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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;
                a symlink in <filename>/etc</filename> with the same name as a link file in
                <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
                disables the link file entirely.</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; and 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.</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>Path=</varname></term>
                                <listitem>
                                <para>The persistent path, as exposed by the
                                udev property <literal>ID_PATH</literal>. May
                                contain shell style globs.</para>
                        </listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><varname>Driver=</varname></term>
                                <listitem>
                                        <para>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>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.</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.</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
                                        commandline. 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>
                                </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>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>magic</literal></term>
                                                        <listitem>
                                                                <para>Wake on receipt of a magic packet.
                                                                </para>
                                                        </listitem>
                                                </varlistentry>
                                                <varlistentry>
                                                        <term><literal>off</literal></term>
                                                        <listitem>
                                                                <para>Never wake.</para>
                                                        </listitem>
                                                </varlistentry>
                                        </variablelist>
                                </listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Example</title>
                <example>
                        <title>/etc/systemd/network/wireless.link</title>

                        <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>
                </para>
        </refsect1>

</refentry>
