systemd Developer Tom Gundersen systemd.link 5 systemd.link Network device configuration link.link Network link configuration is performed by the net_setup_link udev builtin. The link files are read from the files located in the system network directory /usr/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 /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"). 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 udevadm8 for diagnosing problems with .link files. A link file is said to match a device if each of the entries in the [Match] section matches, or if the section is empty. The following keys are accepted: MACAddress= The hardware address. 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. 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 DRIVER of its parent device, or if that is not set, the driver as exposed by ethtool -i of the device itself. Type= A whitespace-separated list of shell-style globs matching the device type, as exposed by the udev property DEVTYPE. Host= Matches against the hostname or machine ID of the host. See ConditionHost= in systemd.unit5 for details. Virtualization= Checks whether the system is executed in a virtualized environment and optionally test whether it is a specific implementation. See ConditionVirtualization= in systemd.unit5 for details. KernelCommandLine= Checks whether a specific kernel command line option is set (or if prefixed with the exclamation mark unset). See ConditionKernelCommandLine= in systemd.unit5 for details. KernelVersion= Checks whether the kernel version (as reported by uname -r) matches a certain expression (or if prefixed with the exclamation mark does not match it). See ConditionKernelVersion= in systemd.unit5 for details. Architecture= Checks whether the system is running on a specific architecture. See ConditionArchitecture= in systemd.unit5 for details. The [Link] section accepts the following keys: Description= A description of the device. Alias= The ifalias 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 rule to set NAME. If the name has already been set by userspace, no renaming is performed. 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. 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. path The name is set based on the device's physical location, as exported by the udev property ID_NET_NAME_PATH. mac The name is set based on the device's persistent MAC address, as exported by the udev property ID_NET_NAME_MAC. Name= The interface name to use in case all the policies specified in NamePolicy= fail, or in case NamePolicy= is missing or disabled. 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. 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= Enables or disables automatic negotiation of transmission parameters. Autonegotiation is a procedure by which two connected ethernet devices choose common transmission parameters, such as speed, duplex mode, and flow control. Takes a boolean value. Unset by default, which means that the kernel default will be used. Note that if autonegotiation is enabled, speed and duplex settings are read-only. If autonegotation is disabled, speed and duplex settings are writable if the driver supports multiple link modes. 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. TCPSegmentationOffload= The TCP Segmentation Offload (TSO) when true enables TCP segmentation offload. Takes a boolean value. Defaults to "unset". TCP6SegmentationOffload= The TCP6 Segmentation Offload (tx-tcp6-segmentation) when true enables TCP6 segmentation offload. Takes a boolean value. Defaults to "unset". GenericSegmentationOffload= The Generic Segmentation Offload (GSO) when true enables generic segmentation offload. Takes a boolean value. Defaults to "unset". GenericReceiveOffload= The Generic Receive Offload (GRO) when true enables generic receive offload. Takes a boolean value. Defaults to "unset". LargeReceiveOffload= The Large Receive Offload (LRO) when true enables large receive offload. Takes a boolean value. Defaults to "unset". 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 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 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 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 systemd-udevd.service8 , udevadm8 , systemd.netdev5 , systemd.network5