[Bug 1937117] Re: misconfigured networkd may break after networkd restart

[Dan Streetman]

Attached debdiffs for b/f.

Note that Bionic only needs a revert of the patch for the original bug,
but Focal requires the patch revert plus an additional small patch to
restore the previous behavior, due to other patches added for bug
1664844.

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https://bugs.launchpad.net/bugs/1937117

Title:
  misconfigured networkd may break after networkd restart

Status in systemd package in Ubuntu:
  Invalid
Status in systemd source package in Bionic:
  In Progress
Status in systemd source package in Focal:
  In Progress

Bug description:
  [impact]

  bug 1929560 fixed an error in setting/clearing an interface 'master'
  during systemd-networkd startup if the interface was already up,
  however for any users who have been relying on that bug to manually
  set an interface 'master' for an interface that's otherwise controlled
  by systemd-networkd, this will break their system by removing the
  interface from the 'master' on a systemd-networkd restart.

  For example, if a user has interface 'eth0' configured to be
  controlled by systemd-networkd, but then manually does 'ip l set dev
  eth0 master br0' to add it to the 'br0' bridge, previous behavior
  would leave 'eth0' in the bridge after a systemd-networkd restart,
  even though it should not be in the bridge based on the systemd-
  networkd configuration. The new behavior matches systemd-networkd
  behavior for all other aspects (i.e. networkd removes all addresses,
  routes, and other settings it doesn't know about or that are different
  from its configuration), however it breaks users expecting the
  previous incorrect behavior.

  [test case]

  configure systemd-networkd to control an interface that isn't set with
  any 'master', and start networkd. For example, an instance configured
  for DHCP. After networkd has started, manually set the interface
  'master' to an existing bridge. Restart networkd, and check if the
  interface still has the bridge 'master' or not.

  e.g., in a newly created cloud-image container, with normal netplan,
  check the networkd control of eth0:

  root at lp1937117-f:~# networkctl list eth0
  IDX LINK TYPE  OPERATIONAL SETUP     
  159 eth0 ether routable    configured

  create a new bridge and manually place eth0 into it:

  root at lp1937117-f:~# ip l add dev br0 type bridge
  root at lp1937117-f:~# ip l set up dev br0
  root at lp1937117-f:~# ip l set dev eth0 master br0
  root at lp1937117-f:~# ip l show dev eth0
  159: eth0 at if160: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br0 state UP mode DEFAULT group default qlen 1000
      link/ether 00:16:3e:56:f8:07 brd ff:ff:ff:ff:ff:ff link-netnsid 0

  now restart systemd-networkd, and check if eth0 remains under br0:

  root at lp1937117-f:~# systemctl restart systemd-networkd
  root at lp1937117-f:~# ip l show dev eth0
  159: eth0 at if160: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
      link/ether 00:16:3e:56:f8:07 brd ff:ff:ff:ff:ff:ff link-netnsid 0

  
  [regression potential]

  the main potential is for problems for those with misconfigured
  systems, relying on specific previous incorrect behavior, specifically
  interfaces controlled by systemd-networkd in part, but also with
  manual configuration performed outside of systemd-networkd
  configuration.

  [scope]

  this is needed for bionic and focal to restore the previous incorrect
  behavior; the correct behavior should remain in impish, and (arguably)
  should remain in hirsute as well

  [other info]

  see bug 1929560 for details of the original bug

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