Images

A container image represents binary data that encapsulates an application and all its software dependencies. Container images are executable software bundles that can run standalone and that make very well defined assumptions about their runtime environment.

You typically create a container image of your application and push it to a registry before referring to it in a Pod.

This page provides an outline of the container image concept.

Image names

Container images are usually given a name such as pause, example/mycontainer, or kube-apiserver. Images can also include a registry hostname; for example: fictional.registry.example/imagename, and possibly a port number as well; for example: fictional.registry.example:10443/imagename.

If you don't specify a registry hostname, Kubernetes assumes that you mean the Docker public registry. You can change this behaviour by setting default image registry in container runtime configuration.

After the image name part you can add a tag or digest (in the same way you would when using with commands like docker or podman). Tags let you identify different versions of the same series of images. Digests are a unique identifier for a specific version of an image. Digests are hashes of the image's content, and are immutable. Tags can be moved to point to different images, but digests are fixed.

Image tags consist of lowercase and uppercase letters, digits, underscores (_), periods (.), and dashes (-). It can be up to 128 characters long. And must follow the next regex pattern: [a-zA-Z0-9_][a-zA-Z0-9._-]{0,127} You can read more about and find validation regex in the OCI Distribution Specification. If you don't specify a tag, Kubernetes assumes you mean the tag latest.

Image digests consists of a hash algorithm (such as sha256) and a hash value. For example: sha256:1ff6c18fbef2045af6b9c16bf034cc421a29027b800e4f9b68ae9b1cb3e9ae07 You can find more information about digests format in the OCI Image Specification.

Some image name examples that Kubernetes can use are:

• busybox - Image name only, no tag or digest. Kubernetes will use Docker public registry and latest tag. (Same as docker.io/library/busybox:latest)
• busybox:1.32.0 - Image name with tag. Kubernetes will use Docker public registry. (Same as docker.io/library/busybox:1.32.0)
• registry.k8s.io/pause:latest - Image name with a custom registry and latest tag.
• registry.k8s.io/pause:3.5 - Image name with a custom registry and non-latest tag.
• registry.k8s.io/pause@sha256:1ff6c18fbef2045af6b9c16bf034cc421a29027b800e4f9b68ae9b1cb3e9ae07 - Image name with digest.
• registry.k8s.io/pause:3.5@sha256:1ff6c18fbef2045af6b9c16bf034cc421a29027b800e4f9b68ae9b1cb3e9ae07 - Image name with tag and digest. Only digest will be used for pulling.

Updating images

When you first create a Deployment, StatefulSet, Pod, or other object that includes a Pod template, then by default the pull policy of all containers in that pod will be set to IfNotPresent if it is not explicitly specified. This policy causes the kubelet to skip pulling an image if it already exists.

Image pull policy

The imagePullPolicy for a container and the tag of the image affect when the kubelet attempts to pull (download) the specified image.

Here's a list of the values you can set for imagePullPolicy and the effects these values have:

IfNotPresent

the image is pulled only if it is not already present locally.

Always

every time the kubelet launches a container, the kubelet queries the container image registry to resolve the name to an image digest. If the kubelet has a container image with that exact digest cached locally, the kubelet uses its cached image; otherwise, the kubelet pulls the image with the resolved digest, and uses that image to launch the container.

Never

the kubelet does not try fetching the image. If the image is somehow already present locally, the kubelet attempts to start the container; otherwise, startup fails. See pre-pulled images for more details.

The caching semantics of the underlying image provider make even imagePullPolicy: Always efficient, as long as the registry is reliably accessible. Your container runtime can notice that the image layers already exist on the node so that they don't need to be downloaded again.

To make sure the Pod always uses the same version of a container image, you can specify the image's digest; replace <image-name>:<tag> with <image-name>@<digest> (for example, image@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2).

When using image tags, if the image registry were to change the code that the tag on that image represents, you might end up with a mix of Pods running the old and new code. An image digest uniquely identifies a specific version of the image, so Kubernetes runs the same code every time it starts a container with that image name and digest specified. Specifying an image by digest fixes the code that you run so that a change at the registry cannot lead to that mix of versions.

