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

[Nick Rosbrook]

I have uploaded these changes to oracular and noble.

Confirmed with Chengen on MM that (a) my PPA builds worked as expected
according to the test plan, and (b) that we are not going to fix Jammy
or Focal. Updating the bug tasks to reflect that.

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

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

** Changed in: systemd (Ubuntu Jammy)
       Status: New => Won't Fix

** Changed in: systemd (Ubuntu Focal)
       Status: New => Won't Fix

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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:
  Won't Fix
Status in systemd source package in Jammy:
  Won't Fix
Status in systemd source package in Noble:
  In Progress
Status in systemd source package in Oracular:
  In Progress
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.

  <Verify that the IP address is removed when KeepConfiguration=no is set>
  1. Check the name of the current network configuration file:
  > ls /run/systemd/network/
  10-netplan-enp0s9.link 10-netplan-enp0s9.network
  2. Create a drop-in directory and override the KeepConfiguration setting:
  > mkdir -p /etc/systemd/network/10-netplan-enp0s9.network.d
  > vim /etc/systemd/network/10-netplan-enp0s9.network.d/10-netplan-enp0s9.network.conf
  ===
  [Network]
  KeepConfiguration=no
  ===
  3. Restart the systemd-networkd service to apply the changes:
  > systemctl restart systemd-networkd
  4. Stop the systemd-networkd service and verify the IP address is dropped:
  > systemctl stop systemd-networkd
  > ip a

  [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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