We've all been there - you're testing out a new Kubernetes deployment only to be greeted with frustrating errors like "imagepullbackoff" or "errImagePull". I have faced these two errors many times; I know the frustration of just wanting your pods to run seamlessly. 

In this article, I'll walk through some common causes for image pull failures, how to troubleshoot and fix these errors, and how to avoid them in the future.

What Causes ImagePullBackOff & ErrImagePull Errors?

The ImagePullBackOff and ErrImagePull errors are two of the most common pod failures in Kubernetes. They both mean that the pod cannot start because the container image cannot be pulled from the registry. The difference between them is that ErrImagePull is the initial error, and ImagePullBackOff is the subsequent error after Kubernetes retries to pull the image several times and fails.

Below are 5 possible causes of these errors. 

Cause 1: Network Issues Preventing Image Pull

One of the possible causes of the ImagePullBackOff or ErrImagePull errors is network issues that prevent Pods and nodes from accessing the remote container image registries. This can be due to the following:

• The registry URL is incorrect or unreachable.
• The network or firewall configuration is blocking the connection to the registry.
• The proxy settings are not configured properly.

To troubleshoot this cause, do the following:

1. Check network connectivity 

Validate that the Pods and nodes can access the remote container image registries by using the `curl` or `wget` commands. If the command returns a valid response, it means that the network connectivity is fine. But if the command returns an error or times out, it means that there is a network issue that needs to be fixed.

2. Check firewall rules

If you have a network firewall, make sure that it allows outbound access to the required ports for the registry. For instance, if your registry is Docker Hub, you must connect to port 443 for HTTPS. You can use commands like `iptables` or  `firewall-cmd` to see and change the firewall rules on your nodes. If the firewall rules are not configured properly, you need to update them to allow the connection to the registry

3. Check proxy settings

If you are pulling images through a proxy, make sure that you configure the HTTPS proxy settings on your nodes and Pods. You can use the `https_proxy` environment variable to set the proxy URL on your nodes and Pods. You can also use the `imagePullSecrets` field in your Pod spec to provide the proxy credentials to your Pods.

For example, you can create a secret named `my-proxy-secret` with the proxy credentials and then use it in your Pod spec as shown below:

  apiVersion: v1
   kind: Pod
   metadata:
     name: my-pod
   spec:
     containers:
     - name: my-container
       image: my-image:latest
       imagePullPolicy: Always
     imagePullSecrets:
     - name: my-proxy-secret

If the proxy settings are wrong, you need to update them to enable the image pull through the proxy.

Cause 2: Invalid Image Names or Tags

Another reason for these errors is the mismatch between the image names or tags used and the image names and tags in the registry. This might be because there are issues with image names or tags, like typos, mismatch with the registry or using the latest tag which might cause unexpected updates. 

To fix this problem, do the following:

1. Validate image names and tags

Check that all the image names and tags in your Pod specs are correct and match the images in the registry. Use the `kubectl get pod` and `kubectl describe pod` commands to check the image names and tags in your Pod specs. If the image name or tag is incorrect, you need to fix it in your Pod spec and redeploy your Pod.

2. Pull image manually

Try pulling the image directly from the command line interface (CLI) to verify that the image name and tag are valid and exist in the registry. You can use the docker pull or podman pull commands to pull the image from the registry. 

If the command succeeds, it means that the image name and tag are valid and exist in the registry. If the command fails, it means that the image name or tag is invalid or does not exist in the registry. You need to fix the image name or tag in your Pod spec or push the image to the registry if it does not exist.

3. Check for misspelled names or tags

Sometimes, the image name or tag can be misspelled. For example, you might have typed my-image:lates instead of my-image:latest. To avoid this, you should use descriptive and consistent image names and tags. Additionally, avoid using the latest tag, which can cause unexpected image updates and inconsistencies.

If you are not familiar with the Pod spec or the deployment config, you can refer to our article: Kubernetes Architecture Explained: Overview for DevOps Enthusiasts

Cause 3: Insufficient Storage or Disk Issues

Another potential cause of these errors is insufficient storage or disk issues on the nodes. This will prevent the image from being downloaded and stored. This occurs when:

• The node disk is full or has insufficient space for the image
• The node disk is slow or has high I/O latency
• The image is too large or has too many layers

To diagnose this as the potential root cause, you should:

1. Check available storage capacity

Pods may fail to start if there is insufficient disk space on the node to store the image. Run the df -h command to check the available storage capacity on the node. If the command shows that the disk is full or has low free space, then you have to delete unused files, images, and Pods to free space or simply add more disk space to the node.

