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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="systemd.link"
          xmlns:xi="http://www.w3.org/2001/XInclude">
  <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>A plain ini-style text file that encodes configuration for matching network devices, used by
    <citerefentry><refentrytitle>systemd-udevd</refentrytitle><manvolnum>8</manvolnum></citerefentry> and in
    particular its <command>net_setup_link</command> builtin. See
    <citerefentry><refentrytitle>systemd.syntax</refentrytitle><manvolnum>7</manvolnum></citerefentry> for a
    general description of the syntax.</para>

    <para>The <filename>.link</filename> files are read from the files located in the system network
    directory <filename>/usr/lib/systemd/network</filename> and
    <filename>/usr/local/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>. All configuration files are collectively sorted and
    processed in alphanumeric order, regardless of the directories in which they live. However, files
    with identical filenames replace each other. It is recommended that each filename is prefixed with
    a number smaller than <literal>70</literal> (e.g. <filename>10-eth0.link</filename>). Otherwise, the
    default <filename>.link</filename> files or those generated by
    <citerefentry><refentrytitle>systemd-network-generator.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    may take precedence over user configured files. 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>Along with the link file <filename>foo.link</filename>, a "drop-in" directory
    <filename>foo.link.d/</filename> may exist. All files with the suffix <literal>.conf</literal>
    from this directory will be merged in the alphanumeric order and parsed after the main file itself
    has been parsed. This is useful to alter or add configuration settings, without having to modify
    the main configuration file. Each drop-in file must have appropriate section headers.</para>

    <para>In addition to <filename>/etc/systemd/network</filename>, drop-in <literal>.d</literal>
    directories can be placed in <filename>/usr/lib/systemd/network</filename> or
    <filename>/run/systemd/network</filename> directories. Drop-in files in <filename>/etc/</filename>
    take precedence over those in <filename>/run/</filename> which in turn take precedence over those
    in <filename>/usr/lib/</filename>. Drop-in files under any of these directories take precedence
    over the main link file wherever located.</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 an interface if all matches specified by the [Match] section are
    satisfied. When a link file does not contain valid settings in [Match] section, then the file will
    match all interfaces 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 first (in alphanumeric order) of the link files that matches a given interface is applied, all
    later files are ignored, even if they match as well. The following keys are accepted:</para>

    <variablelist class='network-directives'>
      <!-- This list is reused in systemd.network(3), hence maintain a specific order:
           1. device matches shared between the two lists
           2. non-shared settings
           3. host matches shared between the two lists
        -->

      <varlistentry id='mac-address'>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>A whitespace-separated list of hardware addresses. The acceptable formats are:</para>

          <variablelist>
            <varlistentry>
              <term><option>colon-delimited hexadecimal</option></term>
              <listitem><para>
                Each field must be one byte.
                E.g. <literal>12:34:56:78:90:ab</literal> or <literal>AA:BB:CC:DD:EE:FF</literal>.
              </para>

                <xi:include href="version-info.xml" xpointer="v250"/></listitem>
            </varlistentry>
            <varlistentry>
              <term><option>hyphen-delimited hexadecimal</option></term>
              <listitem><para>
                Each field must be one byte.
                E.g. <literal>12-34-56-78-90-ab</literal> or <literal>AA-BB-CC-DD-EE-FF</literal>.
              </para>

                <xi:include href="version-info.xml" xpointer="v250"/></listitem>
            </varlistentry>
            <varlistentry>
              <term><option>dot-delimited hexadecimal</option></term>
              <listitem><para>
                Each field must be two bytes.
                E.g. <literal>1234.5678.90ab</literal> or <literal>AABB.CCDD.EEFF</literal>.
              </para>

                <xi:include href="version-info.xml" xpointer="v250"/></listitem>
            </varlistentry>
            <varlistentry>
              <term><option>IPv4 address format</option></term>
              <listitem><para>
                E.g. <literal>127.0.0.1</literal> or <literal>192.168.0.1</literal>.
              </para>

