Within a Kubernetes environment, there are a huge amount of possible errors that your system could return, with any number of them appearing in your system at the same time. While many of these are serious and require immediate attention, others fall into a more informal category. One of these more accidental errors is ImagePullBackOff, which is caused by not getting a specific image source on a node.
Typically, on every node within a Kubernetes cluster, there is a kubelect that runs all the individual containers within that node. This kubelect becomes the point of communication between the containers, the node, and the central Kubernetes API, creating a stream that allows messaging to run smoothly.
When attempting to fetch an image, if the image is not already on the node itself, the central messaging system, kubelet, will instruct something called container runtime to locate and deploy it. When this image cannot be retrieved, kubelet returns an ImagePullBackOff error.
Whenever you get this error, you know that there is a mismatch between the specific image permissions and access a node has and the tasks you have given it. Finding the cause of this blockage and fixing it will fix this error and allow your system to continue as efficiently as possible.
What causes an ImagePullBackOff error within Kubernetes?
Within Kubernetes, the ImagePullBackOff error may occur due to various specific causes. Since the error is directly related to image errors, it will throw an error if there is a mismatch between what your nodes can access and the images themselves. Likewise, since you know that this error is specifically related to an image, you already have a big piece of the troubleshooting puzzle.
There are three common causes of this error, each of which you can solve in turn to find out how to fix your problem:
- Your references do not match
- You have no permissions
- Your registry blocked your access
Let’s break these down further.
Your references do not match
With potentially thousands of different image sources within your Kubernetes environment, chances are one of the images is slipping through your net. When you create an image, you must tag it that does not currently exist. While many people tend to do this sequentially, starting with their first image as image1, and the next as image2, this doesn’t always protect you.
Sometimes developers forget to add the correct title or tag to the image, which means it cannot be found with the name you intended. When a pod tries to retrieve a specific image with a particular name, but can’t find it, it returns the ImagePullBackOff error because it simply cannot access the image, because that filename doesn’t exist.
You may also have made a minor typo when configuring the title of the image the pod is trying to pull. Instead of writing Image2829, you could have written 2992, with this simple mix of a letter that moves the image to a completely different file. While the pod may be able to fetch that file, at best it won’t be the image you wanted, and at worst you’ll receive the ImagePullBackOff error.
Whenever one of your pods returns this error, you should always first check that you have connected that pod to an image that:
- Really exists within your ecosystem
- Is titled as you think, with close attention to the exact format of the title or tag
If any of these things aren’t right, all you need to do is make a minor change to get your system back on track.
Your registry blocked your access
Within Kubernetes are many companies will often partner with Docker Hub to retrieve a range of different image attributes. While this usually works to get you started, Docker Hub has two plans, one free and one Pro, for businesses to select. With the free plan, when connecting your Kubernetes ecosystem, you get up to 200 container image requests every 6 hours.
However, if you exceed those 200 container image requests, you will need to upgrade to the next tier. If you don’t upgrade, Docker Hub will block your request, causing you to receive an ImagePullBackOff error.
Fortunately, this is a very easy mistake to check as all you need to do is check the current permissions your Docker Hub account has. Once you’ve reached the limit, you’ll need to upgrade your account to the next level.
You have no permissions
When companies create their own Kubernetes environment, they often like to use custom images that belong only to their company. With this, they create a so-called private container image registry within their ecosystem. While companies can just publish their files on Docker Hub, that would mean making them public, meaning other companies can look at them.
Instead of doing this, they use a private container image registry, which means you need to give your Kubernetes nodes permission to access the registry to find the specific images they’re looking for.
Again, this is a pretty simple bug to fix, where users just need to create a so-called “secret” in the node. This file gives the node permission to fetch from the private registry, which then allows it to bypass the previous block and retrieve the desired image.
It is difficult to go through an entire development cycle within a Kubernetes environment without making a small overview here and there. Usually, as with the ImagePullBackOff error, this isn’t a big deal. Rather than panicking, the solution to your problem will most likely be one of the three above.