What is ErrImagePull or ImagePullBackOff
Kubernetes pods sometimes experience issues when trying to pull container images from a container registry. If an error occurs, the pod goes into the ImagePullBackOff state.
When a Kubernetes cluster creates a new deployment, or updates an existing deployment, it typically needs to pull an image. This is done by the kubelet process on each worker node. For the kubelet to successfully pull the images, they need to be accessible from all nodes in the cluster that match the scheduling request.
ImagePullBackOff error occurs when the image path is incorrect, the network fails, or the kubelet does not succeed in authenticating with the container registry. Kubernetes initially throws the
ErrImagePull error, and then after retrying a few times, “pulls back” and schedules another download attempt. For each unsuccessful attempt, the delay increases exponentially, up to a maximum of 5 minutes.
To identify the
ImagePullBackOff error: run the
kubectl get pods command. The pod status will show the error like so:
NAME READY STATUS RESTARTS AGE my-pod-1 0/1 ImagePullBackOff 0 2m34s
We’ll provide best practices for diagnosing and resolving simple cases of these errors, but more complex cases will require advanced diagnosis and troubleshooting, which is beyond the scope of this article.
ImagePullBackOff and ErrImagePull Errors: Common Causes
|Pod specification provides the wrong repository name||Edit pod specification and provide the correct registry|
|Pod specification provides the wrong image name, or an unqualified image name||Edit pod specification and provide the correct image name|
|Pod specification provides an invalid tag, or fails to provide a tag||Edit pod specification and provide the correct tag. If the image does not have a latest tag, you must provide a valid tag|
|Container registry is not accessible||Restore network connection and allow the pod to retry pulling the image|
|The pod does not have the appropriate credentials to access the image||Add a Secret with the appropriate credentials and reference it in the pod specification|
ImagePullBackOff and ErrImagePull Errors: Diagnosis and Resolution
As mentioned, an
ImagePullBackOff is the result of repeat
ErrImagePull errors, meaning the kubelet tried to pull a container image several times and failed. This indicates a persistent problem that needs to be addressed.
Step 1: Gather information
kubectl describe pod [name] and save the content to a text file for future reference:
kubectl describe pod [name] /tmp/troubleshooting_describe_pod.txt
Step 2: Examine Events section in describe output
Check the Events section of the describe pod text file, and look for one of the following messages:
Repository ... does not exist or no pull access
Manifest ... not found
The image below shows examples of how each of these messages appears in the Events output.
Step 3: Troubleshoot
If the error is
Repository ... does not exist or no pull access:
- This means that the repository specified in the pod does not exist in the Docker registry the cluster is using
- By default, images are pulled from Docker Hub, but your cluster may be using one or more private registries
- The error may occur because the pod does not specify the correct repository name, or does not specify the correct fully qualified image name (e.g.
- To resolve it, double check the pod specification and ensure that the repository and image are specified correctly.
- If this still doesn’t work, there may be a network issue preventing access to the container registry. Look in the describe pod text file to obtain the hostname of the Kubernetes node. Log into the node and try to download the image manually.
error is Manifest ... not found:
- This means that the specific version of the requested image was not found.
- If you specified a tag, this means the tag was incorrect. Double-check that the tag in the pod specification is correct, and exists in the repository. Keep in mind that tags in the repo may have changed.
- If you did not specify a tag, check if the image has a
latesttag. Images that do not have a
latesttag will not be returned, if you do not specify a valid tag. In this case, you must specify a valid tag.
If the error is
- The issue is that the container registry, or the specific image you requested, cannot be accessed using the credentials you provided.
- To resolve this, create a Secret with the appropriate credentials, and reference the Secret in the pod specification.
- If you already have a Secret with credentials, ensure those credentials have permission to access the required image, or grant access in the container repository.
As above, please note that this procedure will only resolve the most common causes of
ImagePullBackOff. If one of the quick fixes above did not work, you’ll need to undertake a more complex, non-linear diagnosis procedure to identify which parts of the Kubernetes environment contribute to the problem and resolve them.
Also, in many cases, after this error is resolved, when you run
kubectl get pods again, you’ll discover another issue preventing the pod from functioning properly. If so, you’ll need to troubleshoot the new issue.
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