                <xi:include href="version-info.xml" xpointer="v250"/></listitem>
            </varlistentry>
            <varlistentry>
              <term><option>IPv6 address format</option></term>
              <listitem><para>
                E.g. <literal>2001:0db8:85a3::8a2e:0370:7334</literal> or <literal>::1</literal>.
              </para>

                <xi:include href="version-info.xml" xpointer="v250"/></listitem>
            </varlistentry>
          </variablelist>

          <para>The total length of each MAC address must be 4 (for IPv4 tunnel), 6 (for Ethernet), 16
          (for IPv6 tunnel), or 20 (for InfiniBand). 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. Defaults to unset.</para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='permanent-mac-address'>
        <term><varname>PermanentMACAddress=</varname></term>
        <listitem>
          <para>A whitespace-separated list of hardware's permanent addresses. While
          <varname>MACAddress=</varname> matches the device's current MAC address, this matches the
          device's permanent MAC address, which may be different from the current one. Use full
          colon-, hyphen- or dot-delimited hexadecimal, or IPv4 or IPv6 address format. 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. Defaults to
          unset.</para>

          <xi:include href="version-info.xml" xpointer="v245"/>
        </listitem>
      </varlistentry>

      <varlistentry id='path'>
        <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>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='driver'>
        <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.
          If the list is prefixed with a "!", the test is inverted.</para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='type'>
        <term><varname>Type=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching the device type, as exposed by
          <command>networkctl list</command>. If the list is prefixed with a "!", the test is inverted.
          Some valid values are <literal>ether</literal>, <literal>loopback</literal>, <literal>wlan</literal>, <literal>wwan</literal>.
          Valid types are named either from the udev <literal>DEVTYPE</literal> attribute, or
          <literal>ARPHRD_</literal> macros in <filename>linux/if_arp.h</filename>, so this is not comprehensive.
          </para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='kind'>
        <term><varname>Kind=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching the device kind, as exposed by
          <command>networkctl status <replaceable>INTERFACE</replaceable></command> or
          <command>ip -d link show <replaceable>INTERFACE</replaceable></command>. If the list is
          prefixed with a "!", the test is inverted. Some valid values are <literal>bond</literal>,
          <literal>bridge</literal>, <literal>gre</literal>, <literal>tun</literal>,
          <literal>veth</literal>. Valid kinds are given by netlink's <literal>IFLA_INFO_KIND</literal>
          attribute, so this is not comprehensive.
          </para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry id='property'>
        <term><varname>Property=</varname></term>
        <listitem>
          <para>A whitespace-separated list of udev property names with their values after equals sign
          (<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>

          <xi:include href="version-info.xml" xpointer="v243"/>
        </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>

          <xi:include href="version-info.xml" xpointer="v218"/>
        </listitem>
      </varlistentry>

      <varlistentry id='host'>
        <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, the previously assigned value is cleared.
          </para>

        <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='virtualization'>
        <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, the previously assigned value is cleared.
          </para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='kernel-command-line'>
        <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, the previously assigned value is cleared.
          </para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='kernel-version'>
        <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, the previously assigned value is cleared.
          </para>

        <xi:include href="version-info.xml" xpointer="v237"/>
        </listitem>
      </varlistentry>

      <varlistentry id='credential'>
        <term><varname>Credential=</varname></term>
        <listitem>
          <para>Checks whether the specified credential was passed to the
          <filename>systemd-udevd.service</filename> service. See <ulink
          url="https://systemd.io/CREDENTIALS">System and Service Credentials</ulink> for details. When
          prefixed with an exclamation mark (<literal>!</literal>), the result is negated.  If an empty
          string is assigned, the previously assigned value is cleared.
          </para>

          <xi:include href="version-info.xml" xpointer="v252"/>
        </listitem>
      </varlistentry>

      <varlistentry id='architecture'>
        <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, the previously assigned value is cleared.
          </para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>

      <varlistentry id='firmware'>
        <term><varname>Firmware=</varname></term>
        <listitem>
          <para>Checks whether the system is running on a machine with the specified firmware. See
          <varname>ConditionFirmware=</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, the previously assigned value is cleared.
          </para>

