[Bug 1986536] Re: Running ifup@ for all interfaces breaks no-auto-down

[Gaelan Steele]

Can reproduce on Jammy.

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Title:
  Running ifup@ for all interfaces breaks no-auto-down

Status in ifupdown package in Ubuntu:
  New

Bug description:
  We (mythic-beasts.com) run a "Raspberry Pi cloud", consisting of
  Raspberry Pis with no SD card and an NFS root file system. This
  requires special considerations for shutting down, as bringing down
  eth0 will break the file system and cause the shutdown process to hang
  forever.

  ifupdown has a config option for exactly this situation: no-auto-down.
  We set this flag in /etc/network/interfaces.d/eth0.

  However, this flag only prevents `ifdown -a` from bringing down the
  interface; `ifdown eth0` still does. Unfortunately, ifup at eth0.service
  does exactly this when bringing down the system.

  This isn't an issue on Debian, because Debian only starts
  ifup at .service for services marked "allow-hotplug"; non-hotplug
  services are started and stopped by networking.service, which just
  uses `if{up,down} -a`. Ubuntu, on the other hand, brings up
  ifup at .service for all services marked auto. (This was an intentional
  change - see the Ubuntu ifupdown changelog.)

  As such, unless I'm missing something, there's no way on Ubuntu to
  have an interface that's automatically brought up on boot, but not
  automatically brought down on shutdown.

  Tested on bionic and focal:

  root at rpi-bionic-arm64:~# lsb_release -rd
  Description:	Ubuntu 18.04.6 LTS
  Release:	18.04
  root at rpi-bionic-arm64:~# apt-cache policy ifupdown
  ifupdown:
    Installed: 0.8.17ubuntu1.1
    Candidate: 0.8.17ubuntu1.1
    Version table:
   *** 0.8.17ubuntu1.1 500
          500 http://ports.ubuntu.com/ubuntu-ports bionic-updates/main arm64 Packages
          100 /var/lib/dpkg/status
       0.8.17ubuntu1 500
          500 http://ports.ubuntu.com/ubuntu-ports bionic/main arm64 Packages

  root at rpi-focal-arm64:~# lsb_release -rd
  Description:	Ubuntu 20.04.3 LTS
  Release:	20.04
  root at rpi-focal-arm64:~# apt-cache policy ifupdown
  ifupdown:
    Installed: 0.8.35ubuntu1
    Candidate: 0.8.35ubuntu1
    Version table:
   *** 0.8.35ubuntu1 500
          500 http://ports.ubuntu.com/ubuntu-ports focal/universe arm64 Packages
          100 /var/lib/dpkg/status

  I can't easily test on jammy, as we're not producing jammy images yet;
  but I'm happy to create one (or try upgrading a pi from focal) if
  people think it's likely to do something interesting.

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