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 <terminal inline>ImagePullBackOff<terminal inline>. 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 <terminal inline>ImagePullBackOff<terminal inline> 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 <terminal inline>ImagePull<terminal inline> part of the <terminal inline>ImagePullBackOff<terminal inline> error primarily relates to your Kubernetes container runtime being unable to pull the image from a private or public container registry. The <terminal inline>Backoff<terminal inline> 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, containerd, 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 <terminal inline>ImagePullBackOff<terminal inline> 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 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.
As you can see, the pod is stuck in an <terminal inline>ImagePullBackOff<terminal inline> because the image doesn’t exist and we cannot pull the image.
To understand the root cause and find more details about this error, use the <terminal inline>kubectl describe<terminal inline> 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:
Which confirms that the image doesn’t exist.
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 <terminal inline>ImagePullBackOff<terminal inline> state, as seen in the following code snippet.
We have deliberately entered the wrong tag name, <terminal inline>lates<terminal inline> instead of <terminal inline>latest<terminal inline>, to replicate this issue.
In the following output, the message indicates that tag <terminal inline>lates<terminal inline> doesn’t exist for image <terminal inline>nginx<terminal inline>.
Hence the image pull is unsuccessful.
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.
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:
To resolve this error, create a secret using the following <terminal inline>kubectl<terminal inline> command. The following <terminal inline>kubectl<terminal inline> command creates a secret for a private Docker registry.
Add your secret to your pod definition, as explained in the following snippet.
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.
In the preceding output, the message indicates that there is a network issue.
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 <terminal inline>ImagePullBackOff<terminal inline> error.
Monitor ImagePullBackoffs With ContainIQ
Using ContainIQ, you can monitor, track, and alert on`ImagePullBackoff` events.
ContainIQ, a tool for monitoring Kubernetes clusters, allows users to view and graph `ImagePullBackoff` events over time. Users can also track the events leading up to the backoff, like `ErrImagePull`, to get alerted as the image pull fails but before the backoff event fires.
ContainIQ provides tooling to set alerts on `ImagePullBackoff` events by pod and get notified in Slack when they occur. Users can also use the filtering features to view other related Warning events as they happen or during a specific period of time.
For example, a ContainIQ user could be alerted if a pod’s state becomes `ImagePullBackoff` because the image is private, and there is an authentication failure, or if the registry / tag name is incorrect. On the other hand, users can also view and graph Normal events such as an image pulling successfully. With this information you could see how often certain images are being pulled and which applications are being deployed the most frequently.
Using the New Monitor button, users can set alerts on `ImagePullBackoff events` for specific pods, or across all pods. Alerts can be toggled on and off with one click from the Monitors tab. A user can also alert on other events like job failures, pod evictions, or health check failures.
In this article, you learned some possible reasons why a pod would get stuck in an <terminal inline>ImagePullBackOff<terminal inline> state. You checked out some different examples to understand the error better and troubleshoot it with commands like <terminal inline>kubectl describe<terminal inline>.
If you’re confident there is no typo in the image, registry, or tag name, then <terminal inline>kubectl describe<terminal inline> will reveal the chain of events that led to the failure. In some cases, you may be able to pull the image using <terminal inline>docker pull<terminal inline>, but your cluster can’t, then that probably means there’s a network issue.