systemd/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>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 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>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 a device 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 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'>
      <!-- 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></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></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></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></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></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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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 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, then previously assigned value is cleared.
          </para>
        </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, then previously assigned value is cleared.
          </para>
        </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, then previously assigned value is cleared.
          </para>
        </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, then previously assigned value is cleared.
          </para>
        </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, then previously assigned value is cleared.
          </para>
        </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, 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. Or use the MAC address specified in
                <varname>MACAddress=</varname>.</para>
              </listitem>
            </varlistentry>
          </variablelist>

          <para>An empty string assignment is equivalent to setting <literal>none</literal>.</para>
        </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>
        </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>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>
        </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>
        </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>
        </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 autonegotiation 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. 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>
              </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 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>LoadCredential=</varname>/<varname>SetCredential=</varname> in
                <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>1</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>
              </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>
        </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>
        </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>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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </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>
        </listitem>
      </varlistentry>

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