Naming of virtio-net interfaces is not consistent in RHEL 7

Environment

• Red Hat Enterprise Linux 7
• KVM or RHEV/RHV virtual machine guest using:
  • One or more paravirtual NIC type using virtio-net driver
  • One or more other emulated NIC type such as Intel e1000 or Realtek rtl8139

Issue

• The name of a virtio-net interface is seen to be different or unstable after a reboot.

• virtio and e1000 or rtl8139 NICs change device names from eth to ens names randomly or unpredictably.

Resolution

Enable persistent network device naming for virtio devices:

• Edit the /etc/default/grub file

• Add net.ifnames=1 to the end of the GRUB_CMDLINE_LINUX parameter

• Rebuild the bootloader config with grub2-mkconfig > /boot/grub2/grub.cfg

• Reboot the system.

• Once rebooted, inspect device names and MAC addresses with ip addr

• If there is a mismatch where a kernel device name does not match the /etc/sysconfig/network-scripts/ifcfg-* file parameters DEVICE or NAME, then correct the ifcfg-* files so that all three (kernel device, DEVICE parameter, NAME parameter) are the same.

• Reboot again so the desired naming and interface configuration applies to each interface.

Workaround

Ensure that every interface presented to the guest is of the same type, and never use disparate network device types in the same system.

This is done by changing the virtual device type in the libvirt or RHV management interface.

Ideally, all interfaces should be virtio-net in a KVM/RHV VM.

Root Cause

• The option to apply systemd persistent naming to virtio-devices was added in RHEL 7.2 with Bug 1259015 - persistent interface names for virtio interfaces, which introduced the following patch:

  • Content from github.com is not included.udev: make naming for virtio devices opt-in

• This behaviour is not applied by default, but is opt-in, requiring net.ifnames=1 on the GRUB kernel boot line.

• The most common cause of unstable names for virtio interfaces is a result of having a mix of virtio interfaces and some other interface type such as e1000 or rtl8139 within the same guest. Standardizing on one interface type for all guest interfaces can also avoid the issue without needing persistent naming.

• Booting any RHEL 7 system, guest or not, with the kernel command line parameter net.ifnames=0 set is known to cause naming issues. Please see the following article for more information:

  • Is it safe to set net.ifnames=0 in RHEL7?

Diagnostic Steps

• The type of NIC can be verified from within the guest with the ethtool command. It will list the name of the driver the interface uses:

  [root@rhel_guest ~]# ethtool -i eth0
  driver: virtio_net
  version: 1.0.0
  firmware-version: 
  expansion-rom-version: 
  bus-info: 0000:00:03.0
  supports-statistics: no
  supports-test: no
  supports-eeprom-access: no
  supports-register-dump: no
  supports-priv-flags: no
  

• From the hypervisor side, the type of NIC presented to the guest can be verified from virt-manager and by using virsh to review a guest's XML definition.

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