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-udevd(8) and in particular its net_setup_link builtin. See systemd.syntax(7) for a general description of the syntax.

Note that some distributions may incorporate .link files in their early boot facilities (e.g. by including copies of the .link files in initramfs). As such it may be necessary to take manual steps to ensure that any local changes are consistent with early-boot storage facilities. The relevant distribution-specific documentation should be consulted.

The .link files are read from the files located in the system network directory /usr/lib/systemd/network and /usr/local/lib/systemd/network [1], 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 smaller than "70" (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 /usr/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 /usr/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 /usr/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 lexicographic 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 lexicographically 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".

        Added in version 250.

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

        Added in version 250.

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

        Added in version 250.

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

        Added in version 250.

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

        Added in version 250.

    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.

    Added in version 211.

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.

    Added in version 245.

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

    Added in version 211.

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.

    Added in version 211.

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.

    Added in version 211.

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.

    Added in version 251.

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.

    Added in version 243.

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.

    Added in version 218.

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, the previously assigned value is cleared.

    Added in version 211.

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, the previously assigned value is cleared.

    Added in version 211.

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, the previously assigned value is cleared.

    Added in version 211.

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, the previously assigned value is cleared.

    Added in version 237.

Version=
    Checks whether a software version matches a certain expression. See ConditionVersion= in systemd.unit(5) for details. When prefixed with an exclamation mark ("!"), the result is negated. If an empty string is assigned, the previously assigned value is cleared.

    Added in version 258.

Credential=
    Checks whether the specified credential was passed to the systemd-udevd.service service. See System and Service Credentials[2] for details. When prefixed with an exclamation mark ("!"), the result is negated. If an empty string is assigned, the previously assigned value is cleared.

    Added in version 252.

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, the previously assigned value is cleared.

    Added in version 211.

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, the previously assigned value is cleared.

    Added in version 249.

[LINK] SECTION OPTIONS

The [Link] section accepts the following keys:

Description=
    A description of the device.

    Added in version 211.

Property=
    Set specified udev properties. This takes space separated list of key-value pairs concatenated with equal sign ("="). Example:

        Property=HOGE=foo BAR=baz

    This option supports simple specifier expansion, see the Specifiers section below. This option can be specified multiple times. If an empty string is assigned, then the all previous assignments are cleared.

    This setting is useful to configure the "ID_NET_MANAGED_BY=" property which declares which network management service shall manage the interface, which is respected by systemd-networkd(8) and others. Use

        Property=ID_NET_MANAGED_BY=io.systemd.Network

    to declare explicitly that systemd-networkd shall manage the interface, or set the property to something else to declare explicitly it shall not do so. See systemd.network(5) for details how this property is used to match interface names.

    Added in version 256.

ImportProperty=
    Import specified udev properties from the saved database. This takes space separated list of property names. Example:

        ImportProperty=HOGE BAR

    This option supports simple specifier expansion, see the Specifiers section below. This option can be specified multiple times. If an empty string is assigned, then the all previous assignments are cleared.

    If the same property is also set in Property= in the above, then the imported property value will be overridden by the value specified in Property=.

    Added in version 256.

UnsetProperty=
    Unset specified udev properties. This takes space separated list of property names. Example:

        UnsetProperty=HOGE BAR

    This option supports simple specifier expansion, see the Specifiers section below. This option can be specified multiple times. If an empty string is assigned, then the all previous assignments are cleared.

    This setting is applied after ImportProperty= and Property= are applied. Hence, if the same property is specified in ImportProperty= or Property=, then the imported or specified property value will be ignored, and the property will be unset.

    Added in version 256.

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

    Added in version 211.

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.

        Added in version 211.

    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.

        Added in version 211.

    none
        Keeps the MAC address assigned by the kernel. Or use the MAC address specified in MACAddress=.

        Added in version 227.

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

    Added in version 211.

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

    Added in version 211.

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.

        Added in version 216.

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

        Added in version 211.

    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).

        Added in version 211.

    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).

        Added in version 211.

    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).

        Added in version 211.

    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).

        Added in version 211.

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

        Added in version 241.

    Added in version 211.

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".