          <xi:include href="version-info.xml" xpointer="v249"/>
        </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>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Property=</varname></term>
        <listitem>
          <para>Set specified udev properties. This takes space separated list of key-value pairs
          concatenated with equal sign (<literal>=</literal>). Example:
          <programlisting>Property=HOGE=foo BAR=baz</programlisting>
          This option supports simple specifier expansion, see the Specifiers section below.
          This option can be specified multiple times. If an empty string is assigned, then the all previous
          assignments are cleared.</para>

          <para>This setting is useful to configure the <literal>ID_NET_MANAGED_BY=</literal> property which
          declares which network management service shall manage the interface, which is respected by
          systemd-networkd and others. Use
          <programlisting>Property=ID_NET_MANAGED_BY=io.systemd.Network</programlisting>
          to declare explicitly that <command>systemd-networkd</command> shall manage the interface, or set
          the property to something else to declare explicitly it shall not do so. See
          <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details how this property is used to match interface names.</para>

          <xi:include href="version-info.xml" xpointer="v256"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>ImportProperty=</varname></term>
        <listitem>
          <para>Import specified udev properties from the saved database. This takes space separated list of
          property names. Example: <programlisting>ImportProperty=HOGE BAR</programlisting>
          This option supports simple specifier expansion, see the Specifiers section below.
          This option can be specified multiple times. If an empty string is assigned, then the all previous
          assignments are cleared.</para>
          <para>If the same property is also set in <varname>Property=</varname> in the above, then the
          imported property value will be overridden by the value specified in <varname>Property=</varname>.
          </para>

          <xi:include href="version-info.xml" xpointer="v256"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>UnsetProperty=</varname></term>
        <listitem>
          <para>Unset specified udev properties. This takes space separated list of
          property names. Example: <programlisting>ImportProperty=HOGE BAR</programlisting>
          This option supports simple specifier expansion, see the Specifiers section below.
          This option can be specified multiple times. If an empty string is assigned, then the all previous
          assignments are cleared.</para>
          <para>This setting is applied after <varname>ImportProperty=</varname> and
          <varname>Property=</varname> are applied. Hence, if the same property is specified in
          <varname>ImportProperty=</varname> or <varname>Property=</varname>, then the imported or specified
          property value will be ignored, and the property will be unset.</para>

          <xi:include href="version-info.xml" xpointer="v256"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Alias=</varname></term>
        <listitem>
          <para>The <varname>ifalias</varname> interface property is set to this value.</para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>none</option></term>
              <listitem>
                <para>Keeps the MAC address assigned by the kernel. Or use the MAC address specified in
                <varname>MACAddress=</varname>.</para>

                <xi:include href="version-info.xml" xpointer="v227"/>
              </listitem>
            </varlistentry>
          </variablelist>

          <para>An empty string assignment is equivalent to setting <literal>none</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>The interface MAC address to use. For this setting to take effect,
          <varname>MACAddressPolicy=</varname> must either be unset, empty, or <literal>none</literal>.
          </para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </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>

                <xi:include href="version-info.xml" xpointer="v216"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </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>

                <xi:include href="version-info.xml" xpointer="v241"/>
              </listitem>
            </varlistentry>
          </variablelist>

            <xi:include href="version-info.xml" xpointer="v211"/>
        </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>

          <para>Interface names must have a minimum length of 1 character and a maximum length of 15
          characters, and may contain any 7bit ASCII character, with the exception of control characters,
          <literal>:</literal>, <literal>/</literal> and <literal>%</literal>. While <literal>.</literal> is
          an allowed character, it's recommended to avoid it when naming interfaces as various tools (such as
          <citerefentry><refentrytitle>resolvconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>) use
          it as separator character. Also, fully numeric interface names are not allowed (in order to avoid
          ambiguity with interface specification by numeric indexes), as are the special strings
          <literal>.</literal>, <literal>..</literal>, <literal>all</literal> and
          <literal>default</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </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>

          <xi:include href="version-info.xml" xpointer="v245"/>
        </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>

