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Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Images

You create your Docker image and push it to a registry before referring to it in a Kubernetes pod.

The image property of a container supports the same syntax as the docker command does, including private registries and tags.

Updating Images

The default pull policy is IfNotPresent which causes the Kubelet to not pull an image if it already exists. If you would like to always force a pull you must set a pull image policy of Always or specify a :latest tag on your image.

If you did not specify tag of your image, it will be assumed as :latest, with pull image policy of Always correspondingly.

Note that you should avoid using :latest tag, see Best Practices for Configuration for more information.

Using a Private Registry

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

Using Google Container Registry

Kubernetes has native support for the Google Container Registry (GCR), when running on Google Compute Engine (GCE). If you are running your cluster on GCE or Google Container Engine (GKE), simply use the full image name (e.g. gcr.io/my_project/image:tag).

All pods in a cluster will have read access to images in this registry.

The kubelet will authenticate to GCR using the instance’s Google service account. The service account on the instance will have a https://www.googleapis.com/auth/devstorage.read_only, so it can pull from the project’s GCR, but not push.

Using AWS EC2 Container Registry

Kubernetes has native support for the AWS EC2 Container Registry, when nodes are AWS EC2 instances.

Simply use the full image name (e.g. ACCOUNT.dkr.ecr.REGION.amazonaws.com/imagename:tag) in the Pod definition.

All users of the cluster who can create pods will be able to run pods that use any of the images in the ECR registry.

The kubelet will fetch and periodically refresh ECR credentials. It needs the following permissions to do this:

Requirements:

Troubleshooting:

Using Azure Container Registry (ACR)

When using Azure Container Registry you can authenticate using either an admin user or a service principal. In either case, authentication is done via standard Docker authentication. These instructions assume the azure-cli command line tool.

You first need to create a registry and generate credentials, complete documentation for this can be found in the Azure container registry documentation.

Once you have created your container registry, you will use the following credentials to login: * DOCKER_USER : service principal, or admin username * DOCKER_PASSWORD: service principal password, or admin user password * DOCKER_REGISTRY_SERVER: ${some-registry-name}.azurecr.io * DOCKER_EMAIL: ${some-email-address}

Once you have those variables filled in you can configure a Kubernetes Secret and use it to deploy a Pod.

Configuring Nodes to Authenticate to a Private Repository

Note: if you are running on Google Container Engine (GKE), there will already be a .dockercfg on each node with credentials for Google Container Registry. You cannot use this approach.

Note: if you are running on AWS EC2 and are using the EC2 Container Registry (ECR), the kubelet on each node will manage and update the ECR login credentials. You cannot use this approach.

Note: this approach is suitable if you can control node configuration. It will not work reliably on GCE, and any other cloud provider that does automatic node replacement.

Docker stores keys for private registries in the $HOME/.dockercfg or $HOME/.docker/config.json file. If you put this in the $HOME of user root on a kubelet, then docker will use it.

Here are the recommended steps to configuring your nodes to use a private registry. In this example, run these on your desktop/laptop:

  1. Run docker login [server] for each set of credentials you want to use. This updates $HOME/.docker/config.json.
  2. View $HOME/.docker/config.json in an editor to ensure it contains just the credentials you want to use.
  3. Get a list of your nodes, for example:
    • if you want the names: nodes=$(kubectl get nodes -o jsonpath='{range.items[*].metadata}{.name} {end}')
    • if you want to get the IPs: nodes=$(kubectl get nodes -o jsonpath='{range .items[*].status.addresses[?(@.type=="ExternalIP")]}{.address} {end}')
  4. Copy your local .docker/config.json to the home directory of root on each node.
    • for example: for n in $nodes; do scp ~/.docker/config.json root@$n:/root/.docker/config.json; done

Verify by creating a pod that uses a private image, e.g.:

$ cat <<EOF > /tmp/private-image-test-1.yaml
apiVersion: v1
kind: Pod
metadata:
  name: private-image-test-1
spec:
  containers:
    - name: uses-private-image
      image: $PRIVATE_IMAGE_NAME
      imagePullPolicy: Always
      command: [ "echo", "SUCCESS" ]
EOF
$ kubectl create -f /tmp/private-image-test-1.yaml
pod "private-image-test-1" created
$

