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Troubleshooting Kubernetes ImagePullBackOff

Troubleshooting Kubernetes ImagePullBackOff

Nov 10, 2021
6 min read

Troubleshooting an issue on a distributed system like Kubernetes can be challenging at times. There are just so many things that can go wrong with a distributed system. It can be even more challenging when a particular error has multiple reasons for occurring.

One such error is ImagePullBackOff. It typically shows up when the kubelet agent instructs the container runtime and can’t pull the image from the container registry for various reasons.

This article will provide an in-depth overview of possible causes for your pod entering into ImagePullBackOff state while starting your container. More importantly, you’ll learn how to troubleshoot and solve this notorious error.

What does an ImagePullBackOff error mean?

The ImagePull part of the ImagePullBackOff error primarily relates to your Kubernetes container runtime being unable to pull the image from a private or public container registry. The Backoff part indicates that Kubernetes will continuously pull the image with an increasing backoff delay. Kubernetes will keep on increasing the delay with each attempt until it reaches the limit of five minutes.

It seems like a generalized statement to say that container runtime (be it Docker, containers, etc.) fails to pull the image from the registry, but let’s try to understand the possible causes for this issue.

Here are some of the possible causes behind your pod getting stuck in the ImagePullBackOff state:

  • Image doesn’t exist.
  • Image tag or name is incorrect.
  • Image is private, and there is an authentication failure.
  • Network issue.
  • Registry name is incorrect.
  • Container registry rate limits.

How can you troubleshoot ImagePullBackOff?

Let’s try to troubleshoot each of the possible causes in that bulleted list.

Image doesn’t exist, or the name is incorrect

In most cases, the error could be either from a typo or the image was not pushed to the container registry, and you’re referring to an image that doesn’t exist. Let’s try to replicate this by creating a pod with a fake image name.

go

As you can see, the pod is stuck in an ImagePullBackOff because the image doesn’t exist and we cannot pull the image.

$ kubectl get pod

NAMEREADYSTATUSRESTARTSAGE
myapp0/1ImagePullBackOff04m59s

To understand the root cause and find more details about this error, use the kubectl describe command. The command itself gives a verbose output, so we’ll just show the parts of output that are relevant to our discussion.

In the following output under Events in the Message column, you can see the actual error message:

go

Which confirms that the image doesn’t exist.

$ kubectl describe pod myapp

........

Events:

TypeReasonAgeFromMessage
-------------------------
NormalScheduled2m54sdefault-schedulerSuccessfully assigned default/myapp to minikube
NormalPulling71s (x4 over 2m53s)kubeletPulling image "myimage/myimage:latest"
WarningFailed67s (x4 over 2m49s)kubeletFailed to pull image "myimage/myimage:latest": rpc error: code = Unknown desc = Error response from daemon: pull access denied for myimage/myimage, repository does not exist or may require 'docker login': denied: requested access to the resource is denied
WarningFailed67s (x4 over 2m49s)kubeletError: ErrImagePull
WarningFailed54s (x6 over 2m48s)kubeletError: ImagePullBackOff
NormalBackoff41s (x7 over 2m48s)kubeletBack-off pulling image "myimage/myimage:latest"

Tag doesn’t exist

There could be cases where the image tag you’re trying to pull is retired, or you entered the wrong tag name. In those cases, your pod will again get stuck in the ImagePullBackOff state, as seen in the following code snippet.

We have deliberately entered the wrong tag name, lates instead of latest, to replicate this issue.

go

kubectl get pod

NAMEREADYSTATUSRESTARTSAGE
nginx0/1ImagePullBackOff03m3s

In the following output, the message indicates that tag lates doesn’t exist for image nginx.

plaintext

Hence the image pull is unsuccessful.

$ kubectl describe pod nginx

........

Events:

TypeReasonAgeFromMessage
-------------------------
NormalScheduled26sdefault-schedulerSuccessfully assigned default/nginx to minikube
NormalBackoff20skubeletBack-off pulling image "nginx:lates"
WarningFailed20skubeletError: ImagePullBackOff
NormalPulling6s (x2 over 25s)kubeletPulling image "nginx:lates"
WarningFailed2s (x2 over 20s)kubeletFailed to pull image "nginx:lates": rpc error: code = Unknown desc = Error response from daemon: manifest for nginx:lates not found: manifest unknown: manifest unknown
WarningFailed2s (x2 over 20s)kubeletError: ErrImagePull

Private image registry and wrong credentials provided

Most enterprises typically use an internal private container registry instead of DockerHub because they don’t want to push their internal applications to someone outside their organization. Even with DockerHub or any other publicly accessible password-protected registry, you must provide proper credentials to Kubernetes using the secret to pull the image from the registry.

