[Bug 1825470] Re: systemd-networkd: Deconfigures bridge after last attached interface disconnects (ie: LXC)

[Dan Streetman]

** Also affects: systemd (Ubuntu Cosmic)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Eoan)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Disco)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu Eoan)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

** Changed in: systemd (Ubuntu Eoan)
       Status: New => In Progress

** Changed in: systemd (Ubuntu Eoan)
   Importance: Undecided => Medium

** Tags added: ddstreet-next

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Title:
  systemd-networkd: Deconfigures bridge after last attached interface
  disconnects (ie: LXC)

Status in systemd package in Ubuntu:
  In Progress
Status in systemd source package in Bionic:
  New
Status in systemd source package in Cosmic:
  New
Status in systemd source package in Disco:
  New
Status in systemd source package in Eoan:
  In Progress

Bug description:
  Ubuntu 18.04 Bionic Beaver

  An issue was reported to systemd-networkd on GitHub -
  https://github.com/systemd/systemd/issues/11650 - and all the thorough
  details are there.

  Using anonymous bridges (ie: not attached to physical NICs) is
  problematic with systemd-networkd as they get deconfigured as soon as
  the last attached device disconnects (ie: a LXC container). Static IP
  configuration must be done again manually.

  The root cause is that systemd-networkd assumes that every network
  interface have a carrier signal. There's no notion of carrier signal
  on anonymous bridges, a case not properly handled by systemd-networkd.

  The systemd dev team provided a patch to address the issue and it
  would be nice to be integrated on the Ubuntu package.

  The PR is here :
  https://github.com/systemd/systemd/commit/93b4dab57e2e13bd804cbee999241be65a443e2e

  To get the proper fix, you'll need to combine 2 patches :

  - The one from the PR above
  - Another patch [1] which fixes a segfault introduced by the PR.

  [1]
  https://github.com/systemd/systemd/pull/11741/commits/a294af6810df3c18909a96b556deadba0e2ab0a9

  On my test environment, I rebuild the Ubuntu package with the 2
  patches above from "237-3ubuntu10.17" to "237-3ubuntu10.20" and made
  thorough tests without hiting a single issue so I think we can assume
  that these 2 patches are safe to use.

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