Ubuntu – Ubuntu Server 20.04 – Netplan configuration for dynamic IPv4 and static IPv6

TL;DR

Does anyone have a working netplan configuration that sets a dynamic IPv4 and a static IPv6 under Ubuntu Server 20.04?

What works

My provider (OVH) gave me a server with this configuration in /etc/netplan/50-cloud-init.yaml out of the box:

# 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:
        ens3:
            dhcp4: true
            match:
                macaddress: xx:yy:zz:aa:bb:cc
            mtu: 1500
            set-name: ens3

So the server is obtaining an IPv4 address through DHCP. Everything works here.

The issue

But, they also gave a static IPv6 (found from the web console management for this server) which is not present in the netplan configuration.

I tried to add it manually, using snippet like:

network:
    version: 2
    ethernets:
        ens3:
            dhcp6: false
            match:
                name: ens3
            addresses:
              - "dead:beef:404:200::cafe/128"
            gateway6: "dead:beef:404:200::1"

But nothing works: if the IPv4 works I have no IPv6, and if I get an IPv6 it revokes my IPv4 and I lose access to the server.

The try

I just tried this exact configuration in /etc/netplan/60-test-askubuntu.yaml after renamed all other files with .yaml.bak:

network:
    version: 2
    ethernets:
        ens3:
            dhcp6: false
            addresses:
              - "2001:41d0:206:cd1d::6153/128"
            gateway6: "2001:41d0:206:cd1d::1"
            dhcp4: true
            mtu: 1500

Result: the IPv4 is still working but I guess it will fail when the DHCP bail will be expired. The IPv6 address is set, but the gateway is not set so I cannot ping6 an IPv6 address:

user@server:/etc/netplan$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether fa:16:3e:23:28:1f brd ff:ff:ff:ff:ff:ff
    inet 51.210.xx.yy/32 scope global dynamic ens3
       valid_lft 86396sec preferred_lft 86396sec
    inet6 2001:41d0:206:cd1d::6153/128 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::f816:3eff:fe23:281f/64 scope link 
       valid_lft forever preferred_lft forever
user@server:/etc/netplan$ ip route
default via 51.210.8.1 dev ens3 proto dhcp src 51.210.xx.yy metric 100 
51.210.8.1 dev ens3 proto dhcp scope link src 51.210.xx.yy metric 100 
user@server:/etc/netplan$ ping google.com
PING google.com (172.217.22.142) 56(84) bytes of data.
64 bytes from par21s12-in-f14.1e100.net (172.217.22.142): icmp_seq=1 ttl=51 time=6.16 ms
64 bytes from par21s12-in-f14.1e100.net (172.217.22.142): icmp_seq=2 ttl=51 time=6.19 ms
^C
--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 6.161/6.177/6.194/0.016 ms
user@server:/etc/netplan$ ping6 google.com
ping6: connect: Network is unreachable
user@server:/etc/netplan$ ip route
default via 51.210.8.1 dev ens3 proto dhcp src 51.210.xx.yy metric 100 
51.210.8.1 dev ens3 proto dhcp scope link src 51.210.xx.yy metric 100 
user@server:/etc/netplan$ ip -6 route
::1 dev lo proto kernel metric 256 pref medium
2001:41d0:206:cd1d::6153 dev ens3 proto kernel metric 256 pref medium
fe80::/64 dev ens3 proto kernel metric 256 pref medium

Question

Does anyone have a working netplan configuration for a dynamic IPv4 and a static IPv6?

It works when I set it manually using ip addr and ip route but I want a permanent configuration using netplan.