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<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
This file is part of systemd.
Copyright 2014 Tom Gundersen
-->
<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>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
<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>KernelVersion=</varname></term>
<listitem>
<para>Checks whether the kernel version (as reported by <command>uname -r</command>) matches a certain
expression (or if prefixed with the exclamation mark does not match it). See
<literal>ConditionKernelVersion=</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>
<para>Defaults to <literal>off</literal>.</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><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>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>
<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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