[Bug 2093164] [NEW] initramfs-tools generates deprecated netplan config gateway4 and gateway6

[Launchpad Bug Tracker]

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[Impact]

According to netplan doc
https://netplan.readthedocs.io/en/latest/netplan-yaml

gateway4/gateway6 are deprecated and the first version deprecates these fields is 0.103,
it should use "routes:" instead.

In scripts/functions:_render_netplan, it still generates the deprecated
fields in /run/netplan/xxx.yaml:

	if [ -n "$gateway4" ]; then
		echo "      gateway4: \"$gateway4\""
	fi
	if [ -n "$gateway6" ]; then
		echo "      gateway6: \"$gateway6\""
	fi

Then casper copies this config to root filesystem and later on used by cloud-init:
https://git.launchpad.net/ubuntu/+source/casper/tree/scripts/casper-bottom/58server_network?h=applied/ubuntu/noble-updates#n47

In this case, cloud-init returns 2 (success with recoverable errors)
rather than '0' (success)

We should modify these fields to use "routes:" based on netplan doc:
https://netplan.readthedocs.io/en/latest/netplan-yaml/#default-routes


[Test Case]

To verify if it generates the correct netplan config, we can specify ip
related config in kernel cmdline in grub menu and check the yaml file in
/run/netplan/

Steps to reproduce the netplan config:

1. launch a test VM in lxd
2. go into grub menu
3. pass ip related info to kernel cmdline, e.g.
ip=10.194.146.91::10.194.146.1:255.255.255.0::::10.194.146.1
4. boot into system and check /run/netplan/xxx.yaml

Originally, it generates something like:
cat /run/netplan/enp5s0.yaml 
network:
  version: 2
  ethernets:
    enp5s0:
      addresses:
        - "10.194.146.91/24"
      gateway4: "10.194.146.1"
      nameservers:
        addresses: ["10.194.146.1"]

After the patch, it generates something like:
cat /run/netplan/enp5s0.yaml 
network:
  version: 2
  ethernets:
    enp5s0:
      addresses:
        - "10.194.146.91/24"
      routes:
        - to: default
          via: "10.194.146.1"
      nameservers:
        addresses: ["10.194.146.1"]


[Where problems could occur]

This patch follows the netplan doc to use "routes:" for default gateway,
if the format is correct, it shouldn't cause any issue. 

[Other Info]
https://bugs.launchpad.net/cloud-init/+bug/1992512

** Affects: initramfs-tools (Ubuntu)
     Importance: Undecided
     Assignee: gerald.yang (gerald-yang-tw)
         Status: In Progress

** Affects: initramfs-tools (Ubuntu Focal)
     Importance: Undecided
     Assignee: gerald.yang (gerald-yang-tw)
         Status: In Progress

** Affects: initramfs-tools (Ubuntu Jammy)
     Importance: Undecided
     Assignee: gerald.yang (gerald-yang-tw)
         Status: In Progress

** Affects: initramfs-tools (Ubuntu Noble)
     Importance: Undecided
     Assignee: gerald.yang (gerald-yang-tw)
         Status: In Progress

** Affects: initramfs-tools (Ubuntu Oracular)
     Importance: Undecided
     Assignee: gerald.yang (gerald-yang-tw)
         Status: In Progress

** Affects: initramfs-tools (Ubuntu Plucky)
     Importance: Undecided
     Assignee: gerald.yang (gerald-yang-tw)
         Status: In Progress


** Tags: patch
-- 
initramfs-tools generates deprecated netplan config gateway4 and gateway6
https://bugs.launchpad.net/bugs/2093164
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