line is compared indivdually, they are
 748       combined by OR. Every match line must start at the first character of
 749       the line.</para>
 750
 751       <para>The match lines are followed by one or more key-value pair lines, which
 752       are recognized by a leading space character. The key name and value are separated
 753       by <literal>=</literal>. An empty line signifies the end
 754       of a record. Lines beginning with <literal>#</literal> are ignored.</para>
 755
 756       <para>The content of all hwdb files is read by
 757       <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 758       and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>.
 759       During runtime only the binary database is used.</para>
 760   </refsect1>
 761
 762   <refsect1><title>Network Link Configuration</title>
 763       <para>Network link configuration is performed by the <literal>net_setup_link</literal>
 764       udev builtin.</para>
 765
 766       <para>The link files are read from the files located in the
 767       system network directory <filename>/usr/lib/systemd/network</filename>,
 768       the volatile runtime network directory <filename>/run/systemd/network</filename>
 769       and the local administration network directory <filename>/etc/systemd/network</filename>.
 770       Link files must have the extension <filename>.link</filename>; other extensions are ignored.
 771       All link files are collectively sorted and processed in lexical order,
 772       regardless of the directories in which they live. However, files with
 773       identical filenames replace each other. Files in <filename>/etc</filename>
 774       have the highest priority, files in <filename>/run</filename> take precedence
 775       over files with the same name in <filename>/usr/lib</filename>. This can be
 776       used to override a system-supplied link file with a local file if needed;
 777       a symlink in <filename>/etc</filename> with the same name as a link file in
 778       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
 779       disables the link file entirely.</para>
 780
 781       <para>The link file contains a <literal>[Match]</literal> section, which
 782       determines if a given link file may be applied to a given device; and a
 783       <literal>[Link]</literal> section specifying how the device should be
 784       configured. The first (in lexical order) of the link files that matches
 785       a given device is applied.</para>
 786
 787       <para>A link file is said to match a device if each of the entries in the
 788       <literal>[Match]</literal> section matches, or if the section is empty.
 789       The following keys are accepted:</para>
 790
 791       <variablelist class='network-directives'>
 792         <varlistentry>
 793           <term><varname>MACAddress</varname></term>
 794           <listitem>
 795             <para>The hardware address.
 796             </para>
 797           </listitem>
 798         </varlistentry>
 799         <varlistentry>
 800           <term><varname>Path</varname></term>
 801           <listitem>
 802             <para>The persistent path, as exposed by the udev property <literal>ID_PATH</literal>.</para>
 803           </listitem>
 804         </varlistentry>
 805         <varlistentry>
 806           <term><varname>Driver</varname></term>
 807           <listitem>
 808             <para>The driver currently bound to the device, as exposed by the
 809             udev property <literal>DRIVER</literal> of its parent device.</para>
 810           </listitem>
 811         </varlistentry>
 812         <varlistentry>
 813           <term><varname>Type</varname></term>
 814           <listitem>
 815             <para>The device type, as exposed by the udev property <literal>DEVTYPE</literal>.</para>
 816           </listitem>
 817         </varlistentry>
 818       </variablelist>
 819
 820       <para>The <literal>[Link]</literal> section accepts the following keys:</para>
 821
 822       <variablelist class='network-directives'>
 823         <varlistentry>
 824           <term><varname>Description</varname></term>
 825           <listitem>
 826             <para>A description of the device.</para>
 827           </listitem>
 828         </varlistentry>
 829         <varlistentry>
 830           <term><varname>Alias</varname></term>
 831           <listitem>
 832             <para>The <literal>ifalias</literal> is set to this value.</para>
 833           </listitem>
 834         </varlistentry>
 835         <varlistentry>
 836           <term><varname>MACAddressPolicy</varname></term>
 837           <listitem>
 838             <para>The policy by which the MAC address should be set. The
 839               available policies are:
 840             </para>
 841             <variablelist>
 842               <varlistentry>
 843                 <term><literal>persistent</literal></term>
 844                 <listitem>
 845                   <para>If the hardware has a persistent MAC address, as most
 846                     hardware should, and this is used by the kernel, nothing is
 847                     done. Otherwise, a new MAC address is generated which is
 848                     guaranteed to be the same on every boot for the given
 849                     machine and the given device, but which is otherwise random.
 850                   </para>
 851                 </listitem>
 852               </varlistentry>
 853               <varlistentry>
 854                 <term><literal>random</literal></term>
 855                 <listitem>
 856                   <para>If the kernel is using a random MAC address, nothing is
 857                     done. Otherwise, a new address is randomly generated each
 858                     time the device appears, typically at boot.
 859                   </para>
 860                 </listitem>
 861               </varlistentry>
 862             </variablelist>
 863           </listitem>
 864         </varlistentry>
 865         <varlistentry>
 866           <term><varname>MACAddress</varname></term>
 867           <listitem>
 868             <para>The MAC address to use, if no <literal>MACAddressPolicy</literal>
 869               is specified.