    Interface names must have a minimum length of 1 character and a maximum length of 15 characters, and may contain any 7bit ASCII character, with the exception of control characters, ":", "/" and "%". While "." is an allowed character, it is recommended to avoid it when naming interfaces as various tools (such as resolvconf(1)) use it as separator character. Also, fully numeric interface names are not allowed (in order to avoid ambiguity with interface specification by numeric indexes), nor are the special strings ".", "..", "all" and "default".

    Added in version 211.

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.

    Added in version 245.

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.

    Alternative interface names may be used to identify interfaces in various tools. In contrast to the primary name (as configured with Name= above) there may be multiple alternative names referring to the same interface. Alternative names may have a maximum length of 127 characters, in contrast to the 15 allowed for the primary interface name, but otherwise are subject to the same naming constraints.

    Added in version 245.

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.

    Added in version 248.

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.

    Added in version 248.

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.

    Added in version 248.

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.

    Added in version 211.

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.

    Added in version 211.

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

    Added in version 211.

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.

    Added in version 233.

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.

        Added in version 211.

    unicast
        Wake on unicast messages.

        Added in version 235.

    multicast
        Wake on multicast messages.

        Added in version 235.

    broadcast
        Wake on broadcast messages.

        Added in version 235.

    arp
        Wake on ARP.

        Added in version 235.

    magic
        Wake on receipt of a magic packet.

        Added in version 211.

    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 ImportCredential=/LoadCredential=/SetCredential= in systemd.exec(5) 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".

        Added in version 235.

    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.

    Added in version 211.

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.

    Added in version 250.

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.

        Added in version 234.

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

        Added in version 234.

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

        Added in version 234.

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

        Added in version 234.

    fibre
        An Ethernet interface using Optical Fibre as the medium.

        Added in version 234.

    Added in version 234.

