[thirdparty/systemd.git] / man / systemd.link.xml

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<refentry id="systemd.link">
  <refentryinfo>
    <title>systemd.link</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.link</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.link</refname>
    <refpurpose>Network device configuration</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename><replaceable>link</replaceable>.link</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>Network link configuration is performed by the
    <command>net_setup_link</command> udev builtin.</para>

    <para>The link files are read from the files located in the system
    network directory <filename>/usr/lib/systemd/network</filename>,
    the volatile runtime network directory
    <filename>/run/systemd/network</filename>, and the local
    administration network directory
    <filename>/etc/systemd/network</filename>. Link files must have
    the extension <filename>.link</filename>; other extensions are
    ignored. All link files are collectively sorted and processed in
    lexical order, regardless of the directories in which they live.
    However, files with identical filenames replace each other. Files
    in <filename>/etc</filename> have the highest priority, files in
    <filename>/run</filename> take precedence over files with the same
    name in <filename>/usr/lib</filename>. This can be used to
    override a system-supplied link file with a local file if needed.
    As a special case, an empty file (file size 0) or symlink with the
    same name pointing to <filename>/dev/null</filename> disables the
    configuration file entirely (it is "masked").</para>

    <para>The link file contains a [Match] section, which determines if a given link file may be applied to a
    given device, as well as a [Link] section specifying how the device should be configured. The first (in
    lexical order) of the link files that matches a given device is applied. Note that a default file
    <filename>99-default.link</filename> is shipped by the system. Any user-supplied
    <filename>.link</filename> should hence have a lexically earlier name to be considered at all.</para>

    <para>See <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
    diagnosing problems with <filename>.link</filename> files.</para>
  </refsect1>

  <refsect1>
    <title>[Match] Section Options</title>

    <para>A link file is said to match a device if all matches specified by the
    <literal>[Match]</literal> section are satisfied. When a link file does not contain valid settings
    in <literal>[Match]</literal> section, then the file will match all devices and
    <command>systemd-udevd</command> warns about that. Hint: to avoid the warning and to make it clear
    that all interfaces shall be matched, add the following:
    <programlisting>OriginalName=*</programlisting>
    The following keys are accepted:</para>

    <variablelist class='network-directives'>
      <varlistentry>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>A whitespace-separated list of hardware addresses. Use full colon-, hyphen- or dot-delimited hexadecimal. See the example below.
          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
          of hardware addresses defined prior to this is reset.</para>

          <para>Example:
          <programlisting>MACAddress=01:23:45:67:89:ab 00-11-22-33-44-55 AABB.CCDD.EEFF</programlisting></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>OriginalName=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the device name, as exposed by the udev property
          "INTERFACE". This cannot be used to match on names that have
          already been changed from userspace. Caution is advised when matching on
          kernel-assigned names, as they are known to be unstable
          between reboots.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Path=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the persistent path, as exposed by the udev property
          <varname>ID_PATH</varname>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Driver=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching the driver currently bound to the
          device, as exposed by the udev property <varname>ID_NET_DRIVER</varname> of its parent device, or if that
          is not set, the driver as exposed by <command>ethtool -i</command> of the device itself.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Type=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the device type, as exposed by the udev
          property <varname>DEVTYPE</varname>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Property=</varname></term>
        <listitem>
          <para>A whitespace-separated list of udev property name with its value after a equal
          (<literal>=</literal>). If multiple properties are specified, the test results are ANDed.
          If the list is prefixed with a "!", the test is inverted. If a value contains white
          spaces, then please quote whole key and value pair. If a value contains quotation, then
          please escape the quotation with <literal>\</literal>.</para>

