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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->
<refentry id="systemd.link">
<refentryinfo>
<title>systemd.link</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd.link</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.link</refname>
<refpurpose>Network device configuration</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename><replaceable>link</replaceable>.link</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>Network link configuration is performed by the
<command>net_setup_link</command> udev builtin.</para>
<para>The link files are read from the files located in the system
network directory <filename>/usr/lib/systemd/network</filename>,
the volatile runtime network directory
<filename>/run/systemd/network</filename>, and the local
administration network directory
<filename>/etc/systemd/network</filename>. Link files must have
the extension <filename>.link</filename>; other extensions are
ignored. All link files are collectively sorted and processed in
lexical order, regardless of the directories in which they live.
However, files with identical filenames replace each other. Files
in <filename>/etc</filename> have the highest priority, files in
<filename>/run</filename> take precedence over files with the same
name in <filename>/usr/lib</filename>. This can be used to
override a system-supplied link file with a local file if needed.
As a special case, an empty file (file size 0) or symlink with the
same name pointing to <filename>/dev/null</filename> disables the
configuration file entirely (it is "masked").</para>
<para>The link file contains a [Match] section, which determines if a given link file may be applied to a
given device, as well as a [Link] section specifying how the device should be configured. The first (in
lexical order) of the link files that matches a given device is applied. Note that a default file
<filename>99-default.link</filename> is shipped by the system. Any user-supplied
<filename>.link</filename> should hence have a lexically earlier name to be considered at all.</para>
<para>See <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
diagnosing problems with <filename>.link</filename> files.</para>
</refsect1>
<refsect1>
<title>[Match] Section Options</title>
<para>A link file is said to match a device if each of the entries in the [Match] 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>A whitespace-separated list of hardware addresses. Use full colon-, hyphen- or dot-delimited hexadecimal. See the example below.
This option may appear more than once, in which case the lists are merged. If the empty string is assigned to this option, the list
of hardware addresses defined prior to this is reset.</para>
<para>Example:
<programlisting>MACAddress=01:23:45:67:89:ab 00-11-22-33-44-55 AABB.CCDD.EEFF</programlisting></para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>OriginalName=</varname></term>
<listitem>
<para>A whitespace-separated list of shell-style globs matching
the device name, as exposed by the udev property
"INTERFACE". This cannot be used to match on names that have
already been changed from userspace. Caution is advised when matching on
kernel-assigned names, as they are known to be unstable
between reboots.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Path=</varname></term>
<listitem>
<para>A whitespace-separated list of shell-style globs matching
the persistent path, as exposed by the udev property
<varname>ID_PATH</varname>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Driver=</varname></term>
<listitem>
<para>A whitespace-separated list of shell-style globs matching the driver currently bound to the
device, as exposed by the udev property <varname>DRIVER</varname> of its parent device, or if that
is not set, the driver as exposed by <command>ethtool -i</command> of the device itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Type=</varname></term>
<listitem>
<para>A whitespace-separated list of shell-style globs matching
the device type, as exposed by the udev
property <varname>DEVTYPE</varname>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Host=</varname></term>
<listitem>
<para>Matches against the hostname or machine ID of the host. See <varname>ConditionHost=</varname> in
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
If an empty string is assigned, then previously assigned value is cleared.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Virtualization=</varname></term>
<listitem>
<para>Checks whether the system is executed in a virtualized environment and optionally test
whether it is a specific implementation. See <varname>ConditionVirtualization=</varname> in
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
If an empty string is assigned, then previously assigned value is cleared.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>KernelCommandLine=</varname></term>
<listitem>
<para>Checks whether a specific kernel command line option is set. See
<varname>ConditionKernelCommandLine=</varname> in
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
If an empty string is assigned, then previously assigned value is cleared.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>KernelVersion=</varname></term>
<listitem>
<para>Checks whether the kernel version (as reported by <command>uname -r</command>) matches a certain
expression. See <varname>ConditionKernelVersion=</varname> in
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
If an empty string is assigned, then previously assigned value is cleared.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Architecture=</varname></term>
<listitem>
<para>Checks whether the system is running on a specific architecture. See
<varname>ConditionArchitecture=</varname> in
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. When prefixed with an exclamation mark (<literal>!</literal>), the result is negated.
