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

[ubuntu-server-builder]

This bug was originally filed in Launchpad as LP: #1980877

Launchpad details

affected_projects = []
assignee = None
assignee_name = None
date_closed = None
date_created = 2022-07-06T20:46:34.682710+00:00
date_fix_committed = None
date_fix_released = None
id = 1980877
importance = low
is_complete = False
lp_url = https://bugs.launchpad.net/cloud-init/+bug/1980877
milestone = None
owner = chad.smith
owner_name = Chad Smith
private = False
status = triaged
submitter = chad.smith
submitter_name = Chad Smith
tags = []
duplicates = []

Launchpad user Chad Smith(chad.smith) wrote on 2022-07-06T20:46:34.682710+00:00

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

[ubuntu-server-builder]

Launchpad user Chad Smith(chad.smith) wrote on 2022-07-06T20:59:36.903153+00:00

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.