          <para>Example: if a .link file has the following:
          <programlisting>Property=ID_MODEL_ID=9999 "ID_VENDOR_FROM_DATABASE=vendor name" "KEY=with \"quotation\""</programlisting>
          then, the .link file matches only when an interface has all the above three properties.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Host=</varname></term>
        <listitem>
          <para>Matches against the hostname or machine ID of the host. See <varname>ConditionHost=</varname> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
          If an empty string is assigned, then previously assigned value is cleared.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Virtualization=</varname></term>
        <listitem>
          <para>Checks whether the system is executed in a virtualized environment and optionally test
          whether it is a specific implementation. See <varname>ConditionVirtualization=</varname> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
          If an empty string is assigned, then previously assigned value is cleared.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>KernelCommandLine=</varname></term>
        <listitem>
          <para>Checks whether a specific kernel command line option is set. See
          <varname>ConditionKernelCommandLine=</varname> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
          If an empty string is assigned, then previously assigned value is cleared.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>KernelVersion=</varname></term>
        <listitem>
          <para>Checks whether the kernel version (as reported by <command>uname -r</command>) matches a certain
          expression. See <varname>ConditionKernelVersion=</varname> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
          details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
          If an empty string is assigned, then previously assigned value is cleared.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Architecture=</varname></term>
        <listitem>
          <para>Checks whether the system is running on a specific architecture. See
          <varname>ConditionArchitecture=</varname> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
          If an empty string is assigned, then previously assigned value is cleared.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1>
    <title>[Link] Section Options</title>

    <para>The [Link] section accepts the following
    keys:</para>

    <variablelist class='network-directives'>
      <varlistentry>
        <term><varname>Description=</varname></term>
        <listitem>
          <para>A description of the device.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Alias=</varname></term>
        <listitem>
          <para>The <varname>ifalias</varname> interface property is set to this value.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>MACAddressPolicy=</varname></term>
        <listitem>
          <para>The policy by which the MAC address should be set. The
          available policies are:
          </para>

          <variablelist>
            <varlistentry>
              <term><option>persistent</option></term>
              <listitem>
                <para>If the hardware has a persistent MAC address, as
                most hardware should, and if it is used by the kernel,
                nothing is done. Otherwise, a new MAC address is
                generated which is guaranteed to be the same on every
                boot for the given machine and the given device, but
                which is otherwise random. This feature depends on ID_NET_NAME_*
                properties to exist for the link. On hardware where these
                properties are not set, the generation of a persistent MAC address
                will fail.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>random</option></term>
              <listitem>
                <para>If the kernel is using a random MAC address,
                nothing is done. Otherwise, a new address is randomly
                generated each time the device appears, typically at
                boot. Either way, the random address will have the
                <literal>unicast</literal> and
                <literal>locally administered</literal> bits set.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>none</option></term>
              <listitem>
                <para>Keeps the MAC address assigned by the kernel.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>The MAC address to use, if no
          <varname>MACAddressPolicy=</varname>
          is specified.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>NamePolicy=</varname></term>
        <listitem>
          <para>An ordered, space-separated list of policies by which the interface name should be set.
          <varname>NamePolicy=</varname> may be disabled by specifying <option>net.ifnames=0</option> on the
          kernel command line. Each of the policies may fail, and the first successful one is used. The name
          is not set directly, but is exported to udev as the property <option>ID_NET_NAME</option>, which
          is, by default, used by a
          <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
          rule to set <varname>NAME</varname>. The available policies are:
          </para>

          <variablelist>
            <varlistentry>
              <term><option>kernel</option></term>
              <listitem>
                <para>If the kernel claims that the name it has set
                for a device is predictable, then no renaming is
                performed.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>database</option></term>
              <listitem>
                <para>The name is set based on entries in the udev's
                Hardware Database with the key
                <varname>ID_NET_NAME_FROM_DATABASE</varname>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>onboard</option></term>
              <listitem>
                <para>The name is set based on information given by
                the firmware for on-board devices, as exported by the
                udev property <varname>ID_NET_NAME_ONBOARD</varname>.
                See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>slot</option></term>
              <listitem>
                <para>The name is set based on information given by
                the firmware for hot-plug devices, as exported by the
                udev property <varname>ID_NET_NAME_SLOT</varname>.
                See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>path</option></term>
              <listitem>
                <para>The name is set based on the device's physical
                location, as exported by the udev property
                <varname>ID_NET_NAME_PATH</varname>.
                See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>mac</option></term>
              <listitem>
                <para>The name is set based on the device's persistent
                MAC address, as exported by the udev property
                <varname>ID_NET_NAME_MAC</varname>.
                See <citerefentry><refentrytitle>systemd.net-naming-scheme</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>keep</option></term>
              <listitem>
                <para>If the device already had a name given by userspace (as part of creation of the device
                or a rename), keep it.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Name=</varname></term>
        <listitem>
          <para>The interface name to use. This option has lower precedence than
          <varname>NamePolicy=</varname>, so for this setting to take effect, <varname>NamePolicy=</varname>
          must either be unset, empty, disabled, or all policies configured there must fail. Also see the
          example below with <literal>Name=dmz0</literal>.</para>

