[Bug 1988119] Re: systemd-udevd: Run net_setup_link on 'change' uevents to prevent DNS outages on Azure

[Milan Barton]

Hi Matthew,
on our production Ubuntu VM in Azure we have problem to ping google.com
The version of prod Ubuntu is:

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.5 LTS"
NAME="Ubuntu"
VERSION="18.04.5 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.5 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

I have installed new test Ubuntu VM now:

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.6 LTS"
NAME="Ubuntu"
VERSION="18.04.6 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.6 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

ping google.com was working fine.

Then I have applied your steps above and ping google.com is still
working fine.

Milan

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https://bugs.launchpad.net/bugs/1988119

Title:
  systemd-udevd: Run net_setup_link on 'change' uevents to prevent DNS
  outages on Azure

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  In Progress

Bug description:
  [Impact]

  A widespread outage was caused on Azure instances earlier today, when
  systemd 237-3ubuntu10.54 was published to the bionic-security pocket.
  Instances could no longer resolve DNS queries, breaking networking.

  For affected users, the following workarounds are available. Use whatever is most convenient.
  - Reboot your instances
  - or -
  - Issue "udevadm trigger -cadd -yeth0 && systemctl restart systemd-networkd" as root

  The trigger was found to be open-vm-tools issuing "udevadm trigger".
  Azure has a specific netplan setup that uses the `driver` match to set
  up networking. If a udevadm trigger is executed, the KV pair that
  contains this info is lost. Next time netplan is executed, the server
  loses it's DNS information.

  This is the same as bug 1902960 experienced on Focal two years ago.

  The root cause was found to be a bug in systemd, where if we receive a
  "Remove" action from a change uevent, we need to run net_setup_link(),
  we need to skip device rename and keep the old name.

  [Testcase]

  Start an instance up on Azure, any type. Simply issue udevadm trigger
  and reload systemd-networkd:

  $ ping google.com
  PING google.com (172.253.62.102) 56(84) bytes of data.
  64 bytes from bc-in-f102.1e100.net (172.253.62.102): icmp_seq=1 ttl=56 time=1.85 ms
  $ sudo udevadm trigger && sudo systemctl restart systemd-networkd
  $ ping google.com
  ping: google.com: Temporary failure in name resolution

  To fix a broken instance, you can run:

  $ sudo udevadm trigger -cadd -yeth0 && sudo systemctl restart systemd-
  networkd

  and then install the test packages below:

  Test packages are available in the following ppa:

  https://launchpad.net/~mruffell/+archive/ubuntu/sf343528-test

  If you install them, the issue should no longer occur.

  [Where problems could occur]

  If a regression were to occur, it would affect systemd-udevd
  processing 'change' events from network devices, which could lead to
  network outages. Since this would happen when systemd-networkd is
  restarted on postinstall, a regression would cause widespread outages
  due to this SRU being targeted to the security pocket, where
  unattended-upgrades will automatically install from.

  Side effects could include incorrect udevd device properties.

  It is very important that this SRU is well tested before release.

  [Other info]

  This was fixed in Systemd 247 with the following commit:

  commit e0e789c1e97e2cdf1cafe0c6b7d7e43fa054f151
  Author: Yu Watanabe <watanabe.yu+github at gmail.com>
  Date: Mon, 14 Sep 2020 15:21:04 +0900
  Subject: udev: re-assign ID_NET_DRIVER=, ID_NET_LINK_FILE=, ID_NET_NAME= properties on non-'add' uevent
  Link: https://github.com/systemd/systemd/commit/e0e789c1e97e2cdf1cafe0c6b7d7e43fa054f151

  This was backported to Focal's systemd 245.4-4ubuntu3.4 in bug 1902960
  two years ago. Focal required a heavy backport, which was performed by
  Dan Streetman. Focals backport can be found in d/p/lp1902960-udev-re-
  assign-ID_NET_DRIVER-ID_NET_LINK_FILE-ID_NET.patch, or the below
  pastebin:

  https://paste.ubuntu.com/p/K5k7bGt3Wx/

  The changes between the Focal backport and the Bionic backport are:

  - We use udev_device_get_action() instead of device_get_action()
  - device_action_from_string() is used to get to enum DeviceAction
  - We return 0 from the "if (a == DEVICE_ACTION_MOVE) " hunk instead of "goto no_rename"
  - log_device_* has been changed to log_*.

  See attached debdiff for Bionic backport.

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