There are third-party admission controllers that mutate Pods (and pod templates) when they are created, so that the running workload is defined based on an image digest rather than a tag. That might be useful if you want to make sure that all your workload is running the same code no matter what tag changes happen at the registry.

Default image pull policy

When you (or a controller) submit a new Pod to the API server, your cluster sets the imagePullPolicy field when specific conditions are met:

• if you omit the imagePullPolicy field, and you specify the digest for the container image, the imagePullPolicy is automatically set to IfNotPresent.
• if you omit the imagePullPolicy field, and the tag for the container image is :latest, imagePullPolicy is automatically set to Always;
• if you omit the imagePullPolicy field, and you don't specify the tag for the container image, imagePullPolicy is automatically set to Always;
• if you omit the imagePullPolicy field, and you specify the tag for the container image that isn't :latest, the imagePullPolicy is automatically set to IfNotPresent.

Required image pull

If you would like to always force a pull, you can do one of the following:

• Set the imagePullPolicy of the container to Always.
• Omit the imagePullPolicy and use :latest as the tag for the image to use; Kubernetes will set the policy to Always when you submit the Pod.
• Omit the imagePullPolicy and the tag for the image to use; Kubernetes will set the policy to Always when you submit the Pod.
• Enable the AlwaysPullImages admission controller.

ImagePullBackOff

When a kubelet starts creating containers for a Pod using a container runtime, it might be possible the container is in Waiting state because of ImagePullBackOff.

The status ImagePullBackOff means that a container could not start because Kubernetes could not pull a container image (for reasons such as invalid image name, or pulling from a private registry without imagePullSecret). The BackOff part indicates that Kubernetes will keep trying to pull the image, with an increasing back-off delay.

Kubernetes raises the delay between each attempt until it reaches a compiled-in limit, which is 300 seconds (5 minutes).

Image pull per runtime class

If you enable the RuntimeClassInImageCriApi feature gate, the kubelet references container images by a tuple of (image name, runtime handler) rather than just the image name or digest. Your container runtime may adapt its behavior based on the selected runtime handler. Pulling images based on runtime class will be helpful for VM based containers like windows hyperV containers.

Serial and parallel image pulls

By default, kubelet pulls images serially. In other words, kubelet sends only one image pull request to the image service at a time. Other image pull requests have to wait until the one being processed is complete.

Nodes make image pull decisions in isolation. Even when you use serialized image pulls, two different nodes can pull the same image in parallel.

If you would like to enable parallel image pulls, you can set the field serializeImagePulls to false in the kubelet configuration. With serializeImagePulls set to false, image pull requests will be sent to the image service immediately, and multiple images will be pulled at the same time.

When enabling parallel image pulls, please make sure the image service of your container runtime can handle parallel image pulls.

The kubelet never pulls multiple images in parallel on behalf of one Pod. For example, if you have a Pod that has an init container and an application container, the image pulls for the two containers will not be parallelized. However, if you have two Pods that use different images, the kubelet pulls the images in parallel on behalf of the two different Pods, when parallel image pulls is enabled.

Maximum parallel image pulls

When serializeImagePulls is set to false, the kubelet defaults to no limit on the maximum number of images being pulled at the same time. If you would like to limit the number of parallel image pulls, you can set the field maxParallelImagePulls in kubelet configuration. With maxParallelImagePulls set to n, only n images can be pulled at the same time, and any image pull beyond n will have to wait until at least one ongoing image pull is complete.

Limiting the number parallel image pulls would prevent image pulling from consuming too much network bandwidth or disk I/O, when parallel image pulling is enabled.

You can set maxParallelImagePulls to a positive number that is greater than or equal to 1. If you set maxParallelImagePulls to be greater than or equal to 2, you must set the serializeImagePulls to false. The kubelet will fail to start with invalid maxParallelImagePulls settings.