In the following example, we’re trying to replicate this issue by spinning up a pod that uses an image from a private registry.

go

We have neither added a secret to Kubernetes nor reference of the secret in pod definition. The pod will again get stuck in the ImagePullBackOff status and the message confirms that access is denied to pull an image from the registry:

plaintext

$ kubectl describe pod mypod

Events:

TypeReasonAgeFromMessage
-------------------------
NormalScheduled39sdefault-schedulerSuccessfully assigned default/mypod to minikube
NormalPulling20s (x2 over 37s)kubeletPulling image "docker.io/hiyou/image"
WarningFailed16s (x2 over 33s)kubeletFailed to pull image "docker.io/hiyou/image": rpc error: code = Unknown desc = Error response from daemon: pull access denied for hiyou/image, repository does not exist or may require 'docker login': denied: requested access to the resource is denied
WarningFailed16s (x2 over 33s)kubeletError: ErrImagePull
NormalBackoff3s (x2 over 32s)kubeletBack-off pulling image "docker.io/hiyou/image"
WarningFailed3s (x2 over 32s)kubeletError: ImagePullBackOff

To resolve this error, create a secret using the following kubectl command. The following kubectl command creates a secret for a private Docker registry.

go

Add your secret to your pod definition, as explained in the following snippet.

yaml

Network issue

There could be a widespread network issue on all the nodes of your Kubernetes cluster, and the container runtime will not be able to pull the image from the container registry. Let’s try to replicate that scenario.

go

$ kubectl describe pod nginx

Events:

TypeReasonAgeFromMessage
-------------------------
NormalScheduled35sdefault-schedulerSuccessfully assigned default/mypod to minikube
NormalPulling19s (x2 over 32s)kubeletPulling image "nginx:latest"
WarningFailed19s (x2 over 32s)kubeletFailed to pull image "nginx:latest": rpc error: code = Unknown desc = failed to pull and unpack image "docker.io/library/nginx:latest": failed to resolve reference "docker.io/library/nginx:latest": failed to do request: Head https://registry-1.docker.io/v2/library/nginx/manifests/latest: dial tcp: lookup registry-1.docker.io on 192.168.64.1:53: server misbehaving
WarningFailed19s (x2 over 32s)kubeletError: ErrImagePull
NormalBackoff5s (x2 over 32s)kubeletBack-off pulling image "nginx:latest"
WarningFailed5s (x2 over 32s)kubeletError: ImagePullBackOff

In the preceding output, the message indicates that there is a network issue.

go

Container registry rate limits

Most container registries have implemented some rate limits (i.e., number of images you can pull) to protect their infrastructure. For example, with Docker Hub, anonymous and free Docker Hub users can only request 100 and 200 container image pull requests per six hours. If you exceed your maximum download limit, you’ll be blocked, resulting in ImagePullBackOff error.

To resolve this for Docker Hub, you would need to upgrade to a Pro or Team account. Many other popular container image registries like GCR or ECR propose similar limitations.

Final thoughts

In this article, you learned some possible reasons why a pod would get stuck in an ImagePullBackOff state. You checked out some different examples to understand the error better and troubleshoot it with commands like kubectl describe. If you’re confident there is no typo in the image, registry, or tag name, then kubectl describe will reveal the chain of events that led to the failure. In some cases, you may be able to pull the image using docker pull, but your cluster can’t, then that probably means there’s a network issue.

To streamline your troubleshooting process by building custom workflows and UIs that help monitor and debug issues, consider using Airplane. Airplane is the developer platform for building custom internal tools.

The basic building blocks of Airplane are Tasks, which are single or multi-step functions that anyone on your team can use. Airplane also offers engineering workflows for use cases that are engineering-centric, such as database backups and incident monitoring. If you're looking to build a dashboard to monitor errors, you can use Airplane Views, which allows you to build custom UIs within minutes.

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Karl Hughes
Karl is the Founder & CEO of Draft where he works to create engaging and thoughtful content for software engineers.

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