          <para>Alternative interface names may be used to identify interfaces in various tools. In contrast
          to the primary name (as configured with <varname>Name=</varname> above) there may be multiple
          alternative names referring to the same interface. Alternative names may have a maximum length of
          127 characters, in contrast to the 15 allowed for the primary interface name, but otherwise are
          subject to the same naming constraints.</para>

          <xi:include href="version-info.xml" xpointer="v245"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TransmitQueues=</varname></term>
        <listitem>
          <para>Specifies the device's number of transmit queues. An integer in the range 1…4096.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v248"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>ReceiveQueues=</varname></term>
        <listitem>
          <para>Specifies the device's number of receive queues. An integer in the range 1…4096.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v248"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TransmitQueueLength=</varname></term>
        <listitem>
          <para>Specifies the transmit queue length of the device in number of packets. An unsigned integer
          in the range 0…4294967294. When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v248"/>
        </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>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </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>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </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>

        <xi:include href="version-info.xml" xpointer="v211"/>
        </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 autonegotiation is disabled, speed and duplex settings are writable
          if the driver supports multiple link modes.</para>

          <xi:include href="version-info.xml" xpointer="v233"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>WakeOnLan=</varname></term>
        <listitem>
          <para>The Wake-on-LAN policy to set for the device. Takes the special value
          <literal>off</literal> which disables Wake-on-LAN, or space separated list of the following
          words:</para>

          <variablelist>
            <varlistentry>
              <term><option>phy</option></term>
              <listitem>
                <para>Wake on PHY activity.</para>

              <xi:include href="version-info.xml" xpointer="v211"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>unicast</option></term>
              <listitem>
                <para>Wake on unicast messages.</para>

              <xi:include href="version-info.xml" xpointer="v235"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>multicast</option></term>
              <listitem>
                <para>Wake on multicast messages.</para>

              <xi:include href="version-info.xml" xpointer="v235"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>broadcast</option></term>
              <listitem>
                <para>Wake on broadcast messages.</para>

              <xi:include href="version-info.xml" xpointer="v235"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>arp</option></term>
              <listitem>
                <para>Wake on ARP.</para>

              <xi:include href="version-info.xml" xpointer="v235"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>magic</option></term>
              <listitem>
                <para>Wake on receipt of a magic packet.
                </para>

                <xi:include href="version-info.xml" xpointer="v211"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>secureon</option></term>
              <listitem>
                <para>Enable SecureOn password for MagicPacket. Implied when
                <varname>WakeOnLanPassword=</varname> is specified. If specified without
                <varname>WakeOnLanPassword=</varname> option, then the password is read from the
                credential <literal><replaceable>LINK</replaceable>.link.wol.password</literal> (e.g.,
                <literal>60-foo.link.wol.password</literal>), and if the credential not found, then
                read from <literal>wol.password</literal>. See
                <varname>ImportCredential=</varname>/<varname>LoadCredential=</varname>/<varname>SetCredential=</varname> in
                <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                for details. The password in the credential, must be 6 bytes in hex format with each
                byte separated by a colon (<literal>:</literal>) like an Ethernet MAC address, e.g.,
                <literal>aa:bb:cc:dd:ee:ff</literal>.</para>

                <xi:include href="version-info.xml" xpointer="v235"/>
              </listitem>
            </varlistentry>
          </variablelist>

          <para>Defaults to unset, and the device's default will be used. This setting can be specified
          multiple times. If an empty string is assigned, then the all previous assignments are
          cleared.</para>

          <xi:include href="version-info.xml" xpointer="v211"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>WakeOnLanPassword=</varname></term>
        <listitem>
          <para>Specifies the SecureOn password for MagicPacket. Takes an absolute path to a regular
          file or an <constant>AF_UNIX</constant> stream socket, or the plain password. When a path to
          a regular file is specified, the password is read from it. When an
          <constant>AF_UNIX</constant> stream socket is specified, a connection is made to it and the
          password is read from it. The password must be 6 bytes in hex format with each byte separated
          by a colon (<literal>:</literal>) like an Ethernet MAC address, e.g.,
          <literal>aa:bb:cc:dd:ee:ff</literal>. This implies <varname>WakeOnLan=secureon</varname>.
          Defaults to unset, and the current value will not be changed.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </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>

