man/systemd.link.xml

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   5
   6 <refentry id="systemd.link">
   7   <refentryinfo>
   8     <title>systemd.link</title>
   9     <productname>systemd</productname>
  10   </refentryinfo>
  11
  12   <refmeta>
  13     <refentrytitle>systemd.link</refentrytitle>
  14     <manvolnum>5</manvolnum>
  15   </refmeta>
  16
  17   <refnamediv>
  18     <refname>systemd.link</refname>
  19     <refpurpose>Network device configuration</refpurpose>
  20   </refnamediv>
  21
  22   <refsynopsisdiv>
  23     <para><filename><replaceable>link</replaceable>.link</filename></para>
  24   </refsynopsisdiv>
  25
  26   <refsect1>
  27     <title>Description</title>
  28
  29     <para>Network link configuration is performed by the
  30     <command>net_setup_link</command> udev builtin.</para>
  31
  32     <para>The link files are read from the files located in the system
  33     network directory <filename>/usr/lib/systemd/network</filename>,
  34     the volatile runtime network directory
  35     <filename>/run/systemd/network</filename>, and the local
  36     administration network directory
  37     <filename>/etc/systemd/network</filename>. Link files must have
  38     the extension <filename>.link</filename>; other extensions are
  39     ignored. All link files are collectively sorted and processed in
  40     lexical order, regardless of the directories in which they live.
  41     However, files with identical filenames replace each other. Files
  42     in <filename>/etc</filename> have the highest priority, files in
  43     <filename>/run</filename> take precedence over files with the same
  44     name in <filename>/usr/lib</filename>. This can be used to
  45     override a system-supplied link file with a local file if needed.
  46     As a special case, an empty file (file size 0) or symlink with the
  47     same name pointing to <filename>/dev/null</filename> disables the
  48     configuration file entirely (it is "masked").</para>
  49
  50     <para>The link file contains a [Match] section, which determines if a given link file may be applied to a
  51     given device, as well as a [Link] section specifying how the device should be configured. The first (in
  52     lexical order) of the link files that matches a given device is applied. Note that a default file
  53     <filename>99-default.link</filename> is shipped by the system. Any user-supplied
  54     <filename>.link</filename> should hence have a lexically earlier name to be considered at all.</para>
  55
  56     <para>See <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
  57     diagnosing problems with <filename>.link</filename> files.</para>
  58   </refsect1>
  59
  60   <refsect1>
  61     <title>[Match] Section Options</title>
  62
  63     <para>A link file is said to match a device if all matches specified by the
  64     <literal>[Match]</literal> section are satisfied. When a link file does not contain valid settings
  65     in <literal>[Match]</literal> section, then the file will match all devices and
  66     <command>systemd-udevd</command> warns about that. Hint: to avoid the warning and to make it clear
  67     that all interfaces shall be matched, add the following:
  68     <programlisting>OriginalName=*</programlisting>
  69     The following keys are accepted:</para>
  70
  71     <variablelist class='network-directives'>
  72       <varlistentry>
  73         <term><varname>MACAddress=</varname></term>
  74         <listitem>
  75           <para>A whitespace-separated list of hardware addresses. Use full colon-, hyphen- or dot-delimited hexadecimal. See the example below.
