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.

What's included in this sealed-secrets-controller Hardened image

This image contains sealed-secrets-controller, which is able to decrypt and interact with secrets deployed as SealedSecret resources. The entry point for the image is controller which provides the internal CLI used to interact with SealedSecrets.

Start a sealed-secrets-controller instance

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

docker run --rm -it dhi.io/sealed-secrets-controller:<tag> --help

Common sealed-secrets-controller use cases

Install sealed-secrets-controller using Helm

Sealed Secrets is designed to run in a Kubernetes cluster, and is frequently used with GitOps workflows to securely manage Kubernetes Secrets. You can install Sealed Secrets using the official helm chart and replace the image. Replace <your-registry-secret> with your Kubernetes image pull secret⁠ and <tag> with the desired image tag.

helm repo add sealed-secrets https://bitnami.github.io/sealed-secrets
helm repo update

helm install sealed-secrets sealed-secrets/sealed-secrets --namespace kube-system --create-namespace \
  --set "images.pullSecrets[0].name=<your-registry-secret>" \
  --set image.registry=dhi.io \
  --set image.repository=sealed-secrets-controller \
  --set image.tag=<tag> \

This deploys the Sealed Secrets controller in the kube-system namespace using the specified image. This is the default namespace that kubeseal tool for encrypting secrets is expected, so changing the namespace will also require providing --controller-namespace argument to kubeseal whenever encrypting a secret.

Encrypting Secrets

To encrypt a secret, use kubeseal to generate a SealedSecret from the Kuebernetes Secret YAML.

First, create a Kubernetes Secret (if it is not already created)

kubectl create secret generic mysecret --dry-run=client \
  --from-literal=password=secret \
  -o yaml > mysecret.yaml

This will generate mysecret.yaml with the following content

apiVersion: v1
data:
  password: c2VjcmV0
kind: Secret
metadata:
  name: mysecret

Once the Kubernetes secret yaml or json is generated, create the SealedSecret resource and confirm it was applied correctly

kubeseal -f mysecret.yaml -w mysealedsecret.yaml
kubectl apply -f sealedsecret.yaml

kubectl get secrets mysecret

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

Hardened image debugging

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 dhi.io/sealed-secrets-controller

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-container \
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \
  dhi.io/sealed-secrets-controller:<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

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Kubernetes manifests or Docker configurations. At minimum, you must update the base image in your existing deployment 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 CLI usage.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs. The sealed-secrets-controller CLI is typically used in CI/CD pipelines and local development environments.

2. Update your CI/CD pipeline configurations.

   Update the image references in your CI/CD scripts, GitHub Actions, or other automation to use the hardened images.

3. For custom containers, update the base image in your Dockerfile.

   If you're building custom images that include sealed-secrets-controller, ensure that your final image uses the hardened sealed-secrets-controller as the base. For multi-stage builds, use images tagged as dev for build stages and non-dev images for runtime.

4. Test Controller functionality.

   To verify the controller is functioning properly, verify that is it able to read any existing SealedSecrets or generate a new SealedSecret.

   # If the secret is not already generated
   kubectl create secret generic mysecret --dry-run=client \
     --from-literal=password=secret \
     -o yaml > secret.yaml
   
   kubeseal -f mysecret.yaml -w mysealedsecret.yaml
   kubectl apply -f sealedsecret.yaml
   
   kubectl get secret mysecret
   

   Copy

Troubleshoot 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.

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.