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Chapter 1. Implementing consistent network interface naming

The udev device manager implements consistent device naming in Red Hat Enterprise Linux. The device manager supports different naming schemes and, by default, assigns fixed names based on firmware, topology, and location information.

Without consistent device naming, the Linux kernel assigns names to network interfaces by combining a fixed prefix and an index. The index increases as the kernel initializes the network devices. For example, eth0 represents the first Ethernet device being probed on start-up. If you add another network interface controller to the system, the assignment of the kernel device names is no longer fixed because, after a reboot, the devices can initialize in a different order. In that case, the kernel can name the devices differently.

To solve this problem, udev assigns consistent device names. This has the following advantages:

  • Device names are stable across reboots.
  • Device names stay fixed even if you add or remove hardware.
  • Defective hardware can be seamlessly replaced.
  • The network naming is stateless and does not require explicit configuration files.
Warning

Generally, Red Hat does not support systems where consistent device naming is disabled. For exceptions, see the Is it safe to set net.ifnames=0 solution.

1.1. How the udev device manager renames network interfaces

To implement a consistent naming scheme for network interfaces, the udev device manager processes the following rule files in the listed order:

  1. Optional: /usr/lib/udev/rules.d/60-net.rules

    The /usr/lib/udev/rules.d/60-net.rules file defines that the deprecated /usr/lib/udev/rename_device helper utility searches for the HWADDR parameter in /etc/sysconfig/network-scripts/ifcfg-* files. If the value set in the variable matches the MAC address of an interface, the helper utility renames the interface to the name set in the DEVICE parameter of the ifcfg file.

    If the system uses only NetworkManager connection profiles in keyfile format, udev skips this step.

  2. Only on Dell systems: /usr/lib/udev/rules.d/71-biosdevname.rules

    This file exists only if the biosdevname package is installed, and the rules file defines that the biosdevname utility renames the interface according to its naming policy, if it was not renamed in the previous step.

    Note

    Install and use biosdevname only on Dell systems.

  3. /usr/lib/udev/rules.d/75-net-description.rules

    This file defines how udev examines the network interface and sets the properties in udev-internal variables. These variables are then processed in the next step by the /usr/lib/udev/rules.d/80-net-setup-link.rules file. Some of the properties can be undefined.

  4. /usr/lib/udev/rules.d/80-net-setup-link.rules

    This file calls the net_setup_link builtin of the udev service, and udev renames the interface based on the order of the policies in the NamePolicy parameter in the /usr/lib/systemd/network/99-default.link file. For further details, see Network interface naming policies.

    If none of the policies applies, udev does not rename the interface.

Additional resources

1.2. Network interface naming policies

By default, the udev device manager uses the /usr/lib/systemd/network/99-default.link file to determine which device naming policies to apply when it renames interfaces. The NamePolicy parameter in this file defines which policies udev uses and in which order: 

NamePolicy=kernel database onboard slot path

The following table describes the different actions of udev based on which policy matches first as specified by the NamePolicy parameter:

PolicyDescriptionExample name

kernel

If the kernel indicates that a device name is predictable, udev does not rename this device.

lo

database

This policy assigns names based on mappings in the udev hardware database. For details, see the hwdb(7) man page on your system.

idrac

onboard

Device names incorporate firmware or BIOS-provided index numbers for onboard devices.

eno1

slot

Device names incorporate firmware or BIOS-provided PCI Express (PCIe) hot-plug slot-index numbers.

ens1

path

Device names incorporate the physical location of the connector of the hardware.

enp1s0

mac

Device names incorporate the MAC address. By default, Red Hat Enterprise Linux does not use this policy, but administrators can enable it.

enx525400d5e0fb

Additional resources

1.3. Network interface naming schemes

The udev device manager uses certain stable interface attributes that device drivers provide to generate consistent device names.

If a new udev version changes how the service creates names for certain interfaces, Red Hat adds a new scheme version and documents the details in the systemd.net-naming-scheme(7) man page on your system. By default, Red Hat Enterprise Linux (RHEL) 8 uses the rhel-8.0 naming scheme, even if you install or update to a later minor version of RHEL.

If you want to use a scheme other than the default, you can switch the network interface naming scheme.

For further details about the naming schemes for different device types and platforms, see the systemd.net-naming-scheme(7) man page on your system.

1.4. Switching to a different network interface naming scheme

By default, Red Hat Enterprise Linux (RHEL) 8 uses the rhel-8.0 naming scheme, even if you install or update to a later minor version of RHEL. While the default naming scheme fits in most scenarios, there might be reasons to switch to a different scheme version, for example:

  • A new scheme can help to better identify a device if it adds additional attributes, such as a slot number, to an interface name.
  • An new scheme can prevent udev from falling back to the kernel-assigned device names (eth*). This happens if the driver does not provide enough unique attributes for two or more interfaces to generate unique names for them.