              <xi:include href="version-info.xml" xpointer="v234"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>aui</option></term>
              <listitem>
                <para>Attachment Unit Interface (AUI). Normally used with hubs.
                </para>

                <xi:include href="version-info.xml" xpointer="v234"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>bnc</option></term>
              <listitem>
                <para>An Ethernet interface using BNC connectors and co-axial cable.</para>

              <xi:include href="version-info.xml" xpointer="v234"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>mii</option></term>
              <listitem>
                <para>An Ethernet interface using a Media Independent Interface (MII).</para>

              <xi:include href="version-info.xml" xpointer="v234"/>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><option>fibre</option></term>
              <listitem>
                <para>An Ethernet interface using Optical Fibre as the medium.</para>

              <xi:include href="version-info.xml" xpointer="v234"/>
              </listitem>
            </varlistentry>
          </variablelist>

            <xi:include href="version-info.xml" xpointer="v234"/>
        </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>
              <xi:include href="ethtool-link-mode.xml" />
            </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>

          <xi:include href="version-info.xml" xpointer="v240"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>ReceiveChecksumOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, hardware offload for checksumming of ingress
          network packets is enabled. When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v245"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TransmitChecksumOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, hardware offload for checksumming of egress
          network packets is enabled. When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v245"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TCPSegmentationOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, TCP Segmentation Offload (TSO) is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v232"/>
        </listitem>
      </varlistentry>
       <varlistentry>
      <term><varname>TCP6SegmentationOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, TCP6 Segmentation Offload (tx-tcp6-segmentation) is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v235"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericSegmentationOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, Generic Segmentation Offload (GSO) is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v232"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericReceiveOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, Generic Receive Offload (GRO) is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v232"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericReceiveOffloadHardware=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, hardware accelerated Generic Receive Offload (GRO) is
          enabled. When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>LargeReceiveOffload=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, Large Receive Offload (LRO) is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v232"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>ReceivePacketSteeringCPUMask=</varname></term>
        <listitem>
          <para>Configures Receive Packet Steering (RPS) list of CPUs to which RPS may forward traffic.
          Takes a list of CPU indices or ranges separated by either whitespace or commas. Alternatively,
          takes the special value <literal>all</literal> in which will include all available CPUs in the mask.
          CPU ranges are specified by the lower and upper CPU indices separated by a dash (e.g. <literal>2-6</literal>).
          This option may be specified more than once, in which case the specified CPU affinity masks are merged.
          If an empty string is assigned, the mask is reset, all assignments prior to this will have no effect.
          Defaults to unset and RPS CPU list is unchanged. To disable RPS when it was previously enabled, use the
          special value <literal>disable</literal>.</para>

          <xi:include href="version-info.xml" xpointer="v256"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>ReceiveVLANCTAGHardwareAcceleration=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, receive VLAN CTAG hardware acceleration is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TransmitVLANCTAGHardwareAcceleration=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, transmit VLAN CTAG hardware acceleration is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>ReceiveVLANCTAGFilter=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, receive filtering on VLAN CTAGs is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TransmitVLANSTAGHardwareAcceleration=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, transmit VLAN STAG hardware acceleration is enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>NTupleFilter=</varname></term>
        <listitem>
          <para>Takes a boolean. If set to true, receive N-tuple filters and actions are enabled.
          When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>RxChannels=</varname></term>
        <term><varname>TxChannels=</varname></term>
        <term><varname>OtherChannels=</varname></term>
        <term><varname>CombinedChannels=</varname></term>
        <listitem>
          <para>Specifies the number of receive, transmit, other, or combined channels, respectively.
          Takes an unsigned integer in the range 1…4294967295 or <literal>max</literal>. If set to
          <literal>max</literal>, the advertised maximum value of the hardware will be used. When
          unset, the number will not be changed. Defaults to unset.</para>

