Prerequisites

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.

Start an ExternalDNS instance

ExternalDNS synchronizes exposed Kubernetes Services and Ingresses with DNS providers. The default entrypoint displays help information.

Run the following command and replace <tag> with the image variant you want to run.

docker run --rm dhi.io/external-dns:<tag> --help

Common ExternalDNS use cases

Deploy with Helm to Kubernetes

The most common way to deploy ExternalDNS is using Helm in a Kubernetes cluster. First, follow the authentication instructions for DHI in Kubernetes⁠.

Add the official ExternalDNS Helm chart repository:

helm repo add external-dns https://kubernetes-sigs.github.io/external-dns/

Install the chart with the Docker Hardened Image:

helm upgrade --install external-dns external-dns/external-dns \
  --version <helm_chart_version> \
  --set image.repository=dhi.io/external-dns \
  --set image.tag=<tag> \
  --set "imagePullSecrets[0].name=registry-credentials"

Configure with AWS Route53

To use ExternalDNS with AWS Route53, add provider-specific configuration to your Helm installation:

helm upgrade --install external-dns external-dns/external-dns \
  --version <helm_chart_version> \
  --set image.repository=dhi.io/external-dns \
  --set image.tag=<tag> \
  --set "imagePullSecrets[0].name=registry-credentials" \
  --set provider=aws \
  --set aws.region=us-east-1 \
  --set txtOwnerId=my-cluster

Configure with Google Cloud DNS

To use ExternalDNS with Google Cloud DNS, configure the provider and credentials:

helm upgrade --install external-dns external-dns/external-dns \
  --version <helm_chart_version> \
  --set image.repository=dhi.io/external-dns \
  --set image.tag=<tag> \
  --set "imagePullSecrets[0].name=registry-credentials" \
  --set provider=google \
  --set google.project=my-project

Configure with Azure DNS

To use ExternalDNS with Azure DNS, specify Azure as the provider:

helm upgrade --install external-dns external-dns/external-dns \
  --version <helm_chart_version> \
  --set image.repository=dhi.io/external-dns \
  --set image.tag=<tag> \
  --set "imagePullSecrets[0].name=registry-credentials" \
  --set provider=azure \
  --set azure.resourceGroup=my-resource-group

Non-hardened images vs Docker Hardened Images

Key differences

Why no shell or package manager?

Docker Hardened Images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Docker Hardened Images include:

• Docker Debug⁠ to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

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.

For example, you can use Docker Debug:

docker debug <image-name>

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-image \
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \
  dhi.io/external-dns:<tag> /dbg/bin/sh

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.

   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.

Troubleshoot 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.

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.