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-full | 10 | full
    10baset1brr-full | 10 | full
    10baset1l-full | 10 | full
    10baset1s-full | 10 | full
    10baset-half | 10 | half
    10baset1s-half | 10 | half
    10baset1s-p2mp-half | 10 | half
    100basefx-full | 100 | full
    100baset-full | 100 | full
    100baset1-full | 100 | full
    100basefx-half | 100 | half
    100baset-half | 100 | half
    1000basekx-full | 1000 | full
    1000baset-full | 1000 | full
    1000baset1-full | 1000 | full
    1000basex-full | 1000 | full
    1000baset-half | 1000 | half
    2500baset-full | 2500 | full
    2500basex-full | 2500 | full
    5000baset-full | 5000 | full
    10000baser-fec | 10000 |
    10000basecr-full | 10000 | full
    10000baseer-full | 10000 | full
    10000basekr-full | 10000 | full
    10000basekx4-full | 10000 | full
    10000baselr-full | 10000 | full
    10000baselrm-full | 10000 | full
    10000basesr-full | 10000 | full
    10000baset-full | 10000 | full
    20000basekr2-full | 20000 | full
    20000basemld2-full | 20000 | full
    25000basecr-full | 25000 | full
    25000basekr-full | 25000 | full
    25000basesr-full | 25000 | full
    40000basecr4-full | 40000 | full
    40000basekr4-full | 40000 | full
    40000baselr4-full | 40000 | full
    40000basesr4-full | 40000 | full
    50000basecr-full | 50000 | full
    50000basecr2-full | 50000 | full
    50000basedr-full | 50000 | full
    50000basekr-full | 50000 | full
    50000basekr2-full | 50000 | full
    50000baselr-er-fr-full | 50000 | full
    50000basesr-full | 50000 | full
    50000basesr2-full | 50000 | full
    56000basecr4-full | 56000 | full
    56000basekr4-full | 56000 | full
    56000baselr4-full | 56000 | full
    56000basesr4-full | 56000 | full
    100000basecr-full | 100000 | full
    100000basecr2-full | 100000 | full
    100000basecr4-full | 100000 | full
    100000basedr-full | 100000 | full
    100000basedr2-full | 100000 | full
    100000basekr-full | 100000 | full
    100000basekr2-full | 100000 | full
    100000basekr4-full | 100000 | full
    100000baselr-er-fr-full | 100000 | full
    100000baselr2-er2-fr2-full | 100000 | full
    100000baselr4-er4-full | 100000 | full
    100000basesr-full | 100000 | full
    100000basesr2-full | 100000 | full
    100000basesr4-full | 100000 | full
    200000basecr-full | 200000 | full
    200000basecr2-full | 200000 | full
    200000basecr4-full | 200000 | full
    200000basedr-2-full | 200000 | full
    200000basedr-full | 200000 | full
    200000basedr2-full | 200000 | full
    200000basedr4-full | 200000 | full
    200000basekr-full | 200000 | full
    200000basekr2-full | 200000 | full
    200000basekr4-full | 200000 | full
    200000baselr2-er2-fr2-full | 200000 | full
    200000baselr4-er4-fr4-full | 200000 | full
    200000basesr-full | 200000 | full
    200000basesr2-full | 200000 | full
    200000basesr4-full | 200000 | full
    200000basevr-full | 200000 | full
    400000basecr2-full | 400000 | full
    400000basecr4-full | 400000 | full
    400000basecr8-full | 400000 | full
    400000basedr2-2-full | 400000 | full
    400000basedr2-full | 400000 | full
    400000basedr4-full | 400000 | full
    400000basedr8-full | 400000 | full
    400000basekr2-full | 400000 | full
    400000basekr4-full | 400000 | full
    400000basekr8-full | 400000 | full
    400000baselr4-er4-fr4-full | 400000 | full
    400000baselr8-er8-fr8-full | 400000 | full
    400000basesr2-full | 400000 | full
    400000basesr4-full | 400000 | full
    400000basesr8-full | 400000 | full
    400000basevr2-full | 400000 | full
    800000basecr4-full | 800000 | full
    800000basecr8-full | 800000 | full
    800000basedr4-2-full | 800000 | full
    800000basedr4-full | 800000 | full
    800000basedr8-2-full | 800000 | full
    800000basedr8-full | 800000 | full
    800000basekr4-full | 800000 | full
    800000basekr8-full | 800000 | full
    800000basesr4-full | 800000 | full
    800000basesr8-full | 800000 | full
    800000basevr4-full | 800000 | full
    800000basevr8-full | 800000 | full
    1600000basecr8-full | 1600000 | full
    1600000basedr8-2-full | 1600000 | full
    1600000basedr8-full | 1600000 | full
    1600000basekr8-full | 1600000 | full
    asym-pause |  |
    aui |  |
    autonegotiation |  |
    backplane |  |
    bnc |  |
    fec-baser |  |
    fec-llrs |  |
    fec-none |  |
    fec-rs |  |
    fibre |  |
    mii |  |
    pause |  |
    tp |  |

    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.

    Added in version 240.

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.

    Added in version 245.

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.

    Added in version 245.

ScatterGather=
    Takes a boolean. If set to true, enables offload feature where the hardware builds a network packet from multiple non-contiguous memory buffers. When unset, the kernel's default will be used. This corresponds to the command ethtool -K INTERFACE tx-scatter-gather on|off.

    Added in version 260.

ScatterGatherFragmentList=
    Takes a boolean. If set to true, enables offload feature where the hardware builds a network packet from a chained list of socket buffers. When unset, the kernel's default will be used. This corresponds to the command ethtool -K INTERFACE tx-scatter-gather-fraglist on|off.

    Added in version 260.

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

    Added in version 232.

TCPECNSegmentationOffload=
    Takes a boolean. If set to true, TCP Segmentation Offload (TSO) is performed even when ECN (Explicit Congestion Notification) flags are active in the TCP stream. When unset, the kernel's default will be used. This corresponds to the command ethtool -K INTERFACE tx-tcp-ecn-segmentation on|off.

    Added in version 260.

TCPMangleIdSegmentationOffload=
    Takes a boolean. If set to true, TCP Segmentation Offload (TSO) is performed even if the hardware is incapable of correctly incrementing the IPv4 Identification (ID) field for each resulting packet. When unset, the kernel's default will be used. This corresponds to the command ethtool -K INTERFACE tx-tcp-mangleid-segmentation on|off.

    Added in version 260.

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.

    Added in version 235.

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

    Added in version 232.