Multi-architecture images with image indexes

As well as providing binary images, a container registry can also serve a container image index. An image index can point to multiple image manifests for architecture-specific versions of a container. The idea is that you can have a name for an image (for example: pause, example/mycontainer, kube-apiserver) and allow different systems to fetch the right binary image for the machine architecture they are using.

Kubernetes itself typically names container images with a suffix -$(ARCH). For backward compatibility, please generate the older images with suffixes. The idea is to generate say pause image which has the manifest for all the arch(es) and say pause-amd64 which is backwards compatible for older configurations or YAML files which may have hard coded the images with suffixes.

Using a private registry

Private registries may require keys to read images from them.
Credentials can be provided in several ways:

• Configuring Nodes to Authenticate to a Private Registry
  • all pods can read any configured private registries
  • requires node configuration by cluster administrator
• Kubelet Credential Provider to dynamically fetch credentials for private registries
  • kubelet can be configured to use credential provider exec plugin for the respective private registry.
• Pre-pulled Images
  • all pods can use any images cached on a node
  • requires root access to all nodes to set up
• Specifying ImagePullSecrets on a Pod
  • only pods which provide their own keys can access the private registry
• Vendor-specific or local extensions
  • if you're using a custom node configuration, you (or your cloud provider) can implement your mechanism for authenticating the node to the container registry.

These options are explained in more detail below.

Configuring nodes to authenticate to a private registry

Specific instructions for setting credentials depends on the container runtime and registry you chose to use. You should refer to your solution's documentation for the most accurate information.

For an example of configuring a private container image registry, see the Pull an Image from a Private Registry task. That example uses a private registry in Docker Hub.

Kubelet credential provider for authenticated image pulls

You can configure the kubelet to invoke a plugin binary to dynamically fetch registry credentials for a container image. This is the most robust and versatile way to fetch credentials for private registries, but also requires kubelet-level configuration to enable.

See Configure a kubelet image credential provider for more details.

Interpretation of config.json

The interpretation of config.json varies between the original Docker implementation and the Kubernetes interpretation. In Docker, the auths keys can only specify root URLs, whereas Kubernetes allows glob URLs as well as prefix-matched paths. The only limitation is that glob patterns (*) have to include the dot (.) for each subdomain. The amount of matched subdomains has to be equal to the amount of glob patterns (*.), for example:

• *.kubernetes.io will not match kubernetes.io, but abc.kubernetes.io
• *.*.kubernetes.io will not match abc.kubernetes.io, but abc.def.kubernetes.io
• prefix.*.io will match prefix.kubernetes.io
• *-good.kubernetes.io will match prefix-good.kubernetes.io

This means that a config.json like this is valid:

{
    "auths": {
        "my-registry.io/images": { "auth": "…" },
        "*.my-registry.io/images": { "auth": "…" }
    }
}

Image pull operations would now pass the credentials to the CRI container runtime for every valid pattern. For example the following container image names would match successfully:

• my-registry.io/images
• my-registry.io/images/my-image
• my-registry.io/images/another-image
• sub.my-registry.io/images/my-image

But not:

• a.sub.my-registry.io/images/my-image
• a.b.sub.my-registry.io/images/my-image

The kubelet performs image pulls sequentially for every found credential. This means, that multiple entries in config.json for different paths are possible, too:

{
    "auths": {
        "my-registry.io/images": {
            "auth": "…"
        },
        "my-registry.io/images/subpath": {
            "auth": "…"
        }
    }
}

If now a container specifies an image my-registry.io/images/subpath/my-image to be pulled, then the kubelet will try to download them from both authentication sources if one of them fails.

Pre-pulled images

By default, the kubelet tries to pull each image from the specified registry. However, if the imagePullPolicy property of the container is set to IfNotPresent or Never, then a local image is used (preferentially or exclusively, respectively).

If you want to rely on pre-pulled images as a substitute for registry authentication, you must ensure all nodes in the cluster have the same pre-pulled images.

This can be used to preload certain images for speed or as an alternative to authenticating to a private registry.

All pods will have read access to any pre-pulled images.

