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

[Dimitri John Ledkov]

What is the legacy software in question?

Why does it have pre-up / post-up scripts and what do they do?

Can the functionality be replicated using networkd-dispatcher which is
available out of the box on bionic? Specifically the routable.d/ off.d/
directories. See http://manpages.ubuntu.com/manpages/bionic/en/man8
/networkd-dispatcher.8.html

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

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

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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