Prerequisites

  • You have access to the console of the server.

Procedure

  1. List the network interfaces: 

    # ip link show
2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:5e:00:53:1a brd ff:ff:ff:ff:ff:ff
...

    Record the MAC addresses of the interfaces.

  2. Optional: Display the ID_NET_NAMING_SCHEME property of a network interface to identify the naming scheme that RHEL currently uses: 

    # udevadm info --query=property --property=ID_NET_NAMING_SCHEME /sys/class/net/eno1'
ID_NET_NAMING_SCHEME=rhel-8.0

    Note that the property is not available on the lo loopback device.

  3. Append the net.naming-scheme=<scheme> option to the command line of all installed kernels, for example: 

    # grubby --update-kernel=ALL --args=net.naming-scheme=rhel-8.4

  4. Reboot the system. 

    # reboot

  5. Based on the MAC addresses you recorded, identify the new names of network interfaces that have changed due to the different naming scheme: 

    # ip link show
2: eno1np0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:5e:00:53:1a brd ff:ff:ff:ff:ff:ff
...

    After switching the scheme, udev names in this example the device with MAC address 00:00:5e:00:53:1a eno1np0, whereas it was named eno1 before.

  6. Identify which NetworkManager connection profile uses an interface with the previous name: 

    # nmcli -f device,name connection show
DEVICE  NAME
eno1  example_profile
...

  7. Set the connection.interface-name property in the connection profile to the new interface name: 

    # nmcli connection modify example_profile connection.interface-name "eno1np0"

  8. Reactivate the connection profile: 

    # nmcli connection up example_profile

Verification

  • Identify the naming scheme that RHEL now uses by displaying the ID_NET_NAMING_SCHEME property of a network interface: 

    # udevadm info --query=property --property=ID_NET_NAMING_SCHEME /sys/class/net/eno1np0'
ID_NET_NAMING_SCHEME=_rhel-8.4

Additional resources

1.5. Determining a predictable RoCE device name on the IBM Z platform

On Red Hat Enterprise Linux (RHEL) 8.7 and later, the udev device manager sets names for RoCE interfaces on IBM Z as follows:

  • If the host enforces a unique identifier (UID) for a device, udev assigns a consistent device name that is based on the UID, for example eno<UID_in_decimal>.

  • If the host does not enforce a UID for a device, the behavior depends on your settings:

    • By default, udev uses unpredictable names for the device.
    • If you set the net.naming-scheme=rhel-8.7 kernel command line option, udev assigns a consistent device name that is based on the function identifier (FID) of the device, for example ens<FID_in_decimal>.

Manually configure predictable device name for RoCE interfaces on IBM Z in the following cases:

  • Your host runs RHEL 8.6 or earlier and enforces a UID for a device, and you plan to update to RHEL 8.7 or later.

    After an update to RHEL 8.7 or later, udev uses consistent interface names. However, if you used unpredictable device names before the update, NetworkManager connection profiles still use these names and fail to activate until you update the affected profiles.

  • Your host runs RHEL 8.7 or later and does not enforce a UID, and you plan to upgrade to RHEL 9.

Before you can use a udev rule or a systemd link file to rename an interface manually, you must determine a predictable device name.

Prerequisites

  • An RoCE controller is installed in the system.
  • The sysfsutils package is installed.

Procedure

  1. Display the available network devices, and note the names of the RoCE devices: 

    # ip link show
...
2: enP5165p0s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
...

  2. Display the device path in the /sys/ file system: 

    # systool -c net -p
Class = "net"

  Class Device = "enP5165p0s0"
  Class Device path = "/sys/devices/pci142d:00/142d:00:00.0/net/enP5165p0s0"
    Device = "142d:00:00.0"
    Device path = "/sys/devices/pci142d:00/142d:00:00.0"

    Use the path shown in the Device path field in the next steps.

  3. Display the value of the <device_path>/uid_id_unique file, for example: 

    # cat /sys/devices/pci142d:00/142d:00:00.0/uid_id_unique

    The displayed value indicates whether UID uniqueness is enforced or not, and you require this value in later steps.

  4. Determine a unique identifier:

    • If UID uniqueness is enforced (1), display the UID stored in the <device_path>/uid file, for example: 

      # cat /sys/devices/pci142d:00/142d:00:00.0/uid

    • If UID uniqueness is not enforced (0), display the FID stored in the <device_path>/function_id file, for example: 

      # cat /sys/devices/pci142d:00/142d:00:00.0/function_id

    The outputs of the commands display the UID and FID values in hexadecimal.