To learn more on how to remove unused images, check our article on How To Remove Unused And Dangling Docker Images

2. Check disk I/O performance

Saturated disk I/O can cause image pull timeouts or failures, especially if the image is large or has many layers. Run the iostat command to check the disk I/O performance on the node. If the command shows that the disk I/O is high or has high latency, you need to improve the disk I/O performance by reducing the disk load, using faster disks, or optimizing the image size or layers.

Cause 4: Unauthorized Access

Unauthorized access to the registry or the image is another potential cause of this error. This can happen because: 

• The registry requires authentication and the credentials are missing or invalid
• The service account does not have permission to pull the image
• The credentials are expired or revoked

To troubleshoot potential unauthorized access problem, you should:

1. Validate image pull secret

If the registry requires authentication, you need to provide the credentials to the Pod using an image pull secret. You can run the kubectl create secret command to create an image pull secret with the credentials and then use the imagePullSecrets field in your Pod spec to reference it. For example, you can create a secret named my-secret with the credentials for Docker Hub and then use it in your Pod. 

If the image pull secret is invalid, you need to fix it by replacing it with the correct credentials and referencing it in your Pod spec.

2. Ensure the service account has pull permissions

If you are using a service account to pull the image, make sure that it has permission to do so. You can run the following commands to check the role and role binding of the service account my-service-account in the default namespace:

   # Check the role of the service account
   kubectl get role my-role -n default

   # Check the role binding of the service account
   kubectl get rolebinding my-rolebinding -n default

If the role or role binding is missing or incorrect, you need to fix it or create a new one with the correct permissions and reference it in your Pod spec. For instance, you can create a role named my-role with the permission to pull images, and a role binding named my-rolebinding that binds the role to the service account my-service-account, and then use it in your Pod spec like this:

If the credentials are expired or revoked, you need to renew them or create new ones and update the image pull secret or the service account accordingly.

Cause 5: Image Registry Issues

Another possible cause of these errors is image registry issues that prevent the image from being available or accessible. This can happen due to the following reasons:

• The registry is down or unreachable
• The registry does not have the requested image or tag
• The registry has errors or bugs that affect the image pull

When you want to fix this problem, you should:

1. Confirm registry is up and running

Check the status and availability of the registry by using the curl or wget commands to test the registry URLs from your browser or CLI. If the command returns a valid response, it means that the registry is up and running. But if the command returns an error or times out, it means that the registry is down or unreachable. You will need to contact the registry provider or administrator to resolve the issue.

2. Check the registry for requested images

Check the registry for the existence of the requested images and tags by using the registry web interface or API. For example, if you are using Docker Hub as your registry, you can use the following URL to check the image my-image:latest:

https://hub.docker.com/r/my-user/my-image/tags?page=1&ordering=last_updated

If the URL shows the image and tag, it means that the registry has the requested image and tag. If the URL does not show the image and tag, it means that the registry does not have the requested image and tag. You need to push the image and tag to the registry or fix the image name and tag in your pod spec if they are incorrect.

3. Trace the registry logs for errors

Trace the logs on the registry for any errors or bugs that might affect the image pull. You can use the registry web interface or API to access the logs, or contact the registry provider or administrator to get the logs. If the logs show any errors or bugs, you will also need to contact the registry provider or administrator to resolve them.

How to Avoid ImagePullBackOff & ErrImagePull Errors?

Following the best practices below will help you avoid these errors:

• Use descriptive and consistent image names and tags, and avoid using the latest tag.
• Use a reliable and secure registry service, such as Docker Hub, Azure Container Registry, or Amazon Elastic Container Registry, and configure the registry’s URL correctly in your Pod spec.
• Use secrets to store and provide the registry credentials to your Pods, and avoid hard-coding the credentials in your Pod spec or Dockerfile.
• Test your images locally before pushing them to the registry, and make sure they are compatible with your Kubernetes cluster version and architecture.
• Monitor your network and firewall settings, and ensure that your nodes and Pods can communicate with the registry without any issues.
• Monitor your node disk space, and ensure that you have enough space for your images and Pods.

By following these best practices, you can reduce the chances of encountering the ImagePullBackOff or ErrImagePull errors and improve the reliability and performance of your Kubernetes deployments.

Check out our Kubernetes Learning Path to master Kubernetes!

Conclusion

Always be methodical in your approach. Start with networking, then image configuration, credentials, and logs/events for clues. Nine times out of ten, the issue is one of those basics rather than something deeper in the Kubernetes API. I hope these tips help you resolve image pull errors faster so you can get back to developing awesome apps

If you have any questions or feedback, please leave a comment below. 

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