v1 network render to netplan does not render IPv6 global nameserver address

[Chad Smith]

cloud-init v 22.2:

Example config with subnet `type: ipv6_slaac` doesn't render any nameserver information in config /etc/netplan/50-cloud-init.yaml when global nameserver is provided.

Steps to reproduce:

cat > network-config <<EOF
version: 1
config:
    - type: physical
      name: eth0
      mac_address: '06:70:c5:13:04:1e'
      subnets:
      - type: ipv6_slaac
    - type: nameserver
      address:
      - '2a03:7900:2:0:31:3:104:161'
      search:
      - 'demo.de'
EOF
PYTHONPATH=. python3 -m cloudinit.cmd.main devel net-convert --network-data=network-config --kind=yaml -D ubuntu -O netplan -d outputd

# ACTUAL OUTPUT - lacks nameserver
$ cat outputd/etc/netplan/50-cloud-init.yaml
# This file is generated from information provided by the datasource. Changes
# to it will not persist across an instance reboot. To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        eth0:
            dhcp6: true
            match:
                macaddress: 06:70:c5:13:04:1e
            set-name: eth0

It may be that this isn't a bug because ipv6_slaac doesn't provide DNS information and requires that the a separate https://www.networkacademy.io/ccna/ipv6/stateless-dhcpv6 server is running on the network backplane that the node can separately contact to get DNS info.

Currently cloud-init netplan renderer only applies global nameserfver information to subnets which contain an "addresses" config directive an no nameserver" directive.
https://github.com/canonical/cloud-init/blob/main/cloudinit/net/netplan.py#L421-L422

[Chad Smith (chad.smith)]

For this particular issue: I believe adding a dns_nameservers: ['2a03:7900:2:0:31:3:104:161'] to eth0's subnets definition will get you want you overriding default behavior.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3997