  76           This option may appear more than once, in which case the lists are merged. If the empty string is assigned to this option, the list
  77           of hardware addresses defined prior to this is reset.</para>
  78
  79           <para>Example:
  80           <programlisting>MACAddress=01:23:45:67:89:ab 00-11-22-33-44-55 AABB.CCDD.EEFF</programlisting></para>
  81         </listitem>
  82       </varlistentry>
  83       <varlistentry>
  84         <term><varname>OriginalName=</varname></term>
  85         <listitem>
  86           <para>A whitespace-separated list of shell-style globs matching
  87           the device name, as exposed by the udev property
  88           "INTERFACE". This cannot be used to match on names that have
  89           already been changed from userspace. Caution is advised when matching on
  90           kernel-assigned names, as they are known to be unstable
  91           between reboots.</para>
  92         </listitem>
  93       </varlistentry>
  94       <varlistentry>
  95         <term><varname>Path=</varname></term>
  96         <listitem>
  97           <para>A whitespace-separated list of shell-style globs matching
  98           the persistent path, as exposed by the udev property
  99           <varname>ID_PATH</varname>.</para>
 100         </listitem>
 101       </varlistentry>
 102       <varlistentry>
 103         <term><varname>Driver=</varname></term>
 104         <listitem>
 105           <para>A whitespace-separated list of shell-style globs matching the driver currently bound to the
 106           device, as exposed by the udev property <varname>DRIVER</varname> of its parent device, or if that
 107           is not set, the driver as exposed by <command>ethtool -i</command> of the device itself.</para>
 108         </listitem>
 109       </varlistentry>
 110       <varlistentry>
 111         <term><varname>Type=</varname></term>
 112         <listitem>
 113           <para>A whitespace-separated list of shell-style globs matching
 114           the device type, as exposed by the udev
 115           property <varname>DEVTYPE</varname>.</para>
 116         </listitem>
 117       </varlistentry>
 118       <varlistentry>
 119         <term><varname>Host=</varname></term>
 120         <listitem>
 121           <para>Matches against the hostname or machine ID of the host. See <varname>ConditionHost=</varname> in
 122           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 123           for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
 124           If an empty string is assigned, then previously assigned value is cleared.
 125           </para>
 126         </listitem>
 127       </varlistentry>
 128       <varlistentry>
 129         <term><varname>Virtualization=</varname></term>
 130         <listitem>
 131           <para>Checks whether the system is executed in a virtualized environment and optionally test
 132           whether it is a specific implementation. See <varname>ConditionVirtualization=</varname> in
 133           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 134           for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
 135           If an empty string is assigned, then previously assigned value is cleared.
 136           </para>
 137         </listitem>
 138       </varlistentry>
 139       <varlistentry>
 140         <term><varname>KernelCommandLine=</varname></term>
 141         <listitem>
 142           <para>Checks whether a specific kernel command line option is set. See
 143           <varname>ConditionKernelCommandLine=</varname> in
 144           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 145           for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
 146           If an empty string is assigned, then previously assigned value is cleared.
 147           </para>
 148         </listitem>
 149       </varlistentry>
 150       <varlistentry>
 151         <term><varname>KernelVersion=</varname></term>
 152         <listitem>
 153           <para>Checks whether the kernel version (as reported by <command>uname -r</command>) matches a certain
 154           expression. See <varname>ConditionKernelVersion=</varname> in
 155           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 156           details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
 157           If an empty string is assigned, then previously assigned value is cleared.
 158           </para>
 159         </listitem>
 160       </varlistentry>
 161       <varlistentry>
 162         <term><varname>Architecture=</varname></term>
 163         <listitem>
 164           <para>Checks whether the system is running on a specific architecture. See
 165           <varname>ConditionArchitecture=</varname> in
 166           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 167           for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
 168           If an empty string is assigned, then previously assigned value is cleared.
 169           </para>
 170         </listitem>
 171       </varlistentry>
 172     </variablelist>
 173
 174   </refsect1>
 175
 176   <refsect1>
 177     <title>[Link] Section Options</title>
 178
 179     <para>The [Link] section accepts the following
 180     keys:</para>
 181
 182     <variablelist class='network-directives'>
 183       <varlistentry>
 184         <term><varname>Description=</varname></term>
 185         <listitem>
 186           <para>A description of the device.</para>
 187         </listitem>
 188       </varlistentry>
 189       <varlistentry>
 190         <term><varname>Alias=</varname></term>
 191         <listitem>
 192           <para>The <varname>ifalias</varname> interface property is set to this value.</para>
 193         </listitem>
