How to use this image

All examples in this guide use the public image. If you've mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/k8s-cluster-api-controller:<tag>
• Mirrored image: <your-namespace>/dhi-k8s-cluster-api-controller:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

What's included in this Cluster API Controller image

This Docker Hardened Image includes the core controller manager binary (cluster-api-controller) from the Cluster API⁠ project. It is the component that upstream publishes as registry.k8s.io/cluster-api/cluster-api-controller. The binary is also reachable at /manager, matching the path that the upstream release manifests use in their command:.

The kubeadm bootstrap provider, kubeadm control-plane provider, infrastructure providers, and the clusterctl CLI are separate components and are not part of this image.

Run the Cluster API Controller container

The controller is designed to run inside a Kubernetes management cluster with in-cluster configuration, RBAC, and the Cluster API custom resource definitions installed. It is not intended to be used as a standalone docker run process for production workloads.

To verify the image is accessible and list the controller's supported flags, run:

docker run --rm dhi.io/k8s-cluster-api-controller:<tag> --help

The manager binary does not implement a dedicated --version flag; its build metadata is embedded via Go's ldflags. Outside a cluster, the manager prints its usage or exits with an error (it cannot load a kubeconfig), which is expected.

Deploy Cluster API in Kubernetes

The recommended installation method is clusterctl, which installs the core provider together with your chosen bootstrap, control-plane, and infrastructure providers. To use this hardened image for the core provider, configure an image override⁠ in $HOME/.config/cluster-api/clusterctl.yaml:

images:
  cluster-api:
    repository: dhi.io
    name: k8s-cluster-api-controller
    tag: <tag>

Then initialize the management cluster, pinning the core provider to the version this image packages:

clusterctl init --core cluster-api:v<VERSION>

Verify the controller is running:

kubectl -n capi-system get pods

You should see output similar to:

NAME                                       READY   STATUS    RESTARTS   AGE
capi-controller-manager-...                1/1     Running   0          30s

For complete deployment options, provider configuration, and the full quick start, refer to the official documentation at https://cluster-api.sigs.k8s.io/user/quick-start⁠.

FIPS variant

This image includes a FIPS-compliant variant that uses Go's native FIPS 140 cryptographic mode. The FIPS variant is available with the fips tag suffix.

Use the FIPS variant when your deployment environment requires FIPS 140-compliant cryptographic operations. The FIPS module is selected at build time, so the binaries run in Go's lenient FIPS mode (fips140=on) by default rather than strict mode (fips140=only). Lenient mode is required because the controller's connection to the Kubernetes API server negotiates X25519 (not FIPS-approved); strict mode would reject it. FIPS-approved algorithms are still enforced for all other cryptographic operations.

Image variants

Docker Hardened Images come in different variants depending on their intended use. Image variants are identified by their tag.

• Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

  • Run as a nonroot user
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically include dev in the tag name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

  • Run as the root user
  • Include a shell and package manager
  • Are used to build or compile applications

• FIPS variants include fips in the variant name and tag. They come in both runtime and build-time variants. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. For example, usage of MD5 fails in FIPS variants.

To view the image variants and get more information about them, select the Tags tab for this repository, and then select a tag.

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Docker Hardened Image. This and a few other common changes are listed in the following table of migration notes.

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages

   Docker Hardened Images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshooting migration

The following are common issues that you may encounter during migration.

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug⁠ to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary.