systemd.net-naming-scheme - Network device naming schemes
Network interfaces names and MAC addresses may be generated based on certain stable interface attributes. This is possible when there is enough information about the device to generate those attributes and the use of this information is configured. This page describes interface naming, i.e. what possible names may be generated. Those names are generated by the systemd-udevd.service(8) builtin net_id and exported as udev properties (ID_NET_NAME_ONBOARD=, ID_NET_LABEL_ONBOARD=, ID_NET_NAME_PATH=, ID_NET_NAME_SLOT=).
Names and MAC addresses are derived from various stable device metadata attributes. Newer versions of udev take more of these attributes into account, improving (and thus possibly changing) the names and addresses used for the same devices. Different versions of those generation rules are called "naming schemes". The default naming scheme is chosen at compilation time. Usually this will be the latest implemented version, but it is also possible to set one of the older versions to preserve compatibility. This may be useful for example for distributions, which may introduce new versions of systemd in stable releases without changing the naming scheme. The naming scheme may also be overridden using the net.naming-scheme= kernel command line switch, see systemd-udevd.service(8). Available naming schemes are described below.
After the udev properties have been generated, appropriate udev rules may be used to actually rename devices based on those properties. See the description of NamePolicy= and MACAddressPolicy= in systemd.link(5).
Note that while the concept of network interface naming schemes is primarily relevant in the context of systemd-udevd.service, the systemd-nspawn(1) container manager also takes it into account when naming network interfaces, see below.
All names start with a two-character prefix that signifies the interface type.
Table 1. Two
character prefixes based on the type of interface
The udev net_id builtin exports the following udev device properties:
This name is set based on the numeric ordering information given by the firmware for on-board devices. The name consists of the prefix, letter o, and a number specified by the firmware. This is only available for PCI devices.
This property is set based on textual label given by the firmware for on-board devices. The name consists of the prefix concatenated with the label. This is only available for PCI devices.
This name consists of the prefix, letter x, and 12 hexadecimal digits of the MAC address. It is available if the device has a fixed MAC address. Because this name is based on an attribute of the card itself, it remains "stable" when the device is moved (even between machines), but will change when the hardware is replaced.
ID_NET_NAME_SLOT=prefix[Pdomain]sslot[ffunction][nport_name|ddev_port], ID_NET_NAME_SLOT=prefixvslot, ID_NET_NAME_SLOT=prefixxslot, ID_NET_NAME_SLOT=prefix[Pdomain]sslot[ffunction][nport_name|ddev_port]bnumber, ID_NET_NAME_SLOT=prefix[Pdomain]sslot[ffunction][nport_name|ddev_port]uport...[cconfig][iinterface], ID_NET_NAME_SLOT=prefix[Pdomain]sslot[ffunction][nport_name|ddev_port]vslot
This property describes the slot position. Different schemes are used depending on the bus type, as described in the table below. In case of USB, BCMA, and SR-VIO devices, the full name consists of the prefix, PCI slot identifier, and USB or BCMA or SR-VIO slot identifier. The first two parts are denoted as "..." in the table below.
Table 2. Slot
The PCI domain is only prepended when it is not 0. All multi-function PCI devices will carry the ffunction number in the device name, including the function 0 device. For non-multi-function devices, the number is suppressed if 0. The port name port_name is used, or the port number ddev_port if the name is not known.
For BCMA devices, the core number is suppressed when 0.
For USB devices the full chain of port numbers of hubs is composed. If the name gets longer than the maximum number of 15 characters, the name is not exported. The usual USB configuration number 1 and interface number 0 values are suppressed.
SR-IOV virtual devices are named based on the name of the parent interface, with a suffix of v and the virtual device number, with any leading zeros removed. The bus number is ignored.
In some configurations a parent PCI bridge of a given network controller may be associated with a slot. In such case we don't generate this device property to avoid possible naming conflicts.
ID_NET_NAME_PATH=prefixcbus_id, ID_NET_NAME_PATH=prefixavendormodeliinstance, ID_NET_NAME_PATH=prefixiaddressnport_name, ID_NET_NAME_PATH=prefix[Pdomain]pbussslot[ffunction][nphys_port_name|ddev_port], ID_NET_NAME_PATH=prefix[Pdomain]pbussslot[ffunction][nphys_port_name|ddev_port]bnumber, ID_NET_NAME_PATH=prefix[Pdomain]pbussslot[ffunction][nphys_port_name|ddev_port]uport...[cconfig][iinterface]
This property describes the device installation location. Different schemes are used depending on the bus type, as described in the table below. For BCMA and USB devices, PCI path information must known, and the full name consists of the prefix, PCI slot identifier, and USB or BCMA location. The first two parts are denoted as "..." in the table below.
