Ubuntu Manpage: systemd.link - Network device configuration

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-udevd(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 and /usr/local/lib/systemd/network, the volatile runtime network
directory /run/systemd/network, and the local administration network directory
/etc/systemd/network. All configuration files are collectively sorted and processed in
alphanumeric order, regardless of the directories in which they live. However, files with
identical filenames replace each other. It is recommended that each filename is prefixed
with a number (e.g. 10-eth0.link). Otherwise, the default .link files or those generated
by systemd-network-generator.service(8) may take precedence over user configured files.
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").

Along with the link file foo.link, a "drop-in" directory foo.link.d/ may exist. All files
with the suffix ".conf" from this directory will be merged in the alphanumeric order and
parsed after the main file itself has been parsed. This is useful to alter or add
configuration settings, without having to modify the main configuration file. Each drop-in
file must have appropriate section headers.

In addition to /etc/systemd/network, drop-in ".d" directories can be placed in
/lib/systemd/network or /run/systemd/network directories. Drop-in files in /etc/ take
precedence over those in /run/ which in turn take precedence over those in /lib/. Drop-in
files under any of these directories take precedence over the main link file wherever
located.

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 an interface 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 interfaces 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 first (in alphanumeric order) of the link files that matches a given interface is
applied, all later files are ignored, even if they match as well. The following keys are
accepted:

MACAddress=
A whitespace-separated list of hardware addresses. The acceptable formats are:

colon-delimited hexadecimal
Each field must be one byte. E.g. "12:34:56:78:90:ab" or "AA:BB:CC:DD:EE:FF".

hyphen-delimited hexadecimal
Each field must be one byte. E.g. "12-34-56-78-90-ab" or "AA-BB-CC-DD-EE-FF".

dot-delimited hexadecimal
Each field must be two bytes. E.g. "1234.5678.90ab" or "AABB.CCDD.EEFF".

IPv4 address format
E.g. "127.0.0.1" or "192.168.0.1".

IPv6 address format
E.g. "2001:0db8:85a3::8a2e:0370:7334" or "::1".

The total length of each MAC address must be 4 (for IPv4 tunnel), 6 (for Ethernet), 16
(for IPv6 tunnel), or 20 (for InfiniBand). 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. Defaults to unset.

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, or IPv4 or IPv6 address format. 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. Defaults to
unset.

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 list. If the list is prefixed with a "!", the test is inverted. Some
valid values are "ether", "loopback", "wlan", "wwan". Valid types are named either
from the udev "DEVTYPE" attribute, or "ARPHRD_" macros in linux/if_arp.h, so this is
not comprehensive.

Kind=
A whitespace-separated list of shell-style globs matching the device kind, as exposed
by networkctl status INTERFACE or ip -d link show INTERFACE. If the list is prefixed
with a "!", the test is inverted. Some valid values are "bond", "bridge", "gre",
"tun", "veth". Valid kinds are given by netlink's "IFLA_INFO_KIND" attribute, so this
is not comprehensive.

Property=
A whitespace-separated list of udev property names with their values after equals sign
("="). 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.

Firmware=
Checks whether the system is running on a machine with the specified firmware. See
ConditionFirmware= 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. Or use the MAC address specified in
MACAddress=.

An empty string assignment is equivalent to setting "none".

MACAddress=
The interface MAC address to use. For this setting to take effect, MACAddressPolicy=
must either be unset, empty, or "none".

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.

TransmitQueues=
Specifies the device's number of transmit queues. An integer in the range 1...4096.
When unset, the kernel's default will be used.

ReceiveQueues=
Specifies the device's number of receive queues. An integer in the range 1...4096.
When unset, the kernel's default will be used.

TransmitQueueLength=
Specifies the transmit queue length of the device in number of packets. An unsigned
integer in the range 0...4294967294. When unset, the kernel's default will be used.

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. Takes the special value "off" which
disables Wake-on-LAN, or space separated list of the following words:

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 password for MagicPacket. Implied when WakeOnLanPassword= is
specified. If specified without WakeOnLanPassword= option, then the password is
read from the credential "LINK.link.wol.password" (e.g.,
"60-foo.link.wol.password"), and if the credential not found, then read from
"wol.password". See LoadCredential=/SetCredential= in systemd.exec(1) for details.
The password in the credential, must be 6 bytes in hex format with each byte
separated by a colon (":") like an Ethernet MAC address, e.g.,
"aa:bb:cc:dd:ee:ff".

