DevOps Diaries

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IN TODAY'S EDITION

Use Case

• ImagePullBackOff error Troubleshoot and Fix

🚀 Top News

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👀 Remote Jobs

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📚️ Resources

12 Best Microsoft Excel Courses for Beginners

USE CASE

ImagePullBackOff error Troubleshoot and Fix

ImagePullBackOff is a common Kubernetes error which occurs when cluster is unable to pull a container image required by one of your Pods. This error message can be confusing, but you can debug it by following a methodical troubleshooting process.

ImagePullBackOff means there was a problem pulling a container image required by your Pod. Kubernetes “backs off” before retrying, adding a delay that’s intended to provide time for any temporary issues to resolve themselves.

The error is generated by the Kubelet process running on the Kubernetes Node that hosts your Pod. Nodes are responsible for pulling and storing the container images their Pods require. When a Pod is scheduled to a Node, the Node first has to pull the image for each of the Pod’s containers, as defined by the spec.containers[].image fields in your Pod’s manifest. If an image can’t be pulled, then an ImagePullBackOff error will occur.

ImagePullBackOff can appear even when the Node has already stored an image matching the requested tag. Whether this will happen depends on the imagePullPolicy assigned to the Pod. When the Always policy is used, then Kubelet will try to pull the image regardless of an existing version being available. This ensures the latest image version is used, but will trigger an ImagePullBackOff error if the image can’t be pulled.

Causes of ImagePullBackOff errors

ImagePullBackOff occurs whenever Kubernetes is unable to fetch an image it needs. Unfortunately, Kubernetes can’t always tell you the exact cause. There are several possible reasons, ranging from user error to network conditions:

1. Invalid image name

Accidentally referencing the wrong image, or inserting a typo, will cause your Pod to try to pull an image that doesn’t exist. This is an easy mistake to make, and it could occur in any part of the image name—either the repository (example.com), image name (my-app), or tag (latest).

2. Image removed from the registry 

Sometimes, you might believe you’re using the correct image name, but the image has, in fact, been removed from the registry. Kubernetes can’t pull something that doesn’t exist.

3. Missing credentials for the image registry

You need to supply access credentials when you’re using images from a private registry. Missing credentials will lead to access denied errors that prevent the image being pulled.

4. Hitting registry rate limits 

Image pulls can be blocked when the registry implements rate limiting and you’ve used up your request quota.

Notably, Docker Hub significantly reduced its rate limit quotas in 2020; active Kubernetes clusters that pull many Docker Hub images without authenticating may hit the cap of 100 requests per six hours.

5. Registry inaccessible

ImagePullBackOff can point to the registry being inaccessible, such as when it’s offline for maintenance or your cluster’s network connectivity is degraded.

You’ll notice that none of these situations are particularly complicated. ImagePullBackOff is one of the simplest Kubernetes problems to resolve, once you’ve worked out which root cause you’re facing.

during an ImagePullBackOff…

When a Pod enters the ImagePullBackOff status, Kubernetes will periodically retry the image pull after an exponentially increasing delay. First, it will wait 5 seconds, then 10 seconds, then 20 seconds, and so on, up to a maximum of 5 minutes between attempts.

The increasing delay is designed to give transient issues such as flaky network connectivity a chance to resolve themselves.

• If the error is being caused by one of these temporary problems, then the image will eventually be pulled successfully and the Pod startup procedure will continue.

• if the error is actually due to a permanent issue—such as a typo in the image’s name–then the Pod will be stuck displaying ImagePullBackOff forever, until you take action to resolve the error.

Fixing ImagePullBackOff Error

Steps to resolve ImagPullBackOff error: (Common Approach)

Let’s demonstrate how to troubleshoot an ImagePullBackOff error

1. create a pod.yaml file with some type error in the image name. like

The spec.containers.image field intentionally contains a typo. Kubernetes will try to pull a non-existent image, which will lead to an ImagePullBackOff error.

2. Apply the manifest to the K8s Cluster: using

   Kubectl apply -f pod.yaml

3. Run get pods command:

   kubectl get pods

4. Inspect the pod with command:

   kubectl describe pod podname

To inspect why this has happened, you can use the describe pod command to view the Pod’s event history. The events are displayed in a table at the bottom of the command’s output:

The event history explains what happened to the Pod. The ErrImagePull status is assigned, then a retry occurs. This also fails, and Kubernetes moves the Pod to the ImagePullBackOff status.

Resolutions:

Other issue reesolution may be from one of the given list:

✅ Double-check the Image Name: Ensure the name and tag are accurate.
✅ Authenticate Properly: Set up secrets for accessing private registries.
✅ Check Network Settings: Verify DNS configurations and network policies.
✅ Review Node Resources: Confirm the node has sufficient resources.
✅ Use ImagePullPolicy: Set it to IfNotPresent or Always based on your requirements.

🛡️ Preventive Tips:

We can follow these steps so that can prevent our cluster to keep out from this error.

1️⃣ Always test image names and tags locally before deploying.
2️⃣ Use versioned tags rather than latest to ensure consistency.
3️⃣ Monitor your cluster for resource utilization.
4️⃣ Keep registry credentials updated and securely stored.
5️⃣ Use Kubernetes tools like kubectl describe pod to quickly identify issues.