          <para>Note that specifying a name that the kernel might use for another
          interface (for example <literal>eth0</literal>) is dangerous because the
          name assignment done by udev will race with the assignment done by the
          kernel, and only one interface may use the name. Depending on the order of
          operations, either udev or the kernel will win, making the naming
          unpredictable. It is best to use some different prefix, for example
          <literal>internal0</literal>/<literal>external0</literal> or
          <literal>lan0</literal>/<literal>lan1</literal>/<literal>lan3</literal>.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>AlternativeNamesPolicy=</varname></term>
        <listitem>
          <para>A space-separated list of policies by which the interface's alternative names
          should be set. Each of the policies may fail, and all successful policies are used. The
          available policies are <literal>database</literal>, <literal>onboard</literal>,
          <literal>slot</literal>, <literal>path</literal>, and <literal>mac</literal>. If the
          kernel does not support the alternative names, then this setting will be ignored.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>AlternativeName=</varname></term>
        <listitem>
          <para>The alternative interface name to use. This option can be specified multiple times.
          If the empty string is assigned to this option, the list is reset, and all prior assignments
          have no effect. If the kernel does not support the alternative names, then this setting will
          be ignored.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>MTUBytes=</varname></term>
        <listitem>
          <para>The maximum transmission unit in bytes to set for the
          device. The usual suffixes K, M, G, are supported and are
          understood to the base of 1024.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>BitsPerSecond=</varname></term>
        <listitem>
          <para>The speed to set for the device, the value is rounded
          down to the nearest Mbps. The usual suffixes K, M, G, are
          supported and are understood to the base of 1000.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Duplex=</varname></term>
        <listitem>
          <para>The duplex mode to set for the device. The accepted values are <option>half</option> and
          <option>full</option>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>AutoNegotiation=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to yes, automatic negotiation of transmission parameters is enabled.
          Autonegotiation is a procedure by which two connected ethernet devices choose
          common transmission parameters, such as speed, duplex mode, and flow control.
          When unset, the kernel's default will be used.</para>

          <para>Note that if autonegotiation is enabled, speed and duplex settings are
          read-only. If autonegotation is disabled, speed and duplex settings are writable
          if the driver supports multiple link modes.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>WakeOnLan=</varname></term>
        <listitem>
          <para>The Wake-on-LAN policy to set for the device. The
          supported values are:</para>

          <variablelist>
            <varlistentry>
              <term><option>phy</option></term>
              <listitem>
                <para>Wake on PHY activity.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>unicast</option></term>
              <listitem>
                <para>Wake on unicast messages.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>multicast</option></term>
              <listitem>
                <para>Wake on multicast messages.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>broadcast</option></term>
              <listitem>
                <para>Wake on broadcast messages.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>arp</option></term>
              <listitem>
                <para>Wake on ARP.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>magic</option></term>
              <listitem>
                <para>Wake on receipt of a magic packet.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>secureon</option></term>
              <listitem>
                <para>Enable secureon(tm) password for MagicPacket(tm).
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>off</option></term>
              <listitem>
                <para>Never wake.</para>
              </listitem>
            </varlistentry>
          </variablelist>

          <para>Defaults to <option>off</option>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Port=</varname></term>
        <listitem>
          <para>The port option is used to select the device port. The
          supported values are:</para>