Defaults to unset, and the device's default will be used. This setting can be
specified multiple times. If an empty string is assigned, then the all previous
assignments are cleared.

WakeOnLanPassword=
Specifies the SecureOn password for MagicPacket. Takes an absolute path to a regular
file or an AF_UNIX stream socket, or the plain password. When a path to a regular file
is specified, the password is read from it. When an AF_UNIX stream socket is
specified, a connection is made to it and the password is read from it. The password
must be 6 bytes in hex format with each byte separated by a colon (":") like an
Ethernet MAC address, e.g., "aa:bb:cc:dd:ee:ff". This implies WakeOnLan=secureon.
Defaults to unset, and the current value will not be changed.

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, 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, 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, TCP Segmentation Offload (TSO) is enabled. When
unset, the kernel's default will be used.

TCP6SegmentationOffload=
Takes a boolean. If set to true, 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, Generic Segmentation Offload (GSO) is enabled. When
unset, the kernel's default will be used.

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

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

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

ReceiveVLANCTAGHardwareAcceleration=
Takes a boolean. If set to true, receive VLAN CTAG hardware acceleration is enabled.
When unset, the kernel's default will be used.

TransmitVLANCTAGHardwareAcceleration=
Takes a boolean. If set to true, transmit VLAN CTAG hardware acceleration is enabled.
When unset, the kernel's default will be used.

ReceiveVLANCTAGFilter=
Takes a boolean. If set to true, receive filtering on VLAN CTAGs is enabled. When
unset, the kernel's default will be used.

TransmitVLANSTAGHardwareAcceleration=
Takes a boolean. If set to true, transmit VLAN STAG hardware acceleration is enabled.
When unset, the kernel's default will be used.

NTupleFilter=
Takes a boolean. If set to true, receive N-tuple filters and actions are enabled. When
unset, the kernel's default will be used.

RxChannels=, TxChannels=, OtherChannels=, CombinedChannels=
Specifies the number of receive, transmit, other, or combined channels, respectively.
Takes an unsigned integer in the range 1...4294967295 or "max". If set to "max", the
advertised maximum value of the hardware will be used. When unset, the number will not
be changed. Defaults to unset.

RxBufferSize=, RxMiniBufferSize=, RxJumboBufferSize=, TxBufferSize=
Specifies the maximum number of pending packets in the NIC receive buffer, mini
receive buffer, jumbo receive buffer, or transmit buffer, respectively. Takes an
unsigned integer in the range 1...4294967295 or "max". If set to "max", the advertised
maximum value of the hardware will be used. When unset, the number will not be
changed. Defaults to unset.

RxFlowControl=
Takes a boolean. When set, enables receive flow control, also known as the ethernet
receive PAUSE message (generate and send ethernet PAUSE frames). When unset, the
kernel's default will be used.

TxFlowControl=
Takes a boolean. When set, enables transmit flow control, also known as the ethernet
transmit PAUSE message (respond to received ethernet PAUSE frames). When unset, the
kernel's default will be used.

AutoNegotiationFlowControl=
Takes a boolean. When set, auto negotiation enables the interface to exchange state
advertisements with the connected peer so that the two devices can agree on the
ethernet PAUSE configuration. When unset, the kernel's default will be used.

GenericSegmentOffloadMaxBytes=
Specifies the maximum size of a Generic Segment Offload (GSO) packet the device should
accept. The usual suffixes K, M, G are supported and are understood to the base of
1024. An unsigned integer in the range 1...65536. Defaults to unset.

GenericSegmentOffloadMaxSegments=
Specifies the maximum number of Generic Segment Offload (GSO) segments the device
should accept. An unsigned integer in the range 1...65535. Defaults to unset.

UseAdaptiveRxCoalesce=, UseAdaptiveTxCoalesce=
Boolean properties that, when set, enable/disable adaptive Rx/Tx coalescing if the
hardware supports it. When unset, the kernel's default will be used.

RxCoalesceSec=, RxCoalesceIrqSec=, RxCoalesceLowSec=, RxCoalesceHighSec=, TxCoalesceSec=,
TxCoalesceIrqSec=, TxCoalesceLowSec=, TxCoalesceHighSec=
These properties configure the delay before Rx/Tx interrupts are generated after a
packet is sent/received. The "Irq" properties come into effect when the host is
servicing an IRQ. The "Low" and "High" properties come into effect when the packet
rate drops below the low packet rate threshold or exceeds the high packet rate
threshold respectively if adaptive Rx/Tx coalescing is enabled. When unset, the
kernel's defaults will be used.

