Updating from CoreOS Container Linux

If you already have CoreOS Container Linux clusters and can’t or don’t want to freshly install Flatcar Container Linux, you can update to Flatcar Container Linux directly from CoreOS Container Linux by performing the following steps.

NOTE: General differences when migrating from CoreOS Container Linux also apply.

At the end of the section you can find the update-to-flatcar.sh script that does all steps for you.

Getting the public update key

First, you need to get Flatcar’s public update key:

curl -L -o /tmp/key https://raw.githubusercontent.com/flatcar-linux/coreos-overlay/flatcar-master/coreos-base/coreos-au-key/files/official-v2.pub.pem

Since the /usr partition is read-only, to allow the updater to use the new key, you need to bind-mount it:

sudo mount --bind /tmp/key /usr/share/update_engine/update-payload-key.pub.pem

Modifying the configuration files

Migrate your /etc/coreos/update.conf settings but keep a symlink:

sudo mv /etc/coreos /etc/flatcar
sudo ln -s flatcar /etc/coreos

Now, you need to point update_engine to Flatcar’s update server by setting the SERVER configuration option in /etc/flatcar/update.conf:


To make sure you get an update even if you’re running the same CoreOS Container Linux version as the latest Flatcar Container Linux, you need to force an update by clearing the current version number from the release file. This file also lives in the /usr partition so you need to do a bind-mount again:

cp /usr/share/coreos/release /tmp
sudo mount --bind /tmp/release /usr/share/coreos/release

Then, you need to edit /usr/share/coreos/release and replace the value of COREOS_RELEASE_VERSION with 0.0.0:


Migrate any cloud-config user_data if it exists in /var/lib/coreos-install/user_data (e.g., in bare metal installations):

test -d /var/lib/coreos-install && sudo ln -sn /var/lib/coreos-install /var/lib/flatcar-install

Restart service and reboot

After that, restart the update service so it rescans the edited configuration. Initiate an immediate update. This takes some time. Afterwards remove the SERVER parameter from the update.conf file because it is already specified in the /usr/share/flatcar/update.conf file on the new partititon. Usually 5 minutes after the update finished, the system will reboot into Flatcar Container Linux, but you can also reboot manually:

sudo systemctl restart update-engine
sudo update_engine_client -update
sudo sed -i "/SERVER=.*/d" /etc/flatcar/update.conf
sudo systemctl reboot

All steps in one script

The update-to-flatcar.sh script does all required steps mentioned above for you:

# To be run on the node via SSH
[email protected] ~ $ wget https://raw.githubusercontent.com/kinvolk/flatcar-docs/main/update-to-flatcar.sh
[email protected] ~ $ less update-to-flatcar.sh # Double check the content of the script
[email protected] ~ $ chmod +x update-to-flatcar.sh
[email protected] ~ $ ./update-to-flatcar.sh
Done, please reboot now
[email protected] ~ $ sudo systemctl reboot

Before you reboot, check that you migrated the variable names as written in Migrating from CoreOS Container Linux .

Going back to CoreOS Container Linux

You can also go the other way.

Manual rollback

If you just updated to Flatcar (and haven’t done any additional updates), CoreOS Container Linux will still be on your disk, you just need to roll back to the other partition.

To do that, just use this command composition:

sudo cgpt prioritize "$(sudo cgpt find -t flatcar-usr | grep --invert-match "$(rootdev -s /usr)")"

Now you can reboot and you’ll be back to CoreOS Container Linux. Remember to undo your changes in your /etc/coreos/update.conf after rolling back if you want to keep getting CoreOS Container Linux updates.

For more information about manual rollbacks, check Performing a manual rollback .

Force an update to CoreOS Container Linux

This procedure is similar to updating from CoreOS Container Linux to Flatcar Container Linux. You need to get CoreOS Container Linux’s public key, point update_engine to CoreOS Container Linux’s update server, and force an update.

Get CoreOS Container Linux’s public key:

curl -L -o /tmp/key https://raw.githubusercontent.com/coreos/coreos-overlay/master/coreos-base/coreos-au-key/files/official-v2.pub.pem

Bind-mount it:

sudo mount --bind /tmp/key /usr/share/update_engine/update-payload-key.pub.pem

Create an /etc/flatcar directory and copy the current update configuration:

sudo mkdir -p /etc/flatcar
sudo cp /etc/coreos/update.conf /etc/flatcar/

Change the SERVER field in /etc/flatcar/update.conf:


Bind-mount the release file:

cp /usr/share/flatcar/release /tmp
sudo mount --bind /tmp/release /usr/share/flatcar/release

Edit FLATCAR_RELEASE_VERSION to force an update:


After that, restart the update service so it rescans the edited configuration and initiates an update. The system will reboot into CoreOS Container Linux:

sudo systemctl restart update-engine
sudo update_engine_client -update