 194       </varlistentry>
 195       <varlistentry>
 196         <term><varname>MACAddressPolicy=</varname></term>
 197         <listitem>
 198           <para>The policy by which the MAC address should be set. The
 199           available policies are:
 200           </para>
 201
 202           <variablelist>
 203             <varlistentry>
 204               <term><option>persistent</option></term>
 205               <listitem>
 206                 <para>If the hardware has a persistent MAC address, as
 207                 most hardware should, and if it is used by the kernel,
 208                 nothing is done. Otherwise, a new MAC address is
 209                 generated which is guaranteed to be the same on every
 210                 boot for the given machine and the given device, but
 211                 which is otherwise random. This feature depends on ID_NET_NAME_*
 212                 properties to exist for the link. On hardware where these
 213                 properties are not set, the generation of a persistent MAC address
 214                 will fail.</para>
 215               </listitem>
 216             </varlistentry>
 217             <varlistentry>
 218               <term><option>random</option></term>
 219               <listitem>
 220                 <para>If the kernel is using a random MAC address,
 221                 nothing is done. Otherwise, a new address is randomly
 222                 generated each time the device appears, typically at
 223                 boot. Either way, the random address will have the
 224                 <literal>unicast</literal> and
 225                 <literal>locally administered</literal> bits set.</para>
 226               </listitem>
 227             </varlistentry>
 228             <varlistentry>
 229               <term><option>none</option></term>
 230               <listitem>
 231                 <para>Keeps the MAC address assigned by the kernel.</para>
 232               </listitem>
 233             </varlistentry>
 234           </variablelist>
 235         </listitem>
 236       </varlistentry>
 237       <varlistentry>
 238         <term><varname>MACAddress=</varname></term>
 239         <listitem>
 240           <para>The MAC address to use, if no
 241           <varname>MACAddressPolicy=</varname>
 242           is specified.</para>
 243         </listitem>
 244       </varlistentry>
 245       <varlistentry>
 246         <term><varname>NamePolicy=</varname></term>
 247         <listitem>
 248           <para>An ordered, space-separated list of policies by which the interface name should be set.
 249           <varname>NamePolicy=</varname> may be disabled by specifying <option>net.ifnames=0</option> on the
 250           kernel command line. Each of the policies may fail, and the first successful one is used. The name
 251           is not set directly, but is exported to udev as the property <option>ID_NET_NAME</option>, which
 252           is, by default, used by a
 253           <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 254           rule to set <varname>NAME</varname>. The available policies are:
 255           </para>
 256
 257           <variablelist>
 258             <varlistentry>
 259               <term><option>kernel</option></term>
 260               <listitem>
 261                 <para>If the kernel claims that the name it has set
 262                 for a device is predictable, then no renaming is
 263                 performed.</para>
 264               </listitem>
 265             </varlistentry>
 266             <varlistentry>
 267               <term><option>database</option></term>
 268               <listitem>
 269                 <para>The name is set based on entries in the udev's
 270                 Hardware Database with the key
 271                 <varname>ID_NET_NAME_FROM_DATABASE</varname>.
 272                 </para>
 273               </listitem>
 274             </varlistentry>
 275             <varlistentry>
 276               <term><option>onboard</option></term>
 277               <listitem>
 278                 <para>The name is set based on information given by
 279                 the firmware for on-board devices, as exported by the
 280                 udev property <varname>ID_NET_NAME_ONBOARD</varname>.
 281                 See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 282                 </para>
 283               </listitem>
 284             </varlistentry>
 285             <varlistentry>
 286               <term><option>slot</option></term>
 287               <listitem>
 288                 <para>The name is set based on information given by
 289                 the firmware for hot-plug devices, as exported by the
 290                 udev property <varname>ID_NET_NAME_SLOT</varname>.
 291                 See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 292                 </para>
 293               </listitem>
 294             </varlistentry>
 295             <varlistentry>
 296               <term><option>path</option></term>
 297               <listitem>
 298                 <para>The name is set based on the device's physical
 299                 location, as exported by the udev property
 300                 <varname>ID_NET_NAME_PATH</varname>.
 301                 See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 302                 </para>
 303               </listitem>
 304             </varlistentry>
 305             <varlistentry>
 306               <term><option>mac</option></term>
 307               <listitem>
 308                 <para>The name is set based on the device's persistent
 309                 MAC address, as exported by the udev property
 310                 <varname>ID_NET_NAME_MAC</varname>.