RxMaxCoalescedFrames=, RxMaxCoalescedIrqFrames=, RxMaxCoalescedLowFrames=,
RxMaxCoalescedHighFrames=, TxMaxCoalescedFrames=, TxMaxCoalescedIrqFrames=,
TxMaxCoalescedLowFrames=, TxMaxCoalescedHighFrames=
These properties configure the maximum number of frames that are sent/received before
a Rx/Tx interrupt is generated. The "Irq" properties come into effect when the host is
servicing an IRQ. The "Low" and "High" properties come into effect when the packet
rate drops below the low packet rate threshold or exceeds the high packet rate
threshold respectively if adaptive Rx/Tx coalescing is enabled. When unset, the
kernel's defaults will be used.

CoalescePacketRateLow=, CoalescePacketRateHigh=
These properties configure the low and high packet rate (expressed in packets per
second) threshold respectively and are used to determine when the corresponding
coalescing settings for low and high packet rates come into effect if adaptive Rx/Tx
coalescing is enabled. If unset, the kernel's defaults will be used.

CoalescePacketRateSampleIntervalSec=
Configures how often to sample the packet rate used for adaptive Rx/Tx coalescing.
This property cannot be zero. This lowest time granularity supported by this property
is seconds. Partial seconds will be rounded up before being passed to the kernel. If
unset, the kernel's default will be used.

StatisticsBlockCoalesceSec=
How long to delay driver in-memory statistics block updates. If the driver does not
have an in-memory statistic block, this property is ignored. This property cannot be
zero. If unset, the kernel's default will be used.

MDI=
Specifies the medium dependent interface (MDI) mode for the interface. A MDI describes
the interface from a physical layer implementation to the physical medium used to
carry the transmission. Takes one of the following words: "straight" (or equivalently:
"mdi"), "crossover" (or equivalently: "mdi-x", "mdix"), and "auto". When "straight",
the MDI straight through mode will be used. When "crossover", the MDI crossover
(MDI-X) mode will be used. When "auto", the MDI status is automatically detected.
Defaults to unset, and the kernel's default will be used.

SR-IOVVirtualFunctions=
Specifies the number of SR-IOV virtual functions. Takes an integer in the range
0...2147483647. Defaults to unset, and automatically determined from the values
specified in the VirtualFunction= settings in the [SR-IOV] sections.

[SR-IOV] SECTION OPTIONS

       The [SR-IOV] section accepts the following keys. Specify several [SR-IOV] sections to
       configure several SR-IOVs. SR-IOV provides the ability to partition a single physical PCI
       resource into virtual PCI functions which can then be injected into a VM. In the case of
       network VFs, SR-IOV improves north-south network performance (that is, traffic with
       endpoints outside the host machine) by allowing traffic to bypass the host machine’s
       network stack.

       VirtualFunction=
           Specifies a Virtual Function (VF), lightweight PCIe function designed solely to move
           data in and out. Takes an integer in the range 0...2147483646. This option is
           compulsory.

       VLANId=
           Specifies VLAN ID of the virtual function. Takes an integer in the range 1...4095.

       QualityOfService=
           Specifies quality of service of the virtual function. Takes an integer in the range
           1...4294967294.

       VLANProtocol=
           Specifies VLAN protocol of the virtual function. Takes "802.1Q" or "802.1ad".

       MACSpoofCheck=
           Takes a boolean. Controls the MAC spoof checking. When unset, the kernel's default
           will be used.

       QueryReceiveSideScaling=
           Takes a boolean. Toggle the ability of querying the receive side scaling (RSS)
           configuration of the virtual function (VF). The VF RSS information like RSS hash key
           may be considered sensitive on some devices where this information is shared between
           VF and the physical function (PF). When unset, the kernel's default will be used.

       Trust=
           Takes a boolean. Allows one to set trust mode of the virtual function (VF). When set,
           VF users can set a specific feature which may impact security and/or performance. When
           unset, the kernel's default will be used.

       LinkState=
           Allows one to set the link state of the virtual function (VF). Takes a boolean or a
           special value "auto". Setting to "auto" means a reflection of the physical function
           (PF) link state, "yes" lets the VF to communicate with other VFs on this host even if
           the PF link state is down, "no" causes the hardware to drop any packets sent by the
           VF. When unset, the kernel's default will be used.

       MACAddress=
           Specifies the MAC address for the virtual function.

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