If an empty string is assigned, then previously assigned value is cleared.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>[Link] Section Options</title>
<para>The [Link] section accepts the following
keys:</para>
<variablelist class='network-directives'>
<varlistentry>
<term><varname>Description=</varname></term>
<listitem>
<para>A description of the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Alias=</varname></term>
<listitem>
<para>The <varname>ifalias</varname> interface property is set to this value.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>MACAddressPolicy=</varname></term>
<listitem>
<para>The policy by which the MAC address should be set. The
available policies are:
</para>
<variablelist>
<varlistentry>
<term><option>persistent</option></term>
<listitem>
<para>If the hardware has a persistent MAC address, as
most hardware should, and if it is used by the kernel,
nothing is done. Otherwise, a new MAC address is
generated which is guaranteed to be the same on every
boot for the given machine and the given device, but
which is otherwise random. This feature depends on ID_NET_NAME_*
properties to exist for the link. On hardware where these
properties are not set, the generation of a persistent MAC address
will fail.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>random</option></term>
<listitem>
<para>If the kernel is using a random MAC address,
nothing is done. Otherwise, a new address is randomly
generated each time the device appears, typically at
boot. Either way, the random address will have the
<literal>unicast</literal> and
<literal>locally administered</literal> bits set.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>none</option></term>
<listitem>
<para>Keeps the MAC address assigned by the kernel.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>MACAddress=</varname></term>
<listitem>
<para>The MAC address to use, if no
<varname>MACAddressPolicy=</varname>
is specified.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>NamePolicy=</varname></term>
<listitem>
<para>An ordered, space-separated list of policies by which the interface name should be set.
<varname>NamePolicy=</varname> may be disabled by specifying <option>net.ifnames=0</option> on the
kernel command line. Each of the policies may fail, and the first successful one is used. The name
is not set directly, but is exported to udev as the property <option>ID_NET_NAME</option>, which
is, by default, used by a udev 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>.
</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>.
</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>.</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>.</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 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 <option>half</option> and
<option>full</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>AutoNegotiation=</varname></term>
<listitem>
<para>Takes a boolean. If set to yes, automatic negotiation of transmission parameters is enabled.
Autonegotiation is a procedure by which two connected ethernet devices choose
common transmission parameters, such as speed, duplex mode, and flow control.
When unset, the kernel's default will be used.</para>
<para>Note that if autonegotiation is enabled, speed and duplex settings are
read-only. If autonegotation is disabled, speed and duplex settings are writable
if the driver supports multiple link modes.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>WakeOnLan=</varname></term>
<listitem>
<para>The Wake-on-LAN policy to set for the device. The
supported values are:</para>
<variablelist>
<varlistentry>
<term><option>phy</option></term>
<listitem>
<para>Wake on PHY activity.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>unicast</option></term>
<listitem>
<para>Wake on unicast messages.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>multicast</option></term>
<listitem>
<para>Wake on multicast messages.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>broadcast</option></term>
<listitem>
<para>Wake on broadcast messages.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>arp</option></term>
<listitem>
<para>Wake on ARP.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>magic</option></term>
<listitem>
<para>Wake on receipt of a magic packet.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>secureon</option></term>
<listitem>
<para>Enable secureon(tm) password for MagicPacket(tm).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>off</option></term>
<listitem>
<para>Never wake.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Defaults to <option>off</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Port=</varname></term>
<listitem>
<para>The port option is used to select the device port. The
supported values are:</para>
<variablelist>
<varlistentry>
<term><option>tp</option></term>
<listitem>
<para>An Ethernet interface using Twisted-Pair cable as the medium.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>aui</option></term>
<listitem>
<para>Attachment Unit Interface (AUI). Normally used with hubs.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>bnc</option></term>
<listitem>
<para>An Ethernet interface using BNC connectors and co-axial cable.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>mii</option></term>
<listitem>
<para>An Ethernet interface using a Media Independent Interface (MII).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>fibre</option></term>
<listitem>
<para>An Ethernet interface using Optical Fibre as the medium.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Advertise=</varname></term>
<listitem>
<para>This sets what speeds and duplex modes of operation are advertised for auto-negotiation.
