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

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Tom</firstname>
        <surname>Gundersen</surname>
      </author>
    </authorgroup>
  </refentryinfo>

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

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

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

  <refsect1>
    <title>Description</title>

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

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

    <para>The link file contains a <literal>[Match]</literal> section,
    which determines if a given link file may be applied to a given
    device, as well as a <literal>[Link]</literal> 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 each of the entries
    in the <literal>[Match]</literal> section matches, or if the
    section is empty. The following keys are accepted:</para>

    <variablelist class='network-directives'>
      <varlistentry>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>The hardware address.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>OriginalName=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the device name, as exposed by the udev property
          "INTERFACE". This cannot be used to match on names that have
          already been changed from userspace. Caution is advised when matching on
          kernel-assigned names, as they are known to be unstable
          between reboots.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Path=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the persistent path, as exposed by the udev property
          <literal>ID_PATH</literal>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Driver=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the driver currently bound to the device,
          as exposed by the udev property <literal>DRIVER</literal>
          of its parent device, or if that is not set, the
          driver as exposed by <literal>ethtool -i</literal>
          of the device itself.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Type=</varname></term>
        <listitem>
          <para>A whitespace-separated list of shell-style globs matching
          the device type, as exposed by the udev
          property <literal>DEVTYPE</literal>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Host=</varname></term>
        <listitem>
          <para>Matches against the hostname or machine
          ID of the host. See <literal>ConditionHost=</literal> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Virtualization=</varname></term>
        <listitem>
          <para>Checks whether the system is executed in
          a virtualized environment and optionally test
          whether it is a specific implementation. See
          <literal>ConditionVirtualization=</literal> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>KernelCommandLine=</varname></term>
        <listitem>
          <para>Checks whether a specific kernel command line option
          is set (or if prefixed with the exclamation mark unset). See
          <literal>ConditionKernelCommandLine=</literal> in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Architecture=</varname></term>
        <listitem>
          <para>Checks whether the system is running on a specific
          architecture. See <literal>ConditionArchitecture=</literal>
          in
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

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

    <para>The <literal>[Link]</literal> 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 <literal>ifalias</literal> 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><literal>persistent</literal></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><literal>random</literal></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><literal>none</literal></term>
              <listitem>
                <para>Keeps the MAC address assigned by the kernel.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>MACAddress=</varname></term>
        <listitem>
          <para>The MAC address to use, if no
          <literal>MACAddressPolicy=</literal>
          is specified.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>NamePolicy=</varname></term>
        <listitem>
          <para>An ordered, space-separated list of policies by which
          the interface name should be set.
          <literal>NamePolicy</literal> may be disabled by specifying
          <literal>net.ifnames=0</literal> 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 <literal>ID_NET_NAME</literal>, which
          is, by default, used by a udev rule to set
          <literal>NAME</literal>. If the name has already been set by
          userspace, no renaming is performed. The available policies
          are:</para>

          <variablelist>
            <varlistentry>
              <term><literal>kernel</literal></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><literal>database</literal></term>
              <listitem>
                <para>The name is set based on entries in the udev's
                Hardware Database with the key
                <literal>ID_NET_NAME_FROM_DATABASE</literal>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>onboard</literal></term>
              <listitem>
                <para>The name is set based on information given by
                the firmware for on-board devices, as exported by the
                udev property <literal>ID_NET_NAME_ONBOARD</literal>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>slot</literal></term>
              <listitem>
                <para>The name is set based on information given by
                the firmware for hot-plug devices, as exported by the
                udev property <literal>ID_NET_NAME_SLOT</literal>.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>path</literal></term>
              <listitem>
                <para>The name is set based on the device's physical
                location, as exported by the udev property
                <literal>ID_NET_NAME_PATH</literal>.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>mac</literal></term>
              <listitem>
                <para>The name is set based on the device's persistent
                MAC address, as exported by the udev property
                <literal>ID_NET_NAME_MAC</literal>.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>Name=</varname></term>
        <listitem>
          <para>The interface name to use in case all the
          policies specified in
          <varname>NamePolicy=</varname> fail, or in case
          <varname>NamePolicy=</varname> is missing or
          disabled.</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>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 <literal>half</literal> and
          <literal>full</literal>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>AutoNegotiation=</varname></term>
        <listitem>
          <para>Enables or disables automatic negotiation of transmission parameters.
          Autonegotiation is a procedure by which two connected ethernet devices choose
          common transmission parameters, such as speed, duplex mode, and flow control.
          Takes a boolean value. Unset by default, which means that the kernel default
          will be used.</para>

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

          <variablelist>
            <varlistentry>
              <term><literal>phy</literal></term>
              <listitem>
                <para>Wake on PHY activity.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>unicast</literal></term>
              <listitem>
                <para>Wake on unicast messages.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>multicast</literal></term>
              <listitem>
                <para>Wake on multicast messages.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>broadcast</literal></term>
              <listitem>
                <para>Wake on broadcast messages.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>arp</literal></term>
              <listitem>
                <para>Wake on ARP.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>magic</literal></term>
              <listitem>
                <para>Wake on receipt of a magic packet.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>secureon</literal></term>
              <listitem>
                <para>Enable secureon(tm) password for MagicPacket(tm).
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>off</literal></term>
              <listitem>
                <para>Never wake.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </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><literal>tp</literal></term>
              <listitem>
                <para>An Ethernet interface using Twisted-Pair cable as the medium.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>aui</literal></term>
              <listitem>
                <para>Attachment Unit Interface (AUI). Normally used with hubs.
                </para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>bnc</literal></term>
              <listitem>
                <para>An Ethernet interface using BNC connectors and co-axial cable.</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>mii</literal></term>
              <listitem>
                <para>An Ethernet interface using a Media Independent Interface (MII).</para>
              </listitem>
            </varlistentry>
            <varlistentry>
              <term><literal>fibre</literal></term>
              <listitem>
                <para>An Ethernet interface using Optical Fibre as the medium.</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>TCPSegmentationOffload=</varname></term>
        <listitem>
          <para>The TCP Segmentation Offload (TSO) when true enables
          TCP segmentation offload. Takes a boolean value.
          Defaults to "unset".</para>
        </listitem>
      </varlistentry>
       <varlistentry>
      <term><varname>TCP6SegmentationOffload=</varname></term>
        <listitem>
          <para>The TCP6 Segmentation Offload (tx-tcp6-segmentation) when true enables
          TCP6 segmentation offload. Takes a boolean value.
          Defaults to "unset".</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>GenericSegmentationOffload=</varname></term>
        <listitem>
          <para>The Generic Segmentation Offload (GSO) when true enables
          generic segmentation offload. Takes a boolean value.
          Defaults to "unset".</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>UDPSegmentationOffload=</varname></term>
        <listitem>
          <para>The UDP Segmentation Offload (USO) when true enables
          UDP segmentation offload. Takes a boolean value.
          Defaults to "unset".</para>
        </listitem>
      </varlistentry>
    <varlistentry>
        <term><varname>GenericReceiveOffload=</varname></term>
        <listitem>
          <para>The Generic Receive Offload (GRO) when true enables
          generic receive offload. Takes a boolean value.
          Defaults to "unset".</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><varname>LargeReceiveOffload=</varname></term>
        <listitem>
          <para>The Large Receive Offload (LRO) when true enables
          large receive offload. Takes a boolean value.
          Defaults to "unset".</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>
    </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>