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SYSTEMD.LINK(5) systemd.link SYSTEMD.LINK(5)

NAME
systemd.link - Network device configuration

SYNOPSIS
link.link

DESCRIPTION
A plain ini-style text file that encodes configuration for matching
network devices, used by systemd-udev(8) and in particular its
net_setup_link builtin. See systemd.syntax(7) for a general description
of the syntax.

The link files are read from the files located in the system network
directory /lib/systemd/network, the volatile runtime network directory
/run/systemd/network, and the local administration network directory
/etc/systemd/network. Link files must have the extension .link; 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 /etc have the highest priority, files in /run take precedence over
files with the same name in /lib. 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 /dev/null disables the configuration file entirely (it is
"masked").

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 99-default.link is shipped by the system. Any
user-supplied .link should hence have a lexically earlier name to be
considered at all.

See udevadm(8) for diagnosing problems with .link files.

[MATCH] SECTION OPTIONS
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 systemd-udevd warns about that. Hint: to avoid the warning
and to make it clear that all interfaces shall be matched, add the
following:

OriginalName=*

The following keys are accepted:

MACAddress=
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.

Example:

MACAddress=01:23:45:67:89:ab 00-11-22-33-44-55 AABB.CCDD.EEFF

PermanentMACAddress=
A whitespace-separated list of hardware's permanent addresses.
While MACAddress= 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. 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.

Path=
A whitespace-separated list of shell-style globs matching the
persistent path, as exposed by the udev property ID_PATH.

Driver=
A whitespace-separated list of shell-style globs matching the
driver currently bound to the device, as exposed by the udev
property ID_NET_DRIVER of its parent device, or if that is not set,
the driver as exposed by ethtool -i of the device itself. If the
list is prefixed with a "!", the test is inverted.

Type=
A whitespace-separated list of shell-style globs matching the
device type, as exposed by networkctl status. If the list is
prefixed with a "!", the test is inverted.

Property=
A whitespace-separated list of udev property name with its value
after a equal ("="). 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 "\".

Example: if a .link file has the following:

Property=ID_MODEL_ID=9999 "ID_VENDOR_FROM_DATABASE=vendor name" "KEY=with \"quotation\""

then, the .link file matches only when an interface has all the
above three properties.

OriginalName=
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.

Host=
Matches against the hostname or machine ID of the host. See
ConditionHost= in systemd.unit(5) for details. When prefixed with
an exclamation mark ("!"), the result is negated. If an empty
string is assigned, then previously assigned value is cleared.

Virtualization=
Checks whether the system is executed in a virtualized environment
and optionally test whether it is a specific implementation. See
ConditionVirtualization= in systemd.unit(5) for details. When
prefixed with an exclamation mark ("!"), the result is negated. If
an empty string is assigned, then previously assigned value is
cleared.

KernelCommandLine=
Checks whether a specific kernel command line option is set. See
ConditionKernelCommandLine= in systemd.unit(5) for details. When
prefixed with an exclamation mark ("!"), the result is negated. If
an empty string is assigned, then previously assigned value is
cleared.

KernelVersion=
Checks whether the kernel version (as reported by uname -r) matches
a certain expression. See ConditionKernelVersion= in
systemd.unit(5) for details. When prefixed with an exclamation mark
("!"), the result is negated. If an empty string is assigned, then
previously assigned value is cleared.

Architecture=
Checks whether the system is running on a specific architecture.
See ConditionArchitecture= in systemd.unit(5) for details. When
prefixed with an exclamation mark ("!"), the result is negated. If
an empty string is assigned, then previously assigned value is
cleared.

[LINK] SECTION OPTIONS
The [Link] section accepts the following keys:

Description=
A description of the device.

Alias=
The ifalias interface property is set to this value.

MACAddressPolicy=
The policy by which the MAC address should be set. The available
policies are:

persistent
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.

random
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 "unicast" and "locally administered" bits
set.

none
Keeps the MAC address assigned by the kernel.

