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

[Braiam Peguero]

[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 (may need several reboots as it's a race condition)

[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

[Braiam Peguero (braiampe)]

@xnox There isn't anything indicating in the changelogs that this bug was fixed and 229-4ubuntu17. Remember that the bug is a race condition, so it may or may not happen at random, it depends which is faster, systemd or DAD

[Dimitri John Ledkov (xnox)]

zesty ships v232 hence marked as fix released for zesty.

[Dimitri John Ledkov (xnox)]

This is incomplete request, essentially requesting "backport all of networkd" which is not appropriate for a targetted fixes that SRUs require.

Can you not use zesty on your system?