 311                 See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 312                 </para>
 313               </listitem>
 314             </varlistentry>
 315             <varlistentry>
 316               <term><option>keep</option></term>
 317               <listitem>
 318                 <para>If the device already had a name given by userspace (as part of creation of the device
 319                 or a rename), keep it.</para>
 320               </listitem>
 321             </varlistentry>
 322           </variablelist>
 323         </listitem>
 324       </varlistentry>
 325       <varlistentry>
 326         <term><varname>Name=</varname></term>
 327         <listitem>
 328           <para>The interface name to use. This option has lower precedence than
 329           <varname>NamePolicy=</varname>, so for this setting to take effect, <varname>NamePolicy=</varname>
 330           must either be unset, empty, disabled, or all policies configured there must fail. Also see the
 331           example below with <literal>Name=dmz0</literal>.</para>
 332
 333           <para>Note that specifying a name that the kernel might use for another
 334           interface (for example <literal>eth0</literal>) is dangerous because the
 335           name assignment done by udev will race with the assignment done by the
 336           kernel, and only one interface may use the name. Depending on the order of
 337           operations, either udev or the kernel will win, making the naming
 338           unpredictable. It is best to use some different prefix, for example
 339           <literal>internal0</literal>/<literal>external0</literal> or
 340           <literal>lan0</literal>/<literal>lan1</literal>/<literal>lan3</literal>.
 341           </para>
 342         </listitem>
 343       </varlistentry>
 344       <varlistentry>
 345         <term><varname>MTUBytes=</varname></term>
 346         <listitem>
 347           <para>The maximum transmission unit in bytes to set for the
 348           device. The usual suffixes K, M, G, are supported and are
 349           understood to the base of 1024.</para>
 350         </listitem>
 351       </varlistentry>
 352       <varlistentry>
 353         <term><varname>BitsPerSecond=</varname></term>
 354         <listitem>
 355           <para>The speed to set for the device, the value is rounded
 356           down to the nearest Mbps. The usual suffixes K, M, G, are
 357           supported and are understood to the base of 1000.</para>
 358         </listitem>
 359       </varlistentry>
 360       <varlistentry>
 361         <term><varname>Duplex=</varname></term>
 362         <listitem>
 363           <para>The duplex mode to set for the device. The accepted values are <option>half</option> and
 364           <option>full</option>.</para>
 365         </listitem>
 366       </varlistentry>
 367       <varlistentry>
 368         <term><varname>AutoNegotiation=</varname></term>
 369         <listitem>
 370           <para>Takes a boolean. If set to yes, automatic negotiation of transmission parameters is enabled.
 371           Autonegotiation is a procedure by which two connected ethernet devices choose
 372           common transmission parameters, such as speed, duplex mode, and flow control.
 373           When unset, the kernel's default will be used.</para>
 374
 375           <para>Note that if autonegotiation is enabled, speed and duplex settings are
 376           read-only. If autonegotation is disabled, speed and duplex settings are writable
 377           if the driver supports multiple link modes.</para>
 378         </listitem>
 379       </varlistentry>
 380       <varlistentry>
 381         <term><varname>WakeOnLan=</varname></term>
 382         <listitem>
 383           <para>The Wake-on-LAN policy to set for the device. The
 384           supported values are:</para>
 385
 386           <variablelist>
 387             <varlistentry>
 388               <term><option>phy</option></term>
 389               <listitem>
 390                 <para>Wake on PHY activity.</para>
 391               </listitem>
 392             </varlistentry>
 393             <varlistentry>
 394               <term><option>unicast</option></term>
 395               <listitem>
 396                 <para>Wake on unicast messages.</para>
 397               </listitem>
 398             </varlistentry>
 399             <varlistentry>
 400               <term><option>multicast</option></term>
 401               <listitem>
 402                 <para>Wake on multicast messages.</para>
 403               </listitem>
 404             </varlistentry>
 405             <varlistentry>
 406               <term><option>broadcast</option></term>
 407               <listitem>
 408                 <para>Wake on broadcast messages.</para>
 409               </listitem>
 410             </varlistentry>
 411             <varlistentry>
 412               <term><option>arp</option></term>
 413               <listitem>
 414                 <para>Wake on ARP.</para>
 415               </listitem>
 416             </varlistentry>
 417             <varlistentry>
 418               <term><option>magic</option></term>
 419               <listitem>
 420                 <para>Wake on receipt of a magic packet.
 421                 </para>
 422               </listitem>
 423             </varlistentry>
 424             <varlistentry>
 425               <term><option>secureon</option></term>
 426               <listitem>
 427                 <para>Enable secureon(tm) password for MagicPacket(tm).
