Mike van Riel

Resolving ImagePullBackOff with Registry Authentication Issues

—

by

Mike van Riel
in

Encountering an ImagePullBackOff error in Kubernetes can be a frustrating experience, especially when it’s due to a 401 error from a Docker Registry. This guide will walk you through the steps to diagnose and resolve this issue by updating your registry credentials.

Diagnosing the ImagePullBackOff Error

When a pod fails to pull an image, it often results in an ImagePullBackOff error. To confirm this and get more details, you can inspect the pod using the describe command:

kubectl describe pod <pod_name>

Look for events indicating a 401 error while pulling from Docker Registry. This typically means there’s an authentication issue with the credentials provided in the imagePullSecrets.

Inspecting and Decoding the ImagePullSecrets

The next step is to inspect the secret referenced by the imagePullSecrets variable in the associated Deployment. This secret contains the credentials Kubernetes uses to pull images from the registry. Extract and decode it with the following command:

kubectl -n redirector get secret gitlab-registry -o jsonpath='{.data.\.dockerconfigjson}' | base64 --decode

This will output the docker registry’s config content, which you can then review for accuracy.

Manual Testing using Docker Login

Using the credentials from the decoded secret, attempt to log in to -for example- the GitLab registry manually. This helps confirm whether the credentials are valid:

docker login registry.gitlab.com

If the login fails, it’s clear that there’s an issue with the credentials.

Generating a New Personal Access Token

To resolve the credential issue, you’ll need to generate a new Personal Access Token in GitLab:

  1. Go to your GitLab account settings.
  2. Navigate to Access Tokens.
  3. Create a new token with the read_registry scope for registry access.

It is recommended to put an expiration date on the token and make it as narrow as possible using just the read_registry scope, doing this will benefit the security of your system.

Updating and Encoding the Docker Configuration

Once you have the new token, replace the old credentials in the .dockerconfigjson with the new token. Then, base64 encode the updated .dockerconfigjson:

echo -n '{"auths":{"registry.gitlab.com":{"username":"<username associated with access token>","password":"<access token>"}}}' | base64

Updating the Kubernetes Secret

Now, update the Kubernetes secret with the acquired access token. This ensures Kubernetes uses the updated credentials for pulling images:

kubectl edit secret <secret-name>

Once opened, this allows you to set the .dockerconfigjson in the data fields of the secret that we acquired above.

Restarting the Pod

Finally, delete the pod to let Kubernetes recreate it and retry pulling the image with the new credentials:

kubectl delete pod <pod_name>

Normally this step is superfluous as Kubernetes has already picked up on the change and retried the pod before you get the chance to perform the step above.

Conclusion

By following these steps, you can resolve the ImagePullBackOff error caused by a 401 error from a Docker Registry. Kubernetes should now be able to pull the image successfully, allowing your pod to start without issues.

If you want to know more on how you can pull containers from a private registry using kubernetes, you can also check the official support page at https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/.