PartialGenericSegmentationOffload=
    Takes a boolean. If set to true, Partial Generic Segmentation Offload (GSO_Partial) is enabled. See Partial Generic Segmentation Offload[3] for more details. When unset, the kernel's default will be used.

    Added in version 258.

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

    Added in version 232.

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.

    Added in version 250.

GenericReceiveOffloadList=
    Takes a boolean. If set to true, Generic Receive Offload (GRO) List for UDP is enabled. When unset, the kernel's default will be used. This corresponds to the command ethtool -K INTERFACE rx-gro-list on|off.

    Added in version 260.

GenericReceiveOffloadUDPForwarding=
    Takes a boolean. If set to true, Generic Receive Offload (GRO) for aggregating incoming UDP packets is enabled. When unset, the kernel's default will be used. This corresponds to the command ethtool -K INTERFACE rx-udp-gro-forwarding on|off.

    Added in version 260.

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

    Added in version 232.

ReceivePacketSteeringCPUMask=
    Configures Receive Packet Steering (RPS) list of CPUs to which RPS may forward traffic. Takes a list of CPU indices or ranges separated by either whitespace or commas. Alternatively, takes the special value "all", which will include all available CPUs in the mask. CPU ranges are specified by the lower and upper CPU indices separated by a dash (e.g. "2-6"). This option may be specified more than once, in which case the specified list of CPU ranges are merged. If an empty string is assigned, the list is reset, all assignments prior to this will have no effect. Defaults to unset and RPS CPU list is unchanged. To disable RPS when it was previously enabled, use the special value "disable".

    Added in version 256.

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

    Added in version 250.

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

    Added in version 250.

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

    Added in version 250.

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

    Added in version 250.

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.

    Added in version 250.

ReceiveFCS=
    Takes a boolean. If set to true, pass the Frame Check Sequence (FCS) value up the stack without trimming it, and allow userspace to calculate error rate or link health. When unset, the kernel's default will be used.

    Added in version 258.

ReceiveAll=
    Takes a boolean. If set to true, allow the interface to also receive damaged ethernet frames. When unset, the kernel's default will be used.

    Added in version 258.

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.

    Added in version 239.

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.

    Added in version 244.

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.

    Added in version 246.

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.

    Added in version 246.

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.

    Added in version 246.

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.

    Added in version 248.

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.

    Added in version 248.

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.

    Added in version 250.

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.

    Added in version 250.

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.

    Added in version 250.

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.

    Added in version 250.

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.

    Added in version 250.

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.

    Added in version 250.

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.

    Added in version 251.

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.

    Added in version 251.

[SR-IOV] SECTION OPTIONS

SR-IOV provides the ability to partition a single physical PCI resource into virtual PCI functions which can then be e.g. injected into a VM. In the case of network VFs, SR-IOV reduces latency and CPU utilisation for north-south network traffic (that is, traffic with endpoints outside the host machine), by allowing traffic to bypass the host machine’s network stack.

The presence of an [SR-IOV] section in a .link file will cause the creation and configuration of the specified virtual function. Within a .network file, the specified virtual function will be configured, but must already exist. Specify several [SR-IOV] sections to configure several SR-IOVs.

The [SR-IOV] section accepts the following keys.

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.

    Added in version 251.

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

    Added in version 251.

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

    Added in version 251.

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

    Added in version 251.

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

    Added in version 251.

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.

    Added in version 251.

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.

    Added in version 251.

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.

    Added in version 251.

MACAddress=
    Specifies the MAC address for the virtual function.

    Added in version 251.

[ENERGYEFFICIENTETHERNET] SECTION OPTIONS

The [EnergyEfficientEthernet] section controls the Energy Efficient Ethernet (EEE) feature of the interface, and accepts the following keys.

Enable=
    Takes a boolean argument. When true, the Energy Efficient Ethernet (EEE) feature will be enabled on the interface. Defaults to unset, and the enablement of EEE will be unchanged.

    Added in version 258.

TxLowPowerIdle=
    Takes a boolean argument. When true, the transmit Low Power Idle (Tx-LPI) mode of the Energy Efficient Ethernet feature will be enabled on the interface. Defaults to unset, and the enablement of the mode will be unchanged.

    Added in version 258.