 428                 </para>
 429               </listitem>
 430             </varlistentry>
 431             <varlistentry>
 432               <term><option>off</option></term>
 433               <listitem>
 434                 <para>Never wake.</para>
 435               </listitem>
 436             </varlistentry>
 437           </variablelist>
 438
 439           <para>Defaults to <option>off</option>.</para>
 440         </listitem>
 441       </varlistentry>
 442       <varlistentry>
 443         <term><varname>Port=</varname></term>
 444         <listitem>
 445           <para>The port option is used to select the device port. The
 446           supported values are:</para>
 447
 448           <variablelist>
 449             <varlistentry>
 450               <term><option>tp</option></term>
 451               <listitem>
 452                 <para>An Ethernet interface using Twisted-Pair cable as the medium.</para>
 453               </listitem>
 454             </varlistentry>
 455             <varlistentry>
 456               <term><option>aui</option></term>
 457               <listitem>
 458                 <para>Attachment Unit Interface (AUI). Normally used with hubs.
 459                 </para>
 460               </listitem>
 461             </varlistentry>
 462             <varlistentry>
 463               <term><option>bnc</option></term>
 464               <listitem>
 465                 <para>An Ethernet interface using BNC connectors and co-axial cable.</para>
 466               </listitem>
 467             </varlistentry>
 468             <varlistentry>
 469               <term><option>mii</option></term>
 470               <listitem>
 471                 <para>An Ethernet interface using a Media Independent Interface (MII).</para>
 472               </listitem>
 473             </varlistentry>
 474             <varlistentry>
 475               <term><option>fibre</option></term>
 476               <listitem>
 477                 <para>An Ethernet interface using Optical Fibre as the medium.</para>
 478               </listitem>
 479             </varlistentry>
 480           </variablelist>
 481         </listitem>
 482       </varlistentry>
 483       <varlistentry>
 484         <term><varname>Advertise=</varname></term>
 485         <listitem>
 486           <para>This sets what speeds and duplex modes of operation are advertised for auto-negotiation.
 487           This implies <literal>AutoNegotiation=yes</literal>. The supported values are:
 488
 489           <table>
 490             <title>Supported advertise values</title>
 491             <tgroup cols='3'>
 492               <colspec colname='Advertise' />
 493               <colspec colname='Speed' />
 494               <colspec colname='Duplex Mode' />
 495
 496               <thead><row>
 497                 <entry>Advertise</entry>
 498                 <entry>Speed (Mbps)</entry>
 499                 <entry>Duplex Mode</entry>
 500               </row></thead>
 501               <tbody>
 502                 <row><entry><option>10baset-half</option></entry>
 503                 <entry>10</entry><entry>half</entry></row>
 504
 505                 <row><entry><option>10baset-full</option></entry>
 506                 <entry>10</entry><entry>full</entry></row>
 507
 508                 <row><entry><option>100baset-half</option></entry>
 509                 <entry>100</entry><entry>half</entry></row>
 510
 511                 <row><entry><option>100baset-full</option></entry>
 512                 <entry>100</entry><entry>full</entry></row>
 513
 514                 <row><entry><option>1000baset-half</option></entry>
 515                 <entry>1000</entry><entry>half</entry></row>
 516
 517                 <row><entry><option>1000baset-full</option></entry>
 518                 <entry>1000</entry><entry>full</entry></row>
 519
 520                 <row><entry><option>10000baset-full</option></entry>
 521                 <entry>10000</entry><entry>full</entry></row>
 522
 523                 <row><entry><option>2500basex-full</option></entry>
 524                 <entry>2500</entry><entry>full</entry></row>
 525
 526                 <row><entry><option>1000basekx-full</option></entry>
 527                 <entry>1000</entry><entry>full</entry></row>
 528
 529                 <row><entry><option>10000basekx4-full</option></entry>
 530                 <entry>10000</entry><entry>full</entry></row>
 531
 532                 <row><entry><option>10000basekr-full</option></entry>
 533                 <entry>10000</entry><entry>full</entry></row>
 534
 535                 <row><entry><option>10000baser-fec</option></entry>
 536                 <entry>10000</entry><entry>full</entry></row>
 537
 538                 <row><entry><option>20000basemld2-full</option></entry>
 539                 <entry>20000</entry><entry>full</entry></row>
 540
 541                 <row><entry><option>20000basekr2-full</option></entry>
 542                 <entry>20000</entry><entry>full</entry></row>
 543               </tbody>
 544             </tgroup>
 545           </table>
 546
 547           By default this is unset, i.e. all possible modes will be advertised.
 548           This option may be specified more than once, in which case all specified speeds and modes are advertised.
 549           If the empty string is assigned to this option, the list is reset, and all prior assignments have no effect.