If everything is working, then, after a few moments, you should see:

$ kubectl logs private-image-test-1
SUCCESS

If it failed, then you will see:

$ kubectl describe pods/private-image-test-1 | grep "Failed"
  Fri, 26 Jun 2015 15:36:13 -0700    Fri, 26 Jun 2015 15:39:13 -0700    19    {kubelet node-i2hq}    spec.containers{uses-private-image}    failed        Failed to pull image "user/privaterepo:v1": Error: image user/privaterepo:v1 not found

You must ensure all nodes in the cluster have the same .docker/config.json. Otherwise, pods will run on some nodes and fail to run on others. For example, if you use node autoscaling, then each instance template needs to include the .docker/config.json or mount a drive that contains it.

All pods will have read access to images in any private registry once private registry keys are added to the .docker/config.json.

This was tested with a private docker repository as of 26 June with Kubernetes version v0.19.3. It should also work for a private registry such as quay.io, but that has not been tested.

Pre-pulling Images

Note: if you are running on Google Container Engine (GKE), there will already be a .dockercfg on each node with credentials for Google Container Registry. You cannot use this approach.

Note: this approach is suitable if you can control node configuration. It will not work reliably on GCE, and any other cloud provider that does automatic node replacement.

By default, the kubelet will try 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

Note: This approach is currently the recommended approach for GKE, GCE, and any cloud-providers where node creation is automated.

Kubernetes supports specifying registry keys on a pod.

Creating a Secret with a Docker Config

Run the following command, substituting the appropriate uppercase values:

$ kubectl create secret docker-registry myregistrykey --docker-server=DOCKER_REGISTRY_SERVER --docker-username=DOCKER_USER --docker-password=DOCKER_PASSWORD --docker-email=DOCKER_EMAIL
secret "myregistrykey" created.

If you need access to multiple registries, you can create one secret for each registry. Kubelet will merge any imagePullSecrets into a single virtual .docker/config.json when pulling images for your Pods.

Pods can only reference image pull secrets in their own namespace, so this process needs to be done one time per namespace.

Bypassing kubectl create secrets

If for some reason you need multiple items in a single .docker/config.json or need control not given by the above command, then you can create a secret using json or yaml.

Be sure to:

Example:

apiVersion: v1
kind: Secret
metadata:
  name: myregistrykey
  namespace: awesomeapps
data:
  .dockerconfigjson: UmVhbGx5IHJlYWxseSByZWVlZWVlZWVlZWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGx5eXl5eXl5eXl5eXl5eXl5eXl5eSBsbGxsbGxsbGxsbGxsbG9vb29vb29vb29vb29vb29vb29vb29vb29vb25ubm5ubm5ubm5ubm5ubm5ubm5ubm5ubmdnZ2dnZ2dnZ2dnZ2dnZ2dnZ2cgYXV0aCBrZXlzCg==
type: kubernetes.io/dockerconfigjson

If you get the error message error: no objects passed to create, it may mean the base64 encoded string is invalid. If you get an error message like Secret "myregistrykey" is invalid: data[.dockerconfigjson]: invalid value ... it means the data was successfully un-base64 encoded, but could not be parsed as a .docker/config.json file.

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.

apiVersion: v1
kind: Pod
metadata:
  name: foo
  namespace: awesomeapps
spec:
  containers:
    - name: foo
      image: janedoe/awesomeapp:v1
  imagePullSecrets:
    - name: myregistrykey

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.

You can use this in conjunction with a per-node .docker/config.json. The credentials will be merged. This approach will work on Google Container Engine (GKE).

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 on the Docker hub.
      • no configuration required
      • on GCE/GKE, a local mirror is automatically used for improved speed and availability
  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 Docker registry
      • may be hosted on the Docker Hub, or elsewhere.
      • manually configure .docker/config.json on each node as described above
    • Or, run an internal private registry behind your firewall with open read access.
      • no Kubernetes configuration required
    • Or, when on GCE/GKE, use the project’s Google Container Registry.
      • 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 a 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.
    • tenant adds that secret to imagePullSecrets of each namespace.

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