Fixing DNS resolution with a routed IPv6-only setup on Debian Bullseye

This guide outlines the steps to enable DNS resolution on a Scaleway Instance that uses the Debian Bullseye image, configured with one or more routed IPv6 addresses and without any IPv4 addresses.

Before you start

To complete the actions presented below, you must have:

• A Scaleway account logged into the console
• An Instance running Debian Bullseye (11) with an IPv6-only setup

Technical explanation

When a Scaleway Instance uses routed IP addresses, the IPv6 stack is automatically configured using SLAAC. With this method, the Instance is periodically advertised with various network configuration details, including the DNS server addresses it should use. The Instance is then free to consume these advertisements or not. By default, the operating system images provided by Scaleway are configured to leverage these advertisements to configure the IPv6 networking and the related DNS servers. The Debian Bullseye image is no exception.

When configuring the network at boot time, the cloud-init software detects the appropriate network configuration method used by the system at hand and writes and/or applies the necessary configuration files/parameters. On Debian Bullseye, and because of cloud-init’s built-in order of detection, the primary detected method is ENI, which configures the network through Debian’s well known /etc/network/interfaces set of files, along with the ifupdown toolset.

This configuration method does not interact well with SLAAC’s DNS advertisements. This results in an absence of DNS resolver configuration, thus breaking most of the network activities.

Due to its modern nature and active maintenance, netplan is a favorable option for configuring cloud systems and conveniently addresses the current issue. However, in alignment with our image building policy, which aims to minimize alterations to the official upstream cloud image, netplan is intentionally not enabled by default in Scaleway’s Debian Bullseye image.

Checking if your Instance is concerned

You can check whether your Debian Bullseye Instance is concerned by running the following command, where UUID is the identifier of your Instance:

scw -o json instance server get UUID | jq '.routed_ip_enabled and ([.public_ips[] | select(.family != "inet6")] == [])'

The command will output:

• true if the Instance is concerned by the present procedure.
• false otherwise.

Installing netplan

The netplan package must be installed before you apply this procedure, or it will fail.

Checking for netplan

To check whether your Instance has netplan installed, run the following command:

dpkg-query -W netplan.io

The command should return an output like this, where <VERSION> is the currently installed version of the package, meaning you can skip directly to the first step of the procedure:

netplan.io	<VERSION>

If the tool is not installed, the command will print the following:

dpkg-query: no packages found matching netplan.io

In this situation, proceed with the next section to install netplan before applying the procedure.

Installing the tool

1. (optional) If, and only if, your Instance is already booted using a routed IPv6-only setup, you need to temporarily configure your DNS resolver so that it can reach the Debian repositories, in order to install netplan. The following uses Google’s DNS server:

   > /etc/resolv.conf cat <<EOF
   nameserver 2001:4860:4860::8888
   EOF
   

2. Update your packages lists:

   apt-get update
   

3. Install netplan:

   apt-get -y install netplan.io
   

Fixing the DNS resolution issue

If your Instance is affected by the DNS resolution issue, follow the procedure below to fix it.

1. Force cloud-init to set up the network configuration using netplan.

   > /etc/cloud/cloud.cfg.d/99_scw_ip6dns.cfg cat <<EOF
   system_info:
     network:
       renderers: [netplan, eni]
       activators: [netplan, eni]
   EOF
   

2. Remove any ifupdown configuration from cloud-init.

   rm -f /etc/network/interfaces.d/50-cloud-init
   

3. Optionally, port your custom ifupdown configuration, if any, to netplan.
   Tip

   Refer to the Debian NetworkConfiguration wiki and the netplan documentation if required.

4. Enable the necessary systemd units:

   systemctl enable systemd-networkd-wait-online.service systemd-resolved.service
   

5. Reboot the Instance:

   reboot
   

6. Reconnect to the Instance and ensure DNS resolution is working:

   host -W3 google.com
   

7. Optionally, it is strongly recommended that you upgrade the scaleway-ecosystem package to at least version 0.0.6-10.

   apt-get update
   apt-get -y upgrade scaleway-ecosystem
   reboot