 550           </para>
 551         </listitem>
 552       </varlistentry>
 553       <varlistentry>
 554         <term><varname>TCPSegmentationOffload=</varname></term>
 555         <listitem>
 556           <para>Takes a boolean. If set to true, the TCP Segmentation Offload (TSO) is enabled.
 557           When unset, the kernel's default will be used.</para>
 558         </listitem>
 559       </varlistentry>
 560        <varlistentry>
 561       <term><varname>TCP6SegmentationOffload=</varname></term>
 562         <listitem>
 563           <para>Takes a boolean. If set to true, the TCP6 Segmentation Offload (tx-tcp6-segmentation) is enabled.
 564           When unset, the kernel's default will be used.</para>
 565         </listitem>
 566       </varlistentry>
 567       <varlistentry>
 568         <term><varname>GenericSegmentationOffload=</varname></term>
 569         <listitem>
 570           <para>Takes a boolean. If set to true, the Generic Segmentation Offload (GSO) is enabled.
 571           When unset, the kernel's default will be used.</para>
 572         </listitem>
 573       </varlistentry>
 574     <varlistentry>
 575         <term><varname>GenericReceiveOffload=</varname></term>
 576         <listitem>
 577           <para>Takes a boolean. If set to true, the Generic Receive Offload (GRO) is enabled.
 578           When unset, the kernel's default will be used.</para>
 579         </listitem>
 580       </varlistentry>
 581       <varlistentry>
 582         <term><varname>LargeReceiveOffload=</varname></term>
 583         <listitem>
 584           <para>Takes a boolean. If set to true, the Large Receive Offload (LRO) is enabled.
 585           When unset, the kernel's default will be used.</para>
 586         </listitem>
 587       </varlistentry>
 588       <varlistentry>
 589         <term><varname>RxChannels=</varname></term>
 590         <listitem>
 591           <para>Sets the number of receive channels (a number between 1 and 4294967295) .</para>
 592         </listitem>
 593       </varlistentry>
 594       <varlistentry>
 595         <term><varname>TxChannels=</varname></term>
 596         <listitem>
 597           <para>Sets the number of transmit channels (a number between 1 and 4294967295).</para>
 598         </listitem>
 599       </varlistentry>
 600       <varlistentry>
 601         <term><varname>OtherChannels=</varname></term>
 602         <listitem>
 603           <para>Sets the number of other channels (a number between 1 and 4294967295).</para>
 604         </listitem>
 605       </varlistentry>
 606       <varlistentry>
 607         <term><varname>CombinedChannels=</varname></term>
 608         <listitem>
 609           <para>Sets the number of combined set channels (a number between 1 and 4294967295).</para>
 610         </listitem>
 611       </varlistentry>
 612     </variablelist>
 613   </refsect1>
 614
 615   <refsect1>
 616     <title>Examples</title>
 617
 618     <example>
 619       <title>/usr/lib/systemd/network/99-default.link</title>
 620
 621       <para>The link file <filename>99-default.link</filename> that is
 622       shipped with systemd defines the default naming policy for
 623       links.</para>
 624
 625       <programlisting>[Link]
 626 NamePolicy=kernel database onboard slot path
 627 MACAddressPolicy=persistent</programlisting>
 628     </example>
 629
 630     <example>
 631       <title>/etc/systemd/network/10-dmz.link</title>
 632
 633       <para>This example assigns the fixed name <literal>dmz0</literal> to the interface with the MAC address
 634       00:a0:de:63:7a:e6:</para>
 635
 636       <programlisting>[Match]
 637 MACAddress=00:a0:de:63:7a:e6
 638
 639 [Link]
 640 Name=dmz0</programlisting>
 641
 642       <para><varname>NamePolicy=</varname> is not set, so <varname>Name=</varname> takes effect. We use the
 643       <literal>10-</literal> prefix to order this file early in the list. Note that it needs to before
 644       <literal>99-link</literal>, i.e. it needs a numerical prefix, to have any effect at all.</para>
 645     </example>
 646
 647     <example>
 648       <title>Debugging <varname>NamePolicy=</varname> assignments</title>
 649
 650       <programlisting>$ sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/hub0
 651 …
 652 Parsed configuration file /usr/lib/systemd/network/99-default.link
 653 Parsed configuration file /etc/systemd/network/10-eth0.link
 654 ID_NET_DRIVER=cdc_ether
 655 Config file /etc/systemd/network/10-eth0.link applies to device hub0
 656 link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
 657 hub0: Device has name_assign_type=4
 658 Using default interface naming scheme 'v240'.
 659 hub0: Policies didn't yield a name, using specified Name=hub0.