          <variablelist>
            <varlistentry>
              <term><option>tp</option></term>
              <listitem>
                <para>An Ethernet interface using Twisted-Pair cable as the medium.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>aui</option></term>
              <listitem>
                <para>Attachment Unit Interface (AUI). Normally used with hubs.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>bnc</option></term>
              <listitem>
                <para>An Ethernet interface using BNC connectors and co-axial cable.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>mii</option></term>
              <listitem>
                <para>An Ethernet interface using a Media Independent Interface (MII).</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>fibre</option></term>
              <listitem>
                <para>An Ethernet interface using Optical Fibre as the medium.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Advertise=</varname></term>
        <listitem>
          <para>This sets what speeds and duplex modes of operation are advertised for auto-negotiation.
          This implies <literal>AutoNegotiation=yes</literal>. The supported values are:

          <table>
            <title>Supported advertise values</title>
            <tgroup cols='3'>
              <colspec colname='Advertise' />
              <colspec colname='Speed' />
              <colspec colname='Duplex Mode' />

              <thead><row>
                <entry>Advertise</entry>
                <entry>Speed (Mbps)</entry>
                <entry>Duplex Mode</entry>
              </row></thead>
              <tbody>
                <row><entry><option>10baset-half</option></entry>
                <entry>10</entry><entry>half</entry></row>

                <row><entry><option>10baset-full</option></entry>
                <entry>10</entry><entry>full</entry></row>

                <row><entry><option>100baset-half</option></entry>
                <entry>100</entry><entry>half</entry></row>

                <row><entry><option>100baset-full</option></entry>
                <entry>100</entry><entry>full</entry></row>

                <row><entry><option>1000baset-half</option></entry>
                <entry>1000</entry><entry>half</entry></row>

                <row><entry><option>1000baset-full</option></entry>
                <entry>1000</entry><entry>full</entry></row>

                <row><entry><option>10000baset-full</option></entry>
                <entry>10000</entry><entry>full</entry></row>

                <row><entry><option>2500basex-full</option></entry>
                <entry>2500</entry><entry>full</entry></row>

                <row><entry><option>1000basekx-full</option></entry>
                <entry>1000</entry><entry>full</entry></row>

                <row><entry><option>10000basekx4-full</option></entry>
                <entry>10000</entry><entry>full</entry></row>

                <row><entry><option>10000basekr-full</option></entry>
                <entry>10000</entry><entry>full</entry></row>

                <row><entry><option>10000baser-fec</option></entry>
                <entry>10000</entry><entry>full</entry></row>

                <row><entry><option>20000basemld2-full</option></entry>
                <entry>20000</entry><entry>full</entry></row>

                <row><entry><option>20000basekr2-full</option></entry>
                <entry>20000</entry><entry>full</entry></row>
              </tbody>
            </tgroup>
          </table>

          By default this is unset, i.e. all possible modes will be advertised.
          This option may be specified more than once, in which case all specified speeds and modes are advertised.
          If the empty string is assigned to this option, the list is reset, and all prior assignments have no effect.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TCPSegmentationOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, the TCP Segmentation Offload (TSO) is enabled.
          When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>
       <varlistentry>
      <term><varname>TCP6SegmentationOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, the TCP6 Segmentation Offload (tx-tcp6-segmentation) is enabled.
          When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericSegmentationOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, the Generic Segmentation Offload (GSO) is enabled.
          When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>
    <varlistentry>
        <term><varname>GenericReceiveOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, the Generic Receive Offload (GRO) is enabled.
          When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>LargeReceiveOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, the Large Receive Offload (LRO) is enabled.
          When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>RxChannels=</varname></term>
        <listitem>
          <para>Sets the number of receive channels (a number between 1 and 4294967295) .</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TxChannels=</varname></term>
        <listitem>
          <para>Sets the number of transmit channels (a number between 1 and 4294967295).</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>OtherChannels=</varname></term>
        <listitem>
          <para>Sets the number of other channels (a number between 1 and 4294967295).</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>CombinedChannels=</varname></term>
        <listitem>
          <para>Sets the number of combined set channels (a number between 1 and 4294967295).</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>RxBufferSize=</varname></term>
        <listitem>
          <para>Takes a integer. Specifies the NIC receive ring buffer size. When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TxBufferSize=</varname></term>
        <listitem>
          <para>Takes a integer. Specifies the NIC transmit ring buffer size. When unset, the kernel's default will be used.</para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>/usr/lib/systemd/network/99-default.link</title>

      <para>The link file <filename>99-default.link</filename> that is
      shipped with systemd defines the default naming policy for
      links.</para>