This implies <literal>AutoNegotiation=yes</literal>. The supported values are:
<table>
<title>Supported advertise values</title>
<tgroup cols='3'>
<colspec colname='Advertise' />
<colspec colname='Speed' />
<colspec colname='Duplex Mode' />
<thead><row>
<entry>Advertise</entry>
<entry>Speed (Mbps)</entry>
<entry>Duplex Mode</entry>
</row></thead>
<tbody>
<row><entry><option>10baset-half</option></entry>
<entry>10</entry><entry>half</entry></row>
<row><entry><option>10baset-full</option></entry>
<entry>10</entry><entry>full</entry></row>
<row><entry><option>100baset-half</option></entry>
<entry>100</entry><entry>half</entry></row>
<row><entry><option>100baset-full</option></entry>
<entry>100</entry><entry>full</entry></row>
<row><entry><option>1000baset-half</option></entry>
<entry>1000</entry><entry>half</entry></row>
<row><entry><option>1000baset-full</option></entry>
<entry>1000</entry><entry>full</entry></row>
<row><entry><option>10000baset-full</option></entry>
<entry>10000</entry><entry>full</entry></row>
<row><entry><option>2500basex-full</option></entry>
<entry>2500</entry><entry>full</entry></row>
<row><entry><option>1000basekx-full</option></entry>
<entry>1000</entry><entry>full</entry></row>
<row><entry><option>10000basekx4-full</option></entry>
<entry>10000</entry><entry>full</entry></row>
<row><entry><option>10000basekr-full</option></entry>
<entry>10000</entry><entry>full</entry></row>
<row><entry><option>10000baser-fec</option></entry>
<entry>10000</entry><entry>full</entry></row>
<row><entry><option>20000basemld2-full</option></entry>
<entry>20000</entry><entry>full</entry></row>
<row><entry><option>20000basekr2-full</option></entry>
<entry>20000</entry><entry>full</entry></row>
</tbody>
</tgroup>
</table>
By default this is unset, i.e. all possible modes will be advertised.
This option may be specified more than once, in which case all specified speeds and modes are advertised.
If the empty string is assigned to this option, the list is reset, and all prior assignments have no effect.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TCPSegmentationOffload=</varname></term>
<listitem>
<para>Takes a boolean. If set to true, the TCP Segmentation Offload (TSO) is enabled.
When unset, the kernel's default will be used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TCP6SegmentationOffload=</varname></term>
<listitem>
<para>Takes a boolean. If set to true, the TCP6 Segmentation Offload (tx-tcp6-segmentation) is enabled.
When unset, the kernel's default will be used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>GenericSegmentationOffload=</varname></term>
<listitem>
<para>Takes a boolean. If set to true, the Generic Segmentation Offload (GSO) is enabled.
When unset, the kernel's default will be used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>GenericReceiveOffload=</varname></term>
<listitem>
<para>Takes a boolean. If set to true, the Generic Receive Offload (GRO) is enabled.
When unset, the kernel's default will be used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>LargeReceiveOffload=</varname></term>
<listitem>
<para>Takes a boolean. If set to true, the Large Receive Offload (LRO) is enabled.
When unset, the kernel's default will be used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>RxChannels=</varname></term>
<listitem>
<para>Sets the number of receive channels (a number between 1 and 4294967295) .</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TxChannels=</varname></term>
<listitem>
<para>Sets the number of transmit channels (a number between 1 and 4294967295).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>OtherChannels=</varname></term>
<listitem>
<para>Sets the number of other channels (a number between 1 and 4294967295).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>CombinedChannels=</varname></term>
<listitem>
<para>Sets the number of combined set channels (a number between 1 and 4294967295).</para>
</listitem>
</varlistentry>
</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>
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