[Bug 1673092] [NEW] systemd doesn't wait until the tentative flag isn't removed before firing units depending on network-online.target

[Braiam Peguero]

Public bug reported:

[Impact]

See this issue for details
https://github.com/systemd/systemd/issues/2037

Basically, while DAD is running checks on a ipv6 address, no service can
bind the interface, which could provoke units to fail. Disabling DAD is
a workaround as explained here [1], but ideally systemd should wait until
the tentative flag is removed. v232 doesn't have this issue. Can the relevant
patches be backported to LTS releases?

[Test Case]

1. Configure ipv6 address on one interface
2. Create unit with After=network-online.target that pings a host over
   ipv6
3. Restart the system

[Regression Potential]

* Unknown

[Other Info]

* Seems to have been fixed on v232.
* Using ifupdown or networkd to create the interface seems to be irrelevant
  (upstream bug report uses networkd, while I use ifupdown)

[1]: https://www.agwa.name/blog/post/beware_the_ipv6_dad_race_condition

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1673092

Title:
  systemd doesn't wait until the tentative flag isn't removed before
  firing units depending on network-online.target

Status in systemd package in Ubuntu:
  New

Bug description:
  [Impact]

  See this issue for details
  https://github.com/systemd/systemd/issues/2037

  Basically, while DAD is running checks on a ipv6 address, no service can
  bind the interface, which could provoke units to fail. Disabling DAD is
  a workaround as explained here [1], but ideally systemd should wait until
  the tentative flag is removed. v232 doesn't have this issue. Can the relevant
  patches be backported to LTS releases?

  [Test Case]

  1. Configure ipv6 address on one interface
  2. Create unit with After=network-online.target that pings a host over
     ipv6
  3. Restart the system

  [Regression Potential]

  * Unknown

  [Other Info]

  * Seems to have been fixed on v232.
  * Using ifupdown or networkd to create the interface seems to be irrelevant
    (upstream bug report uses networkd, while I use ifupdown)

  [1]:
  https://www.agwa.name/blog/post/beware_the_ipv6_dad_race_condition

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