 870             </para>
 871           </listitem>
 872         </varlistentry>
 873         <varlistentry>
 874           <term><varname>NamePolicy</varname></term>
 875           <listitem>
 876             <para>An ordered, space-separated list of policies by which the
 877               interface name should be set. <literal>NamePolicy</literal> may
 878               be disabled by specifying <literal>net.ifnames=0</literal> on the
 879               kernel commandline. Each of the policies may fail, and the first
 880               successful one is used. The name is not set directly, but
 881               is exported to udev as the property <literal>ID_NET_NAME</literal>,
 882               which is, by default, used by a udev rule to set
 883               <literal>NAME</literal>. The available policies are:
 884             </para>
 885             <variablelist>
 886               <varlistentry>
 887                 <term><literal>database</literal></term>
 888                 <listitem>
 889                   <para>The name is set based on entries in the Hardware
 890                     Database with the key
 891                     <literal>ID_NET_NAME_FROM_DATABASE</literal>.
 892                   </para>
 893                 </listitem>
 894               </varlistentry>
 895               <varlistentry>
 896                 <term><literal>onboard</literal></term>
 897                 <listitem>
 898                   <para>The name is set based on information given by the
 899                     firmware for on-board devices, as exported by the udev
 900                     property <literal>ID_NET_NAME_ONBOARD</literal>.
 901                   </para>
 902                 </listitem>
 903               </varlistentry>
 904               <varlistentry>
 905                 <term><literal>slot</literal></term>
 906                 <listitem>
 907                   <para>The name is set based on information given by the
 908                     firmware for hot-plug devices, as exported by the udev
 909                     property <literal>ID_NET_NAME_SLOT</literal>.
 910                   </para>
 911                 </listitem>
 912               </varlistentry>
 913               <varlistentry>
 914                 <term><literal>path</literal></term>
 915                 <listitem>
 916                   <para>The name is set based on the device's physical location,
 917                     as exported by the udev property
 918                     <literal>ID_NET_NAME_PATH</literal>.
 919                   </para>
 920                 </listitem>
 921               </varlistentry>
 922               <varlistentry>
 923                 <term><literal>mac</literal></term>
 924                 <listitem>
 925                   <para>The name is set based on the device's persistent MAC
 926                     address, as exported by the udev property
 927                     <literal>ID_NET_NAME_MAC</literal>.
 928                   </para>
 929                 </listitem>
 930               </varlistentry>
 931             </variablelist>
 932           </listitem>
 933         </varlistentry>
 934         <varlistentry>
 935           <term><varname>Name</varname></term>
 936           <listitem>
 937             <para>The interface name to use in case all the policies specified
 938               in <literal>NamePolicy</literal> fail, or in case
 939               <literal>NamePolicy</literal> is missing or disabled.
 940             </para>
 941           </listitem>
 942         </varlistentry>
 943         <varlistentry>
 944           <term><varname>MTU</varname></term>
 945           <listitem>
 946             <para>The MTU to set for the device.</para>
 947           </listitem>
 948         </varlistentry>
 949         <varlistentry>
 950           <term><varname>SpeedMBytes</varname></term>
 951           <listitem>
 952             <para>The speed to set for the device.</para>
 953           </listitem>
 954         </varlistentry>
 955         <varlistentry>
 956           <term><varname>Duplex</varname></term>
 957           <listitem>
 958             <para>The duplex mode to set for the device. The accepted values
 959               are <literal>half</literal> and <literal>full</literal>.
 960             </para>
 961           </listitem>
 962         </varlistentry>
 963         <varlistentry>
 964           <term><varname>WakeOnLan</varname></term>
 965           <listitem>
 966             <para>The Wake-on-LAN policy to set for the device. The supported
 967               values are:
 968             </para>
 969             <variablelist>
 970               <varlistentry>
 971                 <term><literal>phy</literal></term>
 972                 <listitem>
 973                   <para>Wake on PHY activity.</para>
 974                 </listitem>
 975               </varlistentry>
 976               <varlistentry>
 977                 <term><literal>magic</literal></term>
 978                 <listitem>
 979                   <para>Wake on receipt of a magic packet.</para>
 980                 </listitem>
 981               </varlistentry>
 982               <varlistentry>
 983                 <term><literal>off</literal></term>
 984                 <listitem>
 985                   <para>Never wake.</para>
 986                 </listitem>
 987               </varlistentry>
 988             </variablelist>
 989           </listitem>
 990         </varlistentry>
 991       </variablelist>
 992   </refsect1>
 993
 994   <refsect1>
 995     <title>See Also</title>
 996     <para>
 997       <citerefentry>
 998         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 999       </citerefentry>,
1000       <citerefentry>
1001         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
1002       </citerefentry>
1003     </para>
1004   </refsect1>
1005 </refentry>