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

[Dimitri John Ledkov]

@wes234234

A simple "apt install ifupdown" will not undo everything that installer
has done to setup the system, nor redo it for the alternative networking
management tool. Where do you expect ifupdown configs to materialize
from? And how do you expect the netplan configs be removed? Given that
the system is sensitive to all configs present on disk. This is similar
situation to how on the desktop one cannot simply "remove network-
manager & install ifupdown" to switch to ifupdown.

"there are plenty of examples" -> can you please give the examples that
you have experienced, specifically? the ubuntu developers have gone
extensive amount of porting, and integration testing to ensure that as
many things as possible are fixed to work, both on new installs and
upgrades.

I really want to know what is still broken and still needs fixing.

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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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