TxLowPowerIdleSec=
    Takes a timespan. This configures how long the interface should not enter the Low Power Idle mode after transmission. If it is too short, may decrease performance. If it is too long, may not gain energy saving. Defaults to unset, and the timespan will be unchanged.

    Added in version 258.

LinkMode=
    Takes a list of link modes, e.g. "1000baset-full". See the table for Advertise= setting in [Link] section in the above for possible values. This configures the Energy Efficient Ethernet capable connection modes to be advertised. Defaults to unset, and the advertised modes will be unchanged.

    Added in version 258.

SPECIFIERS

Some settings resolve specifiers which may be used to write generic unit files referring to runtime or unit parameters that are replaced when the unit files are loaded. Specifiers must be known and resolvable for the setting to be valid. The following specifiers are understood:

Table 2. Specifiers available in unit files
Specifier | Meaning | Details
"%a" | Architecture | A short string identifying the architecture of the local system. A string such as x86, x86-64 or arm64. See the architectures defined for ConditionArchitecture= in systemd.unit(5) for a full list.
"%A" | Operating system image version | The operating system image version identifier of the running system, as read from the IMAGE_VERSION= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.
"%b" | Boot ID | The boot ID of the running system, formatted as string. See random(4) for more information.
"%B" | Operating system build ID | The operating system build identifier of the running system, as read from the BUILD_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.
"%H" | Host name | The hostname of the running system.
"%l" | Short host name | The hostname of the running system, truncated at the first dot to remove any domain component.
"%m" | Machine ID | The machine ID of the running system, formatted as string. See machine-id(5) for more information.
"%M" | Operating system image identifier | The operating system image identifier of the running system, as read from the IMAGE_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.
"%o" | Operating system ID | The operating system identifier of the running system, as read from the ID= field of /etc/os-release. See os-release(5) for more information.
"%q" | Pretty host name | The pretty hostname of the running system, as read from the PRETTY_HOSTNAME= field of /etc/machine-info. If not set, resolves to the short hostname. See machine-info(5) for more information.
"%T" | Directory for temporary files | This is either /tmp or the path "$TMPDIR", "$TEMP" or "$TMP" are set to. (Note that the directory may be specified without a trailing slash.)
"%v" | Kernel release | Identical to uname -r output.
"%V" | Directory for larger and persistent temporary files | This is either /var/tmp or the path "$TMPDIR", "$TEMP" or "$TMP" are set to. (Note that the directory may be specified without a trailing slash.)
"%w" | Operating system version ID | The operating system version identifier of the running system, as read from the VERSION_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.
"%W" | Operating system variant ID | The operating system variant identifier of the running system, as read from the VARIANT_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.

EXAMPLES

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

The link file 99-default.link that is shipped with systemd defines the default policies for the interface name, alternative names, and MAC address of links.

    [Match]
    OriginalName=*
    [Link]
    NamePolicy=keep kernel database onboard slot path
    AlternativeNamesPolicy=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-default.link, i.e. it needs a numerical prefix, to have any effect at all.

Example 3. (Re-)applying a .link file to an interface

After a new .link file has been created, or an existing .link file modified, the new settings may be applied to the matching interface with the following commands:

    $ sudo udevadm control --reload
    $ sudo ip link set eth0 down
    $ sudo udevadm trigger --verbose --settle --action add /sys/class/net/eth0

You may also need to stop the service that manages the network interface, e.g. systemd-networkd.service(8) or NetworkManager.service before the above operation, and then restart the service after that. For more details about udevadm command, see udevadm(8).

Example 4. Debugging NamePolicy= assignments

    $ sudo SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/hub0
    ...
    Parsed configuration file /usr/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 did not 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 /usr/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 /usr/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=/usr/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=/usr/lib/systemd/network/99-default.link
    ID_NET_NAME=enp0s31f6
    ...

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

Example 5. /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 6. /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-network-generator.service(8)

NOTES

1.  💣💥🧨💥💥💣 Please note that those configuration files must be available at all times. If /usr/local/ is a separate partition, it may not be available during early boot, and must not be used for configuration.

2.  System and Service Credentials
    https://systemd.io/CREDENTIALS

3.  Partial Generic Segmentation Offload
    https://docs.kernel.org/networking/segmentation-offloads.html#partial-generic-segmentation-offload