MACAddress=
The MAC address to use, if no MACAddressPolicy= is specified.

NamePolicy=
An ordered, space-separated list of policies by which the interface
name should be set. NamePolicy= may be disabled by specifying
net.ifnames=0 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 ID_NET_NAME,
which is, by default, used by a udev(7), rule to set NAME. The
available policies are:

kernel
If the kernel claims that the name it has set for a device is
predictable, then no renaming is performed.

database
The name is set based on entries in the udev's Hardware
Database with the key ID_NET_NAME_FROM_DATABASE.

onboard
The name is set based on information given by the firmware for
on-board devices, as exported by the udev property
ID_NET_NAME_ONBOARD. See systemd.net-naming-scheme(7).

slot
The name is set based on information given by the firmware for
hot-plug devices, as exported by the udev property
ID_NET_NAME_SLOT. See systemd.net-naming-scheme(7).

path
The name is set based on the device's physical location, as
exported by the udev property ID_NET_NAME_PATH. See
systemd.net-naming-scheme(7).

mac
The name is set based on the device's persistent MAC address,
as exported by the udev property ID_NET_NAME_MAC. See
systemd.net-naming-scheme(7).

keep
If the device already had a name given by userspace (as part of
creation of the device or a rename), keep it.

Name=
The interface name to use. This option has lower precedence than
NamePolicy=, so for this setting to take effect, NamePolicy= must
either be unset, empty, disabled, or all policies configured there
must fail. Also see the example below with "Name=dmz0".

Note that specifying a name that the kernel might use for another
interface (for example "eth0") 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 "internal0"/"external0" or "lan0"/"lan1"/"lan3".

AlternativeNamesPolicy=
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
"database", "onboard", "slot", "path", and "mac". If the kernel
does not support the alternative names, then this setting will be
ignored.

AlternativeName=
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.

MTUBytes=
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.

BitsPerSecond=
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.

Duplex=
The duplex mode to set for the device. The accepted values are half
and full.

AutoNegotiation=
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.

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.

WakeOnLan=
The Wake-on-LAN policy to set for the device. The supported values
are:

phy
Wake on PHY activity.

unicast
Wake on unicast messages.

multicast
Wake on multicast messages.

broadcast
Wake on broadcast messages.

arp
Wake on ARP.

magic
Wake on receipt of a magic packet.

secureon
Enable secureon(tm) password for MagicPacket(tm).

off
Never wake.

Defaults to off.

Port=
The port option is used to select the device port. The supported
values are:

tp
An Ethernet interface using Twisted-Pair cable as the medium.

aui
Attachment Unit Interface (AUI). Normally used with hubs.

bnc
An Ethernet interface using BNC connectors and co-axial cable.

mii
An Ethernet interface using a Media Independent Interface
(MII).

fibre
An Ethernet interface using Optical Fibre as the medium.

Advertise=
This sets what speeds and duplex modes of operation are advertised
for auto-negotiation. This implies "AutoNegotiation=yes". The
supported values are:

Table 1. Supported advertise values
+-------------------+--------------+-------------+
|Advertise | Speed (Mbps) | Duplex Mode |
+-------------------+--------------+-------------+
|10baset-half | 10 | half |
+-------------------+--------------+-------------+
|10baset-full | 10 | full |
+-------------------+--------------+-------------+
|100baset-half | 100 | half |
+-------------------+--------------+-------------+
|100baset-full | 100 | full |
+-------------------+--------------+-------------+
|1000baset-half | 1000 | half |
+-------------------+--------------+-------------+
|1000baset-full | 1000 | full |
+-------------------+--------------+-------------+
|10000baset-full | 10000 | full |
+-------------------+--------------+-------------+
|2500basex-full | 2500 | full |
+-------------------+--------------+-------------+
|1000basekx-full | 1000 | full |
+-------------------+--------------+-------------+
|10000basekx4-full | 10000 | full |
+-------------------+--------------+-------------+
|10000basekr-full | 10000 | full |
+-------------------+--------------+-------------+
|10000baser-fec | 10000 | full |
+-------------------+--------------+-------------+
|20000basemld2-full | 20000 | full |
+-------------------+--------------+-------------+
|20000basekr2-full | 20000 | full |
+-------------------+--------------+-------------+
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.