  5. Convert the hexadecimal identifier to decimal, for example: 

    # printf "%d\n" 0x00001402
5122

  6. To determine the predictable device name, append the identifier in decimal format to the corresponding prefix based on whether UID uniqueness is enforced or not:

    • If UID uniqueness is enforced, append the identifier to the eno prefix, for example eno5122.
    • If UID uniqueness is not enforced, append the identifier to the ens prefix, for example ens5122.

Next steps

Additional resources

1.6. Customizing the prefix for Ethernet interfaces during installation

If you do not want to use the default device-naming policy for Ethernet interfaces, you can set a custom device prefix during the Red Hat Enterprise Linux (RHEL) installation.

Important

Red Hat supports systems with customized Ethernet prefixes only if you set the prefix during the RHEL installation. Using the prefixdevname utility on already deployed systems is not supported.

If you set a device prefix during the installation, the udev service uses the <prefix><index> format for Ethernet interfaces after the installation. For example, if you set the prefix net, the service assigns the names net0, net1, and so on to the Ethernet interfaces.

The udev service appends the index to the custom prefix, and preserves the index values of known Ethernet interfaces. If you add an interface, udev assigns an index value that is one greater than the previously-assigned index value to the new interface.

Prerequisites

  • The prefix consists of ASCII characters.
  • The prefix is an alphanumeric string.
  • The prefix is shorter than 16 characters.
  • The prefix does not conflict with any other well-known network interface prefix, such as eth, eno, ens, and em.

Procedure

  1. Boot the Red Hat Enterprise Linux installation media.

  2. In the boot manager, follow these steps:

    1. Select the Install Red Hat Enterprise Linux <version> entry.
    2. Press Tab to edit the entry.
    3. Append net.ifnames.prefix=<prefix> to the kernel options.
    4. Press Enter to start the installation program.
  3. Install Red Hat Enterprise Linux.

Verification

  • To verify the interface names, display the network interfaces: 

    # ip link show
...
2: net0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:5e:00:53:1a brd ff:ff:ff:ff:ff:ff
...

Additional resources

1.7. Configuring user-defined network interface names by using udev rules

You can use udev rules to implement custom network interface names that reflect your organization’s requirements.

Procedure

  1. Identify the network interface that you want to rename: 

    # ip link show
...
enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:5e:00:53:1a brd ff:ff:ff:ff:ff:ff
...

    Record the MAC address of the interface.

  2. Display the device type ID of the interface: 

    # cat /sys/class/net/enp1s0/type
1

  3. Create the /etc/udev/rules.d/70-persistent-net.rules file, and add a rule for each interface that you want to rename: 

    SUBSYSTEM=="net",ACTION=="add",ATTR{address}=="<MAC_address>",ATTR{type}=="<device_type_id>",NAME="<new_interface_name>"
    Important

    Use only 70-persistent-net.rules as a file name if you require consistent device names during the boot process. The dracut utility adds a file with this name to the initrd image if you regenerate the RAM disk image.

    For example, use the following rule to rename the interface with MAC address 00:00:5e:00:53:1a to provider0: 

    SUBSYSTEM=="net",ACTION=="add",ATTR{address}=="00:00:5e:00:53:1a",ATTR{type}=="1",NAME="provider0"

  4. Optional: Regenerate the initrd RAM disk image: 

    # dracut -f

    You require this step only if you need networking capabilities in the RAM disk. For example, this is the case if the root file system is stored on a network device, such as iSCSI.

  5. Identify which NetworkManager connection profile uses the interface that you want to rename: 

    # nmcli -f device,name connection show
DEVICE  NAME
enp1s0  example_profile
...

  6. Unset the connection.interface-name property in the connection profile: 

    # nmcli connection modify example_profile connection.interface-name ""

  7. Temporarily, configure the connection profile to match both the new and the previous interface name: 

    # nmcli connection modify example_profile match.interface-name "provider0 enp1s0"

  8. Reboot the system: 

    # reboot

  9. Verify that the device with the MAC address that you specified in the link file has been renamed to provider0: 

    # ip link show
provider0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:5e:00:53:1a brd ff:ff:ff:ff:ff:ff
...

  10. Configure the connection profile to match only the new interface name: 

    # nmcli connection modify example_profile match.interface-name "provider0"

    You have now removed the old interface name from the connection profile.

  11. Reactivate the connection profile: 

    # nmcli connection up example_profile

Additional resources

  • udev(7) man page on your system
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