Specifying imagePullSecrets on a Pod

Kubernetes supports specifying container image registry keys on a Pod. imagePullSecrets must all be in the same namespace as the Pod. The referenced Secrets must be of type kubernetes.io/dockercfg or kubernetes.io/dockerconfigjson.

Creating a Secret with a Docker config

You need to know the username, registry password and client email address for authenticating to the registry, as well as its hostname. Run the following command, substituting the appropriate uppercase values:

kubectl create secret docker-registry <name> \
  --docker-server=DOCKER_REGISTRY_SERVER \
  --docker-username=DOCKER_USER \
  --docker-password=DOCKER_PASSWORD \
  --docker-email=DOCKER_EMAIL

If you already have a Docker credentials file then, rather than using the above command, you can import the credentials file as a Kubernetes Secrets.
Create a Secret based on existing Docker credentials explains how to set this up.

This is particularly useful if you are using multiple private container registries, as kubectl create secret docker-registry creates a Secret that only works with a single private registry.

Referring to an imagePullSecrets on a Pod

Now, you can create pods which reference that secret by adding an imagePullSecrets section to a Pod definition. Each item in the imagePullSecrets array can only reference a Secret in the same namespace.

For example:

cat <<EOF > pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: foo
  namespace: awesomeapps
spec:
  containers:
    - name: foo
      image: janedoe/awesomeapp:v1
  imagePullSecrets:
    - name: myregistrykey
EOF

cat <<EOF >> ./kustomization.yaml
resources:
- pod.yaml
EOF

This needs to be done for each pod that is using a private registry.

However, setting of this field can be automated by setting the imagePullSecrets in a ServiceAccount resource.

Check Add ImagePullSecrets to a Service Account for detailed instructions.

You can use this in conjunction with a per-node .docker/config.json. The credentials will be merged.

Use cases

There are a number of solutions for configuring private registries. Here are some common use cases and suggested solutions.

1. Cluster running only non-proprietary (e.g. open-source) images. No need to hide images.
   • Use public images from a public registry
     • No configuration required.
     • Some cloud providers automatically cache or mirror public images, which improves availability and reduces the time to pull images.
2. Cluster running some proprietary images which should be hidden to those outside the company, but visible to all cluster users.
   • Use a hosted private registry
     • Manual configuration may be required on the nodes that need to access to private registry
   • Or, run an internal private registry behind your firewall with open read access.
     • No Kubernetes configuration is required.
   • Use a hosted container image registry service that controls image access
     • It will work better with cluster autoscaling than manual node configuration.
   • Or, on a cluster where changing the node configuration is inconvenient, use imagePullSecrets.
3. Cluster with proprietary images, a few of which require stricter access control.
   • Ensure AlwaysPullImages admission controller is active. Otherwise, all Pods potentially have access to all images.
   • Move sensitive data into a "Secret" resource, instead of packaging it in an image.
4. A multi-tenant cluster where each tenant needs own private registry.
   • Ensure AlwaysPullImages admission controller is active. Otherwise, all Pods of all tenants potentially have access to all images.
   • Run a private registry with authorization required.
   • Generate registry credential for each tenant, put into secret, and populate secret to each tenant namespace.
   • The tenant adds that secret to imagePullSecrets of each namespace.

If you need access to multiple registries, you can create one secret for each registry.

Legacy built-in kubelet credential provider

In older versions of Kubernetes, the kubelet had a direct integration with cloud provider credentials. This gave it the ability to dynamically fetch credentials for image registries.

There were three built-in implementations of the kubelet credential provider integration: ACR (Azure Container Registry), ECR (Elastic Container Registry), and GCR (Google Container Registry).

For more information on the legacy mechanism, read the documentation for the version of Kubernetes that you are using. Kubernetes v1.26 through to v1.32 do not include the legacy mechanism, so you would need to either:

• configure a kubelet image credential provider on each node
• specify image pull credentials using imagePullSecrets and at least one Secret

What's next

• Read the OCI Image Manifest Specification.
• Learn about container image garbage collection.
• Learn more about pulling an Image from a Private Registry.