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/<repository>:<tag>
• Mirrored image: <your-namespace>/dhi-<repository>:<tag>

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

About Kustomize Controller

The kustomize-controller is a Flux CD component that reconciles Kustomization custom resources in Kubernetes. It manages the full lifecycle of Kustomize-based manifests including applying resources, pruning removed resources, and performing health checks to verify the desired state is achieved.

Deploying with Flux CD

The kustomize-controller is typically deployed as part of the Flux CD toolkit. To use the Docker Hardened Image, update your Flux installation to reference the DHI image instead of the upstream image.

docker run --rm dhi.io/fluxcd-kustomize-controller:<tag> --help

Deployment requirements

The kustomize-controller runs as a Kubernetes controller and requires:

• Access to the Kubernetes API server
• RBAC permissions to manage Kustomization resources and the resources they declare
• Access to a Flux source-controller for fetching Git repository artifacts
• A writable /tmp directory for GnuPG operations when decrypting SOPS secrets

When deployed, the controller watches for Kustomization resources and reconciles the desired manifest state.

Version and Flux compatibility

The image version corresponds to the kustomize-controller component version:

Common configuration options

The controller supports various command-line flags:

• --log-level: Set logging verbosity (debug, info, warn, error)
• --log-encoding: Log format (json, console)
• --enable-leader-election: Enable leader election for HA deployments
• --metrics-addr: Address for exposing Prometheus metrics
• --health-addr: Address for health check endpoints
• --concurrent: Number of concurrent reconciliations

Image variants

Docker Hardened Images come in different variants depending on their intended use.

• 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 the 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 variant 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.

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.

   If you're using a multi-stage build, update the runtime stage to use a non-dev hardened image. This ensures your production containers run with minimal attack surface.

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.

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.