          <xi:include href="version-info.xml" xpointer="v239"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>RxBufferSize=</varname></term>
        <term><varname>RxMiniBufferSize=</varname></term>
        <term><varname>RxJumboBufferSize=</varname></term>
        <term><varname>TxBufferSize=</varname></term>
        <listitem>
          <para>Specifies the maximum number of pending packets in the NIC receive buffer, mini receive
          buffer, jumbo receive buffer, or transmit buffer, respectively. Takes an unsigned integer in
          the range 1…4294967295 or <literal>max</literal>. If set to <literal>max</literal>, the
          advertised maximum value of the hardware will be used. When unset, the number will not be
          changed. Defaults to unset.</para>

          <xi:include href="version-info.xml" xpointer="v244"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>RxFlowControl=</varname></term>
        <listitem>
          <para>Takes a boolean. When set, enables receive flow control, also known as the ethernet
          receive PAUSE message (generate and send ethernet PAUSE frames). When unset, the kernel's
          default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v246"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TxFlowControl=</varname></term>
        <listitem>
          <para>Takes a boolean. When set, enables transmit flow control, also known as the ethernet
          transmit PAUSE message (respond to received ethernet PAUSE frames). When unset, the kernel's
          default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v246"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>AutoNegotiationFlowControl=</varname></term>
        <listitem>
          <para>Takes a boolean. When set, auto negotiation enables the interface to exchange state
          advertisements with the connected peer so that the two devices can agree on the ethernet
          PAUSE configuration. When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v246"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericSegmentOffloadMaxBytes=</varname></term>
        <listitem>
          <para>Specifies the maximum size of a Generic Segment Offload (GSO) packet the
          device should accept. The usual suffixes K, M, G are supported and are
          understood to the base of 1024. An unsigned integer in the range 1…65536.
          Defaults to unset.</para>

          <xi:include href="version-info.xml" xpointer="v248"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericSegmentOffloadMaxSegments=</varname></term>
        <listitem>
          <para>Specifies the maximum number of Generic Segment Offload (GSO) segments the device should
          accept. An unsigned integer in the range 1…65535. Defaults to unset.</para>

