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

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

Start a Rancher Webhook image

Rancher Webhook needs a Kubernetes cluster. It connects through the pod's service account (or KUBECONFIG), creates its serving certificate in the cattle-system namespace, and registers its hooks in the rancher.cattle.io ValidatingWebhookConfiguration and MutatingWebhookConfiguration. Rancher installs it in every cluster it runs or manages, using the rancher-webhook Helm chart at the version each Rancher release pins.

Use the tag line for your Rancher version: 0.11 for Rancher 2.15, 0.10 for 2.14, 0.9 for 2.13 and 0.8 for 2.12. The Debian-based tags also come in the upstream form, for example v0.11.4. The newest Rancher patch of each minor pins the newest webhook patch. With an older Rancher patch, the hardened image is newer than the one Rancher pins.

Environment variables

The chart sets these variables from its values.

Common Rancher Webhook use cases

The FIPS tags come only with a DHI Select or DHI Enterprise subscription, from a mirrored repository. For them, use <your-namespace>/dhi-rancher-webhook instead of dhi.io/rancher-webhook in both setups below, and docker.io as the pull secret's server.

Point Rancher at the hardened image

Rancher passes the Rancher chart's webhook value to the webhook chart it installs, as Helm values. Set the image there, with a pull secret for dhi.io. Replace:

• <version> with a hardened release of your Rancher's line, for example 0.11.4 (0.11.4-fips for FIPS).
• <rancher-repo> with the Helm repository you installed Rancher from.
• <rancher-version> with the Rancher version you run, so the upgrade changes only the webhook values.

$ kubectl -n cattle-system create secret docker-registry dhi-pull \
  --docker-server=dhi.io --docker-username=<username> --docker-password=<access-token>
$ cat > webhook-values.yaml <<'EOF'
webhook: |
  image:
    repository: dhi.io/rancher-webhook
    tag: <version>
  global:
    cattle:
      imagePullSecrets:
        - dhi-pull
EOF
$ helm upgrade rancher <rancher-repo>/rancher --namespace cattle-system --version <rancher-version> \
  --reuse-values -f webhook-values.yaml

Check that Rancher reinstalled the webhook with the hardened image:

$ kubectl -n cattle-system rollout status deployment rancher-webhook
$ kubectl -n cattle-system get deployment rancher-webhook \
  -o jsonpath='{.spec.template.spec.containers[0].image}'

This changes the webhook only on the cluster Rancher runs on. Copy the image into a private registry instead, as in the next section:

• For the clusters Rancher manages, which get their webhook settings separately.
• On Rancher 2.12, 2.13 and 2.14, whose webhook charts have no global.cattle.imagePullSecrets value.
• When Rancher uses a system default registry, which the chart puts in front of dhi.io/rancher-webhook.

Run the webhook from a private registry

With a system default registry or a cluster private registry set, Rancher installs the webhook from <registry>/rancher/rancher-webhook:v<version>, where v<version> is the tag the Rancher release pins. Copy the hardened image of the same line to that path and tag before Rancher installs or upgrades the webhook. Rancher then pulls it with no other change.

Find the tag in the image list of the Rancher release you install or upgrade to. Replace <rancher-version> with that release, for example 2.15.3:

$ curl -fsSL https://github.com/rancher/rancher/releases/download/v<rancher-version>/rancher-images.txt \
  | grep rancher/rancher-webhook
rancher/rancher-webhook:v0.11.4

Copy the newest hardened image of that line into your registry, with every platform. Replace <version> with the version from the list, without the v, and <line> with its first two numbers, for example 0.11 for 0.11.4 (0.11-fips for FIPS):

$ docker buildx imagetools create \
  --tag <registry>/rancher/rancher-webhook:v<version> \
  dhi.io/rancher-webhook:<line>

The chart pulls with imagePullPolicy: IfNotPresent, so a node that already holds the upstream image under that tag keeps running it.

For the resources each validator and mutator checks, see the upstream docs.md⁠.

Non-hardened images vs. Docker Hardened Images

• Entrypoint: the upstream image sets no entrypoint and runs CMD ["webhook"]. The hardened image runs the same binary, /usr/bin/webhook, as its entrypoint and sets no command. The chart sets neither command nor args, so both start the same binary.
• User: the upstream image runs as the user webhook (uid 1000). Runtime variants of the hardened image run as the user webhook with uid 65532.
• Shell: the upstream image ships bash and coreutils. Runtime variants of the hardened image ship no shell; the dev variants do.
• TLS: upstream enables two ChaCha20-Poly1305 cipher suites for TLS 1.2 connections. The FIPS variants refuse them and negotiate only FIPS-approved suites.
• Tags: the Debian-based tags include the upstream tag form, such as v0.11.4, v0.11.4-dev and v0.11.4-fips. The Alpine-based tags do not.

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.