[Bug 1845234] Re: do-release-upgrade reinstalls netplan.io, breaking systemd-networkd configuration

[Steve Langasek]

Users who remove netplan.io are removing a core component of the
definition of a base Ubuntu system (that's what being a dependency of
ubuntu-minimal means).  This should be considered unsupported.

Any affected user should have dealt with not wanting netplan to manage
their network by removing the configuration files from /etc/netplan, not
by removing the package.

** Changed in: netplan.io (Ubuntu)
       Status: New => Invalid

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Title:
  do-release-upgrade reinstalls netplan.io, breaking systemd-networkd
  configuration

Status in netplan.io package in Ubuntu:
  Invalid

Bug description:
  [impact]

  Users who have chosen to remove the 'netplan.io' package and manually
  configure systemd-networkd, when upgrading, will have the 'netplan.io'
  package reinstalled, which will create a new dynamic networkd
  configuration after reboot, which breaks system networking.

  [test case]

  install an Ubuntu release, e.g. Bionic.

  Make sure some 'desktop' metapackage is installed, e.g.:
  $ sudo apt install xubuntu-desktop

  Remove netplan:
  $ sudo apt purge netplan.io

  note that the above command removes not only 'netplan.io' but also
  'ubuntu-minimal'

  then, configure systemd-networkd manually, in some way that would
  conflict with netplan's default of just using dhcp on the main
  interface, e.g.:

  $ grep . /etc/systemd/network/*
  /etc/systemd/network/10-br0.netdev:[NetDev]
  /etc/systemd/network/10-br0.netdev:Name=br0
  /etc/systemd/network/10-br0.netdev:Kind=bridge
  /etc/systemd/network/20-ens3.network:[Match]
  /etc/systemd/network/20-ens3.network:Name=ens3
  /etc/systemd/network/20-ens3.network:[Network]
  /etc/systemd/network/20-ens3.network:Bridge=br0
  /etc/systemd/network/50-br0.network:[Match]
  /etc/systemd/network/50-br0.network:Name=br0
  /etc/systemd/network/50-br0.network:[Network]
  /etc/systemd/network/50-br0.network:Address=192.168.122.200/24
  /etc/systemd/network/50-br0.network:Gateway=192.168.122.1
  /etc/systemd/network/50-br0.network:DNS=192.168.122.1

  (That is a trivial example of networkd config that would conflict with
  netplan's default, real examples can be much more complex, even as far
  as causing systemd-networkd to fail to start)

  reboot the system to discard old netplan networkd configuration, and
  pick up new manual networkd configuration.  If needed, edit /etc
  /update-manager/release-upgrades to change Prompt to 'normal'.  Then
  upgrade to a new release:

  $ sudo do-release-upgrade

  following the prompts to complete the upgrade, including the final
  prompt to reboot after the upgrade.

  Boot into the upgraded system and check if netplan is installed and
  operating:

  $ dpkg -l|grep netplan
  ii  netplan.io                            0.97-0ubuntu1~19.04.1               amd64        YAML network configuration abstraction for various backends
  $ ls -l /run/systemd/network/
  total 8
  -rw-r--r-- 1 root root  69 Sep 24 15:36 10-netplan-ens3.link
  -rw-r--r-- 1 root root 129 Sep 24 15:36 10-netplan-ens3.network

  
  [regression potential]

  TBD

  [other info]

  This appears to require one of the 'desktop' packages installed, e.g.
  'ubuntu-desktop, 'xubuntu-desktop', etc.; I reproduced it with
  'xubuntu-desktop' installed, but could not reproduce it with a stock
  cloud image that has no 'desktop' package installed.

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