Bug report

Attempting to set IPv6 default route using "to: ::/0" under the "routes" section in the network-config. However, cloud-init instead configures a route to "::/64" on non-netplan systems.

Steps to reproduce the problem

cloud-init yaml file to configure the network:

network:
    version: 2
    ethernets:
        ens192:
            addresses:
            - 10.0.0.2/24
            - 2001:db8:1::2/64
            routes:
            - to: 0.0.0.0/0
              via: 10.0.0.1
            - to: ::/0
              via: 2001:db8:1::1

DEBIAN TEST

cloud-init command to convert input yaml into Debian network configuration file: cloud-init devel net-convert -p net.yaml -k yaml --output-kind eni -d out -D debian --debug

/etc/network/interfaces.d/50-cloud-init:

# 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}
auto lo
iface lo inet loopback

auto ens192
iface ens192 inet static
    address 10.0.0.2/24
    post-up route add default gw 10.0.0.1 || true
    pre-down route del default gw 10.0.0.1 || true
    post-up route add -A inet6 ::/64 gw 2001:db8:1::1 || true
    pre-down route del -A inet6 ::/64 gw 2001:db8:1::1 || true

# control-alias ens192
iface ens192 inet6 static
    address 2001:db8:1::2/64

ALMALINUX TEST

cloud-init command to convert input yaml into AlmaLinux network configuration file: cloud-init devel net-convert -p net.yaml -k yaml --output-kind sysconfig -d out -D almalinux --debug

/etc/sysconfig/network-scripts/route6-ens192:

# Created by cloud-init on instance boot automatically, do not edit.
#
::/64 via 2001:db8:1::1  dev ens192

Environment details

Cloud-init version: 22.4.2
Operating System Distribution: Debian & AlmaLinux

More info

It is possible to set the default route using the gateway4 and gateway6 properties, but those are deprecated in Netplan in favour of using the "routes" section. We would like to use a consistent cloud-init network-config regardless of OS distribution (ubuntu / debian / almalinux) so that is why we are specifically trying to set the gateway using the "routes" section in these examples.

canonical / cloud-init