[Bug 2098183] Re: Preserve IPv6 configurations when `KeepConfiguration=dhcp-on-stop` is set

[Nick Rosbrook]

For SRU purposes, we tend to use "In Progress" to indicate that
something has been uploaded to the unapproved queue. I am going to set
to "Triaged" to better reflect where we are at.

We have very recently released a systemd update to noble, and we try to
give enough time between updates so that (a) we are not doing too
frequent of updates, and (b) we can release multiple bug fixes per
update.

So, I am going to tag this so that I catch it in my next round, but I do
not have capacity to get to it in the near future.

** Changed in: systemd (Ubuntu Noble)
       Status: In Progress => Triaged

** Changed in: systemd (Ubuntu Oracular)
       Status: In Progress => Triaged

** Tags added: systemd-sru-next

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Title:
  Preserve IPv6 configurations when `KeepConfiguration=dhcp-on-stop` is
  set

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Focal:
  New
Status in systemd source package in Jammy:
  New
Status in systemd source package in Noble:
  Triaged
Status in systemd source package in Oracular:
  Triaged
Status in systemd source package in Plucky:
  Fix Released

Bug description:
  [Impact]
  The IPv6 configuration is being cleared even though `KeepConfiguration` is set to `yes`, causing the iSCSI backend system to hang during shutdown.

  [Fix]
  Systemd upstream has addressed this issue.
  https://github.com/systemd/systemd/pull/34989

  [Test Plan]
  <Verify the functionality of KeepConfiguration in preserving IPv6 configurations>
  1. Set up a machine with an iSCSI backend in an IPv6-only environment.
  2. Configure the `KeepConfiguration` option for each network device individually using drop-in configuration files (e.g., `/etc/systemd/network/*.network.d/*.conf`) or set the `"critical"` option to `true` in the Netplan configuration.
  3. Power off the machine and verify that the system does not hang.

  <Verify the serialization and deserialization functionality in the systemd-networkd service>
  1. Enable debugging for systemd-networkd
  mkdir -p /etc/systemd/system/systemd-networkd.service.d/
  vim /etc/systemd/system/systemd-networkd.service.d/10-debug.conf
  ===
  [Service]
  Environment=SYSTEMD_LOG_LEVEL=debug
  ===
  systemctl daemon-reload
  2. Restart the systemd-networkd service twice using the `systemctl restart systemd-networkd` command.
  3. Confirm that serialization and deserialization are functioning correctly by checking `/var/log/syslog`
  2025-03-13T00:35:01.540134+00:00 oracular systemd-networkd[2998]: Serializing...
  2025-03-13T00:35:01.540165+00:00 oracular systemd-networkd[2998]: enp0s9: Serializing DHCPv4 address (configured): 10.0.2.100/24 broadcast 10.0.2.255 (valid forever, preferred forever), flags: permanent, scope: global, label: n/a
  2025-03-13T00:35:01.540193+00:00 oracular systemd-networkd[2998]: enp0s9: Serializing DHCPv6 address (configured): 2603:c020:4016:8201:48ee:6b22:d3e4:2e8/128 (valid for 1d 56min 49s, preferred for 23h 56min 49s), flags: no-prefixroute, scope: global
  2025-03-13T00:35:01.540218+00:00 oracular systemd-networkd[2998]: Serialization completed.
  ...
  2025-03-13T00:35:01.596591+00:00 oracular systemd-networkd[3029]: Deserializing...
  2025-03-13T00:35:01.596612+00:00 oracular systemd-networkd[3029]: enp0s9: Deserialized DHCPv4 address (configured): 10.0.2.100/24 broadcast 10.0.2.255 (valid forever, preferred forever), flags: permanent, scope: global, label: n/a
  2025-03-13T00:35:01.596635+00:00 oracular systemd-networkd[3029]: enp0s9: Deserialized DHCPv6 address (configured): 2603:c020:4016:8201:48ee:6b22:d3e4:2e8/128 (valid for 1d 56min 49s, preferred for 23h 56min 49s), flags: no-prefixroute, scope: global
  2025-03-13T00:35:01.596662+00:00 oracular systemd-networkd[3029]: Deserialization completed.

  [Where problems could occur]
  The patches introduce network serialization and deserialization behavior, ensuring that DHCPv6 addresses are preserved when `KeepConfiguration=yes` is set. If any regressions occur, error messages may appear when restarting `systemd-networkd`, but they will be ignored. Both IPv4/IPv6 configuration and the IPv6 connection may be affected.

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