Learn Kubernetes Basics

Concepts

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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 skip pulling an image if it already exists. 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.
  • omit the imagePullPolicy and the tag for the image to use.
  • enable the AlwaysPullImages admission controller.

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

Building Multi-architecture Images with Manifests

Docker CLI now supports the following command docker manifest with sub commands like create, annotate and push. These commands can be used to build and push the manifests. You can use docker manifest inspect to view the manifest.

Please see docker documentation here: https://docs.docker.com/edge/engine/reference/commandline/manifest/

See examples on how we use this in our build harness: https://cs.k8s.io/?q=docker%20manifest%20(create%7Cpush%7Cannotate)&i=nope&files=&repos=

These commands rely on and are implemented purely on the Docker CLI. You will need to either edit the $HOME/.docker/config.json and set experimental key to enabled or you can just set DOCKER_CLI_EXPERIMENTAL environment variable to enabled when you call the CLI commands.

Note: Please use Docker 18.06 or above, versions below that either have bugs or do not support the experimental command line option. Example https://github.com/docker/cli/issues/1135 causes problems under containerd.

If you run into trouble with uploading stale manifests, just clean up the older manifests in $HOME/.docker/manifests to start fresh.

For Kubernetes, we have typically used images with 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:

  • Using Google Container Registry
    • Per-cluster
    • automatically configured on Google Compute Engine or Google Kubernetes Engine
    • all pods can read the project’s private registry
  • Using Amazon Elastic Container Registry (ECR)
    • use IAM roles and policies to control access to ECR repositories
    • automatically refreshes ECR login credentials
  • Using Oracle Cloud Infrastructure Registry (OCIR)
    • use IAM roles and policies to control access to OCIR repositories
  • Using Azure Container Registry (ACR)
  • Using IBM Cloud Container Registry
    • use IAM roles and policies to grant access to IBM Cloud Container Registry
  • Configuring Nodes to Authenticate to a Private Registry
    • all pods can read any configured private registries
    • requires node configuration by cluster administrator
  • Pre-pulled Images
    • all pods can use any images cached on a node
    • requires root access to all nodes to setup
  • Specifying ImagePullSecrets on a Pod
    • only pods which provide own keys can access the private registry

Each option is described in more detail below.

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 Kubernetes Engine, 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 Amazon Elastic Container Registry

Kubernetes has native support for the Amazon Elastic 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:

  • ecr:GetAuthorizationToken
  • ecr:BatchCheckLayerAvailability
  • ecr:GetDownloadUrlForLayer
  • ecr:GetRepositoryPolicy
  • ecr:DescribeRepositories
  • ecr:ListImages
  • ecr:BatchGetImage

Requirements:

  • You must be using kubelet version v1.2.0 or newer. (e.g. run /usr/bin/kubelet --version=true).
  • If your nodes are in region A and your registry is in a different region B, you need version v1.3.0 or newer.
  • ECR must be offered in your region

Troubleshooting:

  • Verify all requirements above.
  • Get $REGION (e.g. us-west-2) credentials on your workstation. SSH into the host and run Docker manually with those creds. Does it work?
  • Verify kubelet is running with --cloud-provider=aws.
  • Increase kubelet log level verbosity to at least 3 and check kubelet logs (e.g. journalctl -u kubelet) for log lines like:
    • aws_credentials.go:109] unable to get ECR credentials from cache, checking ECR API
    • aws_credentials.go:116] Got ECR credentials from ECR API for <AWS account ID for ECR>.dkr.ecr.<AWS region>.amazonaws.com

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.

Using IBM Cloud Container Registry

IBM Cloud Container Registry provides a multi-tenant private image registry that you can use to safely store and share your images. By default, images in your private registry are scanned by the integrated Vulnerability Advisor to detect security issues and potential vulnerabilities. Users in your IBM Cloud account can access your images, or you can use IAM roles and policies to grant access to IBM Cloud Container Registry namespaces.

To install the IBM Cloud Container Registry CLI plug-in and create a namespace for your images, see Getting started with IBM Cloud Container Registry.

If you are using the same account and region, you can deploy images that are stored in IBM Cloud Container Registry into the default namespace of your IBM Cloud Kubernetes Service cluster without any additional configuration, see Building containers from images. For other configuration options, see Understanding how to authorize your cluster to pull images from a registry.

Configuring Nodes to Authenticate to a Private Registry

Note: If you are running on Google Kubernetes Engine, 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.
Note: Kubernetes as of now only supports the auths and HttpHeaders section of docker config. This means credential helpers (credHelpers or credsStore) are not supported.

Docker stores keys for private registries in the $HOME/.dockercfg or $HOME/.docker/config.json file. If you put the same file in the search paths list below, kubelet uses it as the credential provider when pulling images.

  • {--root-dir:-/var/lib/kubelet}/config.json
  • {cwd of kubelet}/config.json
  • ${HOME}/.docker/config.json
  • /.docker/config.json
  • {--root-dir:-/var/lib/kubelet}/.dockercfg
  • {cwd of kubelet}/.dockercfg
  • ${HOME}/.dockercfg
  • /.dockercfg
Note: You may have to set HOME=/root explicitly in your environment file for kubelet.

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 one of the search paths list above.
    • for example: for n in $nodes; do scp ~/.docker/config.json root@$n:/var/lib/kubelet/config.json; done

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

kubectl apply -f - <<EOF
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
pod/private-image-test-1 created

If everything is working, then, after a few moments, you can run:

kubectl logs private-image-test-1

and see that the command outputs:

SUCCESS

If you suspect that the command failed, you can run:

kubectl describe pods/private-image-test-1 | grep 'Failed'

In case of failure, the output is similar to:

  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.

Pre-pulled Images

Note: If you are running on Google Kubernetes Engine, 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 Google Kubernetes Engine, 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 <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 secret. 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 will only work with a single private registry.

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

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.

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. This approach will work on Google Kubernetes Engine.

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/Google Kubernetes Engine, 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.
      • It 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 is required.
    • Or, when on GCE/Google Kubernetes Engine, use the project’s Google Container Registry.
      • 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. Kubelet will merge any imagePullSecrets into a single virtual .docker/config.json

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