[Bug 1771236] Re: forced use of systemd-networkd interferes with ifupdown in 18.04

[Brandon Applegate]

Looking for an update on this - specifically the seemingly unanswered
question of "does removing all netplan related config render systemd-
netword 'toothless'".  I too am very concerned that simply installing
ifupdown doesn't by itself achieve the result that seems to be put forth
by the various pieces of documentation and advice around these issues.

I'm currently fighting vmware guest customizations on 18.04 (a whole
other rabbit hole...) - but my current solution is in jeopardy if
systemd-networkd is "touching" interfaces before I can get ansible to do
it's thing.  I'm currently testing and troubleshooting this.

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https://bugs.launchpad.net/bugs/1771236

Title:
  forced use of systemd-networkd interferes with ifupdown in 18.04

Status in ifupdown package in Ubuntu:
  Incomplete
Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  For several reasons, we are not able to use netplan nor systemd-
  networkd due to legacy applications that expect ifupdown's pre-up and
  post-up script mechanism.  The documentation around 18.04's
  (premature, I feel) wholesale adoption of netplan claims that one can
  revert to old behaviour by merely installing ifupdown (amongst
  assertions that netplan will never offer a mechanism for configuring
  pre-up and post-up actions even for network managers that support
  them).

  However when ifupdown is installed, systemd-networkd still tries to
  manage interfaces.  If you 'systemctl disable systemd-networkd', upon
  next reboot it is automatically re-enabled.  We tried disabling any
  systemd units even remotely related to networking and yet systemd-
  networkd still runs.  If it hasn't been configured, it tries to DHCP.
  On networks that don't provide DHCP this results in a stupendously
  long stall during boot.  Currently it appears to be impossible to tell
  systemd-networkd not to run in a clean manner that won't get reverted
  on package upgrades.

  I sincerely hope this is is a bug/oversight and not intentional.  We
  need to be able to disable systemd-networkd properly.

  Thanks

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