 660 ID_NET_LINK_FILE=/etc/systemd/network/10-eth0.link
 661 ID_NET_NAME=hub0
 662 …</programlisting>
 663
 664      <para>Explicit <varname>Name=</varname> configuration wins in this case.</para>
 665
 666      <programlisting>sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/enp0s31f6
 667 …
 668 Parsed configuration file /usr/lib/systemd/network/99-default.link
 669 Parsed configuration file /etc/systemd/network/10-eth0.link
 670 Created link configuration context.
 671 ID_NET_DRIVER=e1000e
 672 Config file /usr/lib/systemd/network/99-default.link applies to device enp0s31f6
 673 link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
 674 enp0s31f6: Device has name_assign_type=4
 675 Using default interface naming scheme 'v240'.
 676 enp0s31f6: Policy *keep*: keeping existing userspace name
 677 enp0s31f6: Device has addr_assign_type=0
 678 enp0s31f6: MAC on the device already matches policy *persistent*
 679 ID_NET_LINK_FILE=/usr/lib/systemd/network/99-default.link
 680 …
 681 </programlisting>
 682
 683      <para>In this case, the interface was already renamed, so the <option>keep</option> policy specified as
 684      the first option in <filename noindex='true'>99-default.link</filename> means that the existing name is
 685      preserved. If <option>keep</option> was removed, or if were in boot before the renaming has happened,
 686      we might get the following instead:</para>
 687
 688      <programlisting>enp0s31f6: Policy *path* yields "enp0s31f6".
 689 enp0s31f6: Device has addr_assign_type=0
 690 enp0s31f6: MAC on the device already matches policy *persistent*
 691 ID_NET_LINK_FILE=/usr/lib/systemd/network/99-default.link
 692 ID_NET_NAME=enp0s31f6
 693 …
 694 </programlisting>
 695
 696       <para>Please note that the details of output are subject to change.</para>
 697     </example>
 698
 699     <example>
 700       <title>/etc/systemd/network/10-internet.link</title>
 701
 702       <para>This example assigns the fixed name
 703       <literal>internet0</literal> to the interface with the device
 704       path <literal>pci-0000:00:1a.0-*</literal>:</para>
 705
 706       <programlisting>[Match]
 707 Path=pci-0000:00:1a.0-*
 708
 709 [Link]
 710 Name=internet0</programlisting>
 711     </example>
 712
 713     <example>
 714       <title>/etc/systemd/network/25-wireless.link</title>
 715
 716       <para>Here's an overly complex example that shows the use of a large number of [Match] and [Link] settings.</para>
 717
 718       <programlisting>[Match]
 719 MACAddress=12:34:56:78:9a:bc
 720 Driver=brcmsmac
 721 Path=pci-0000:02:00.0-*
 722 Type=wlan
 723 Virtualization=no
 724 Host=my-laptop
 725 Architecture=x86-64
 726
 727 [Link]
 728 Name=wireless0
 729 MTUBytes=1450
 730 BitsPerSecond=10M
 731 WakeOnLan=magic
 732 MACAddress=cb:a9:87:65:43:21</programlisting>
 733     </example>
 734   </refsect1>
 735
 736   <refsect1>
 737     <title>See Also</title>
 738     <para>
 739       <citerefentry>
 740         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 741       </citerefentry>,
 742       <citerefentry>
 743         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 744       </citerefentry>,
 745       <citerefentry>
 746         <refentrytitle>systemd.netdev</refentrytitle><manvolnum>5</manvolnum>
 747       </citerefentry>,
 748       <citerefentry>
 749         <refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum>
 750       </citerefentry>
 751     </para>
 752   </refsect1>
 753
 754 </refentry>