          <xi:include href="version-info.xml" xpointer="v248"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>UseAdaptiveRxCoalesce=</varname></term>
        <term><varname>UseAdaptiveTxCoalesce=</varname></term>
        <listitem>
          <para>Boolean properties that, when set, enable/disable adaptive Rx/Tx coalescing if the hardware
          supports it. When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>RxCoalesceSec=</varname></term>
        <term><varname>RxCoalesceIrqSec=</varname></term>
        <term><varname>RxCoalesceLowSec=</varname></term>
        <term><varname>RxCoalesceHighSec=</varname></term>
        <term><varname>TxCoalesceSec=</varname></term>
        <term><varname>TxCoalesceIrqSec=</varname></term>
        <term><varname>TxCoalesceLowSec=</varname></term>
        <term><varname>TxCoalesceHighSec=</varname></term>
        <listitem>
          <para>These properties configure the delay before Rx/Tx interrupts are generated after a packet is
          sent/received. The <literal>Irq</literal> properties come into effect when the host is servicing an
          IRQ. The <literal>Low</literal> and <literal>High</literal> properties come into effect when the
          packet rate drops below the low packet rate threshold or exceeds the high packet rate threshold
          respectively if adaptive Rx/Tx coalescing is enabled. When unset, the kernel's defaults will be
          used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
        <varlistentry>
        <term><varname>RxMaxCoalescedFrames=</varname></term>
        <term><varname>RxMaxCoalescedIrqFrames=</varname></term>
        <term><varname>RxMaxCoalescedLowFrames=</varname></term>
        <term><varname>RxMaxCoalescedHighFrames=</varname></term>
        <term><varname>TxMaxCoalescedFrames=</varname></term>
        <term><varname>TxMaxCoalescedIrqFrames=</varname></term>
        <term><varname>TxMaxCoalescedLowFrames=</varname></term>
        <term><varname>TxMaxCoalescedHighFrames=</varname></term>
        <listitem>
          <para>These properties configure the maximum number of frames that are sent/received before a Rx/Tx
          interrupt is generated. The <literal>Irq</literal> properties come into effect when the host is
          servicing an IRQ. The <literal>Low</literal> and <literal>High</literal> properties come into
          effect when the packet rate drops below the low packet rate threshold or exceeds the high packet
          rate threshold respectively if adaptive Rx/Tx coalescing is enabled. When unset, the kernel's
          defaults will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>CoalescePacketRateLow=</varname></term>
        <term><varname>CoalescePacketRateHigh=</varname></term>
        <listitem>
          <para>These properties configure the low and high packet rate (expressed in packets per second)
          threshold respectively and are used to determine when the corresponding coalescing settings for low
          and high packet rates come into effect if adaptive Rx/Tx coalescing is enabled. If unset, the
          kernel's defaults will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>CoalescePacketRateSampleIntervalSec=</varname></term>
        <listitem>
          <para>Configures how often to sample the packet rate used for adaptive Rx/Tx coalescing. This
          property cannot be zero. This lowest time granularity supported by this property is seconds.
          Partial seconds will be rounded up before being passed to the kernel. If unset, the kernel's
          default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>StatisticsBlockCoalesceSec=</varname></term>
        <listitem>
          <para>How long to delay driver in-memory statistics block updates. If the driver does not have an
          in-memory statistic block, this property is ignored. This property cannot be zero. If unset, the
          kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v250"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MDI=</varname></term>
        <listitem>
          <para>Specifies the medium dependent interface (MDI) mode for the interface. A MDI describes
          the interface from a physical layer implementation to the physical medium used to carry the
          transmission. Takes one of the following words: <literal>straight</literal> (or equivalently:
          <literal>mdi</literal>), <literal>crossover</literal> (or equivalently:
          <literal>mdi-x</literal>, <literal>mdix</literal>), and <literal>auto</literal>. When
          <literal>straight</literal>, the MDI straight through mode will be used. When
          <literal>crossover</literal>, the MDI crossover (MDI-X) mode will be used. When
          <literal>auto</literal>, the MDI status is automatically detected. Defaults to unset, and the
          kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>SR-IOVVirtualFunctions=</varname></term>
        <listitem>
          <para>Specifies the number of SR-IOV virtual functions. Takes an integer in the range
          0…2147483647. Defaults to unset, and automatically determined from the values specified in
          the <varname>VirtualFunction=</varname> settings in the [SR-IOV] sections.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1 id='sr-iov'>
    <title>[SR-IOV] Section Options</title>
    <para>The [SR-IOV] section accepts the following keys. Specify several [SR-IOV] sections to
    configure several SR-IOVs. SR-IOV provides the ability to partition a single physical PCI resource
    into virtual PCI functions which can then be injected into a VM. In the case of network VFs, SR-IOV
    improves north-south network performance (that is, traffic with endpoints outside the host machine)
    by allowing traffic to bypass the host machine’s network stack.</para>

    <variablelist class='network-directives'>
      <varlistentry>
        <term><varname>VirtualFunction=</varname></term>
        <listitem>
          <para>Specifies a Virtual Function (VF), lightweight PCIe function designed solely to move
          data in and out. Takes an integer in the range 0…2147483646. This option is compulsory.
          </para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>VLANId=</varname></term>
        <listitem>
          <para>Specifies VLAN ID of the virtual function. Takes an integer in the range 1…4095.</para>

