[Bug 1574483] Re: assigns MAC-based names for devices with locally administered MAC address

[Martin Pitt]

I currently run systemd/udev from xenial-proposed, and when enabling
tethering I now get enp0s29u3 as a device name, which is location based.
Before, with 229-4ubuntu4 I got enxXXXXXXXX (MAC based).

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Title:
  assigns MAC-based names for devices with locally administered MAC
  address

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Xenial:
  Fix Committed
Status in systemd source package in Yakkety:
  Fix Released
Status in systemd package in Debian:
  Fix Released

Bug description:
  Originally from https://bugs.debian.org/812575: Our current
  /lib/systemd/network/90-mac-for-usb.link assigns MAC-based names to
  all USB devices. However, this is wrong and pointless for locally
  administered MAC addresses (with the second bit set to '1', see
  https://en.wikipedia.org/wiki/MAC_address#Address_details).

  The old 75-persistent-net generator had this rule:

     ENV{MATCHADDR}=="?[2367abef]:*",        ENV{MATCHADDR}=""

  which ignored these devices. We need to adjust 90-mac-for-usb.link to
  do the same, and only apply to universally administered MAC addresses
  (at least until https://lists.ubuntu.com/archives/ubuntu-
  devel/2016-April/039302.html gets concluded).

  We should simply fall through the default policy of assigning location
  based names. The kernel assigned ones have no meaning whatsoever, so
  this at least provides name stability for use cases where you can rely
  on plugging the device into the same port.

  SRU INFORMATION:
  ================
  Fix: https://anonscm.debian.org/cgit/pkg-systemd/systemd.git/commit/?id=ec7e332

  Test case:
   - Plug in an Android phone and enable USB tethering, or a different USB device which uses locally administered MAC addresses, i. e. they change after every reboot of the device. The first byte of the MAC address must have the second bit set, i. e. the hex number matches ?[2367abef].
   - Check "ip a". With current xenial version the interface name is MAC based, like "enx12345678", and it changes after rebooting the device.
   - With this fix, the name is location based, like "enp0s1u2", and it remains stable as long as you use the same USB port.

  Regression potential: This only affects naming USB network devices. It
  should be tested that devices with universally administered MACs (i.
  e. stable, second bit is zero) keep MAC based names to avoid changing
  existing stable names. For devices with locally administered MACs (i.
  e. random ones) the name currently changes all the time anyway, so any
  /etc/network/interfaces or firewall script that refers to them is
  already broken and there cannot be further regressions there. Also,
  the updated policy is only applied after a computer reboot or
  replugging the device, the name does not change while the device is
  already plugged in.

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