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

[Chengen Du]

Public bug reported:

[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]
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.

[Where problems could occur]
The patches ensure that DHCPv6 addresses are preserved when `KeepConfiguration=yes` is set. However, if any regressions occur, the IPv6 connection may be impacted.

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

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https://bugs.launchpad.net/bugs/2098183

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

Status in systemd package in Ubuntu:
  New

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]
  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.

  [Where problems could occur]
  The patches ensure that DHCPv6 addresses are preserved when `KeepConfiguration=yes` is set. However, if any regressions occur, the IPv6 connection may be impacted.

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