        <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>QualityOfService=</varname></term>
        <listitem>
          <para>Specifies quality of service of the virtual function. Takes an integer in the range
          1…4294967294.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>VLANProtocol=</varname></term>
        <listitem>
          <para>Specifies VLAN protocol of the virtual function. Takes <literal>802.1Q</literal> or
          <literal>802.1ad</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MACSpoofCheck=</varname></term>
        <listitem>
          <para>Takes a boolean. Controls the MAC spoof checking. When unset, the kernel's default will
          be used.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>QueryReceiveSideScaling=</varname></term>
        <listitem>
          <para>Takes a boolean. Toggle the ability of querying the receive side scaling (RSS)
          configuration of the virtual function (VF). The VF RSS information like RSS hash key may be
          considered sensitive on some devices where this information is shared between VF and the
          physical function (PF). When unset, the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>Trust=</varname></term>
        <listitem>
          <para>Takes a boolean. Allows one to set trust mode of the virtual function (VF). When set,
          VF users can set a specific feature which may impact security and/or performance. When unset,
          the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LinkState=</varname></term>
        <listitem>
          <para>Allows one to set the link state of the virtual function (VF). Takes a boolean or a
          special value <literal>auto</literal>. Setting to <literal>auto</literal> means a
          reflection of the physical function (PF) link state, <literal>yes</literal> lets the VF to
          communicate with other VFs on this host even if the PF link state is down,
          <literal>no</literal> causes the hardware to drop any packets sent by the VF. When unset,
          the kernel's default will be used.</para>

          <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>Specifies the MAC address for the virtual function.</para>

        <xi:include href="version-info.xml" xpointer="v251"/>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Specifiers</title>

    <para>Some settings resolve specifiers which may be used to write generic unit files referring to runtime
    or unit parameters that are replaced when the unit files are loaded. Specifiers must be known and
    resolvable for the setting to be valid. The following specifiers are understood:</para>

    <table class='specifiers'>
      <title>Specifiers available in unit files</title>
      <tgroup cols='3' align='left' colsep='1' rowsep='1'>
        <colspec colname="spec" />
        <colspec colname="mean" />
        <colspec colname="detail" />
        <thead>
          <row>
            <entry>Specifier</entry>
            <entry>Meaning</entry>
            <entry>Details</entry>
          </row>
        </thead>
        <tbody>
          <xi:include href="standard-specifiers.xml" xpointer="a"/>
          <xi:include href="standard-specifiers.xml" xpointer="A"/>
          <xi:include href="standard-specifiers.xml" xpointer="b"/>
          <xi:include href="standard-specifiers.xml" xpointer="B"/>
          <xi:include href="standard-specifiers.xml" xpointer="H"/>
          <xi:include href="standard-specifiers.xml" xpointer="l"/>
          <xi:include href="standard-specifiers.xml" xpointer="m"/>
          <xi:include href="standard-specifiers.xml" xpointer="M"/>
          <xi:include href="standard-specifiers.xml" xpointer="o"/>
          <xi:include href="standard-specifiers.xml" xpointer="q"/>
          <xi:include href="standard-specifiers.xml" xpointer="T"/>
          <xi:include href="standard-specifiers.xml" xpointer="v"/>
          <xi:include href="standard-specifiers.xml" xpointer="V"/>
          <xi:include href="standard-specifiers.xml" xpointer="w"/>
          <xi:include href="standard-specifiers.xml" xpointer="W"/>
        </tbody>
      </tgroup>
    </table>
  </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 policies for the interface name, alternative names, and MAC address of links.</para>

      <programlisting>[Match]
OriginalName=*

[Link]
NamePolicy=keep kernel database onboard slot path
AlternativeNamesPolicy=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
      <filename>99-default.link</filename>, i.e. it needs a numerical prefix, to have any effect at all.</para>
    </example>

    <example>
      <title>(Re-)applying a .link file to an interface</title>

      <para>After a new .link file has been created, or an existing .link file modified, the new settings
      may be applied to the matching interface with the following commands:</para>

      <programlisting>$ sudo udevadm control --reload
$ sudo ip link set eth0 down
$ sudo udevadm trigger --verbose --settle --action add /sys/class/net/eth0</programlisting>

      <para>You may also need to stop the service that manages the network interface, e.g.
      <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      or <filename>NetworkManager.service</filename> before the above operation, and then restart the service
      after that. For more details about <command>udevadm</command> command, see
      <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</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><simplelist type="inline">
      <member><citerefentry>
        <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry></member>
      <member><citerefentry>
        <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry></member>
      <member><citerefentry>
        <refentrytitle>systemd.netdev</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry></member>
      <member><citerefentry>
        <refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry></member>
      <member><citerefentry>
        <refentrytitle>systemd-network-generator.service</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>