      <programlisting>[Link]
NamePolicy=kernel database onboard slot path
MACAddressPolicy=persistent</programlisting>
    </example>

    <example>
      <title>/etc/systemd/network/10-dmz.link</title>

      <para>This example assigns the fixed name <literal>dmz0</literal> to the interface with the MAC address
      00:a0:de:63:7a:e6:</para>

      <programlisting>[Match]
MACAddress=00:a0:de:63:7a:e6

[Link]
Name=dmz0</programlisting>

      <para><varname>NamePolicy=</varname> is not set, so <varname>Name=</varname> takes effect. We use the
      <literal>10-</literal> prefix to order this file early in the list. Note that it needs to be before
      <literal>99-link</literal>, i.e. it needs a numerical prefix, to have any effect at all.</para>
    </example>

    <example>
      <title>Debugging <varname>NamePolicy=</varname> assignments</title>

      <programlisting>$ sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/hub0
…
Parsed configuration file /usr/lib/systemd/network/99-default.link
Parsed configuration file /etc/systemd/network/10-eth0.link
ID_NET_DRIVER=cdc_ether
Config file /etc/systemd/network/10-eth0.link applies to device hub0
link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
hub0: Device has name_assign_type=4
Using default interface naming scheme 'v240'.
hub0: Policies didn't yield a name, using specified Name=hub0.
ID_NET_LINK_FILE=/etc/systemd/network/10-eth0.link
ID_NET_NAME=hub0
…</programlisting>

     <para>Explicit <varname>Name=</varname> configuration wins in this case.</para>

     <programlisting>sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/enp0s31f6
…
Parsed configuration file /usr/lib/systemd/network/99-default.link
Parsed configuration file /etc/systemd/network/10-eth0.link
Created link configuration context.
ID_NET_DRIVER=e1000e
Config file /usr/lib/systemd/network/99-default.link applies to device enp0s31f6
link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
enp0s31f6: Device has name_assign_type=4
Using default interface naming scheme 'v240'.
enp0s31f6: Policy *keep*: keeping existing userspace name
enp0s31f6: Device has addr_assign_type=0
enp0s31f6: MAC on the device already matches policy *persistent*
ID_NET_LINK_FILE=/usr/lib/systemd/network/99-default.link
…
</programlisting>

     <para>In this case, the interface was already renamed, so the <option>keep</option> policy specified as
     the first option in <filename index="false">99-default.link</filename> means that the existing name is
     preserved. If <option>keep</option> was removed, or if were in boot before the renaming has happened,
     we might get the following instead:</para>

     <programlisting>enp0s31f6: Policy *path* yields "enp0s31f6".
enp0s31f6: Device has addr_assign_type=0
enp0s31f6: MAC on the device already matches policy *persistent*
ID_NET_LINK_FILE=/usr/lib/systemd/network/99-default.link
ID_NET_NAME=enp0s31f6
…
</programlisting>

      <para>Please note that the details of output are subject to change.</para>
    </example>

    <example>
      <title>/etc/systemd/network/10-internet.link</title>

      <para>This example assigns the fixed name
      <literal>internet0</literal> to the interface with the device
      path <literal>pci-0000:00:1a.0-*</literal>:</para>

      <programlisting>[Match]
Path=pci-0000:00:1a.0-*

[Link]
Name=internet0</programlisting>
    </example>

    <example>
      <title>/etc/systemd/network/25-wireless.link</title>

      <para>Here's an overly complex example that shows the use of a large number of [Match] and [Link] settings.</para>

      <programlisting>[Match]
MACAddress=12:34:56:78:9a:bc
Driver=brcmsmac
Path=pci-0000:02:00.0-*
Type=wlan
Virtualization=no
Host=my-laptop
Architecture=x86-64

[Link]
Name=wireless0
MTUBytes=1450
BitsPerSecond=10M
WakeOnLan=magic
MACAddress=cb:a9:87:65:43:21</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry>
        <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>,
      <citerefentry>
        <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>,
      <citerefentry>
        <refentrytitle>systemd.netdev</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
        <refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>
    </para>
  </refsect1>

</refentry>