Table 3. Path
CCW and grouped CCW devices are found in IBM System Z mainframes. Any leading zeros and dots are suppressed.
For PCI, BCMA, and USB devices, the same rules as described above for slot naming are used.
The following "naming schemes" have been defined (which may be chosen at system boot-up time via the net.naming-scheme= kernel command line switch, see above):
This is the naming scheme that was implemented in systemd 238.
Naming was changed for virtual network interfaces created with SR-IOV and NPAR and for devices where the PCI network controller device does not have a slot number associated.
SR-IOV virtual devices are named based on the name of the parent interface, with a suffix of "vport", where port is the virtual device number. Previously those virtual devices were named as if completely independent.
The ninth and later NPAR virtual devices are named following the scheme used for the first eight NPAR partitions. Previously those devices were not renamed and the kernel default ("ethN") was used.
Names are also generated for PCI devices where the PCI network controller device does not have an associated slot number itself, but one of its parents does. Previously those devices were not renamed and the kernel default was used.
The "ib" prefix and stable names for infiniband devices are introduced. Previously those devices were not renamed.
The ACPI index field (used in ID_NET_NAME_ONBOARD=) is now also used when 0.
A new naming policy NamePolicy=keep was introduced. With this policy, if the network device name was already set by userspace, the device will not be renamed again. Previously, this naming policy applied implicitly, and now it must be explicitly requested. Effectively, this means that network devices will be renamed according to the configuration, even if they have been renamed already, if keep is not specified as the naming policy in the .link file. See systemd.link(5) for a description of NamePolicy=.
MACAddressPolicy=persistent was extended to set MAC addresses based on the device name. Previously addresses were only based on the ID_NET_NAME_* attributes, which meant that interface names would never be generated for virtual devices. Now a persistent address will be generated for most devices, including in particular bridges.
Note: when userspace does not set a MAC address for a bridge device, the kernel will initially assign a random address, and then change it when the first device is enslaved to the bridge. With this naming policy change, bridges get a persistent MAC address based on the bridge name instead of the first enslaved device.
Support for renaming netdevsim (simulated networking) devices was added. Previously those devices were not renamed.
Previously two-letter interface type prefix was prepended to ID_NET_LABEL_ONBOARD=. This is not done anymore.
When systemd-nspawn(1) derives the name for the host side of the network interface created with --network-veth from the container name it previously simply truncated the result at 15 characters if longer (since that's the maximum length for network interface names). From now on, for any interface name that would be longer than 15 characters the last 4 characters are set to a 24bit hash value of the full interface name. This way network interface name collisions between multiple similarly named containers (who only differ in container name suffix) should be less likely (but still possible, since the 24bit hash value is very small).
When a PCI slot is associated with a PCI bridge that has multiple child network controllers, the same value of the ID_NET_NAME_SLOT property might be derived for those controllers. This would cause a naming conflict if the property is selected as the device name. Now, we detect this situation and don't produce the ID_NET_NAME_SLOT property.
PCI hotplug slot names for the s390 PCI driver are a hexadecimal representation of the function_id device attribute. This attribute is now used to build the ID_NET_NAME_SLOT. Before that, all slot names were parsed as decimal numbers, which could either result in an incorrect value of the ID_NET_NAME_SLOT property or none at all.
Some firmware and hypervisor implementations report unreasonably high numbers for the onboard index. To prevent the generation of bogus onbard interface names, index numbers greater than 16381 (2¹⁴-1) were ignored. For s390 PCI devices index values up to 65535 (2¹⁶-1) are valid. To account for that, the limit was increased to 65535.
The udev rule NAME= replaces ":", "/", and "%" with an underscore ("_"), and refuses strings which contain only numerics.
Added naming scheme for Xen netfront "vif" interfaces based on the guest side VIF number set from the Xen config (or the interface index in AWS EC2).
Note that latest may be used to denote the latest scheme known (to this particular version of systemd).
Example 1. Using udevadm test-builtin to display device properties
test-builtin net_id /sys/class/net/enp0s31f6
Using default interface naming scheme 'v243'.
Example 2. PCI Ethernet card with firmware index "1"
ID_NET_NAME_ONBOARD_LABEL=Ethernet Port 1
Example 3. PCI Ethernet card in hotplug slot with firmware index number
Example 4. PCI Ethernet multi-function card with 2 ports
Example 5. PCI WLAN card
Example 6. PCI IB host adapter with 2 ports
Example 7. USB built-in 3G modem
Example 8. USB Android phone
Example 9. s390 grouped CCW interface
Predictable Network Interface Names