ReceiveChecksumOffload=
Takes a boolean. If set to true, the hardware offload for
checksumming of ingress network packets is enabled. When unset, the
kernel's default will be used.

TransmitChecksumOffload=
Takes a boolean. If set to true, the hardware offload for
checksumming of egress network packets is enabled. When unset, the
kernel's default will be used.

TCPSegmentationOffload=
Takes a boolean. If set to true, the TCP Segmentation Offload (TSO)
is enabled. When unset, the kernel's default will be used.

TCP6SegmentationOffload=
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.

GenericSegmentationOffload=
Takes a boolean. If set to true, the Generic Segmentation Offload
(GSO) is enabled. When unset, the kernel's default will be used.

GenericReceiveOffload=
Takes a boolean. If set to true, the Generic Receive Offload (GRO)
is enabled. When unset, the kernel's default will be used.

LargeReceiveOffload=
Takes a boolean. If set to true, the Large Receive Offload (LRO) is
enabled. When unset, the kernel's default will be used.

RxChannels=
Sets the number of receive channels (a number between 1 and
4294967295) .

TxChannels=
Sets the number of transmit channels (a number between 1 and
4294967295).

OtherChannels=
Sets the number of other channels (a number between 1 and
4294967295).

CombinedChannels=
Sets the number of combined set channels (a number between 1 and
4294967295).

RxBufferSize=
Takes a integer. Specifies the NIC receive ring buffer size. When
unset, the kernel's default will be used.

TxBufferSize=
Takes a integer. Specifies the NIC transmit ring buffer size. When
unset, the kernel's default will be used.

EXAMPLES
Example 1. /lib/systemd/network/99-default.link

The link file 99-default.link that is shipped with systemd defines the
default naming policy for links.

[Link]
NamePolicy=kernel database onboard slot path
MACAddressPolicy=persistent

Example 2. /etc/systemd/network/10-dmz.link

This example assigns the fixed name "dmz0" to the interface with the
MAC address 00:a0:de:63:7a:e6:

[Match]
MACAddress=00:a0:de:63:7a:e6

[Link]
Name=dmz0

NamePolicy= is not set, so Name= takes effect. We use the "10-" prefix
to order this file early in the list. Note that it needs to be before
"99-link", i.e. it needs a numerical prefix, to have any effect at all.

Example 3. Debugging NamePolicy= assignments

$ sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/hub0
...
Parsed configuration file /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
...

Explicit Name= configuration wins in this case.

sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/enp0s31f6
...
Parsed configuration file /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 /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=/lib/systemd/network/99-default.link
...

In this case, the interface was already renamed, so the keep policy
specified as the first option in 99-default.link means that the
existing name is preserved. If keep was removed, or if were in boot
before the renaming has happened, we might get the following instead:

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=/lib/systemd/network/99-default.link
ID_NET_NAME=enp0s31f6
...

Please note that the details of output are subject to change.

Example 4. /etc/systemd/network/10-internet.link

This example assigns the fixed name "internet0" to the interface with
the device path "pci-0000:00:1a.0-*":

[Match]
Path=pci-0000:00:1a.0-*

[Link]
Name=internet0

Example 5. /etc/systemd/network/25-wireless.link

Here's an overly complex example that shows the use of a large number
of [Match] and [Link] settings.

[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

SEE ALSO
systemd-udevd.service(8), udevadm(8), systemd.netdev(5),
systemd.network(5)

systemd 245 SYSTEMD.LINK(5)