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

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

Ansible is a batteries-included controller CLI for playbooks and ad-hoc tasks. This image does not listen on a port. Playbook language, modules, and configuration are documented at https://docs.ansible.com/⁠.

What's included in this Ansible image

This Docker Hardened Ansible image includes:

• ansible-community — Prints the Ansible community package version.
• ansible — Ad-hoc task runner for the Ansible community package.
• ansible-playbook — Runs Ansible playbooks.
• ansible-galaxy — Lists and installs Ansible collections and roles.
• ansible-vault — Encrypts and decrypts Ansible vault data.
• ansible-config — Displays Ansible configuration.
• ansible-doc — Shows documentation for Ansible modules and plugins.
• ansible-inventory — Displays and graphs an Ansible inventory.
• ansible-console — Interactive Ansible console.
• ansible-pull — Pulls playbooks from a repository and runs them locally. Needs the dev variant, which has a shell and git.

Start an Ansible image

The image sets no entrypoint. With no arguments, the default command prints the Ansible version.

$ docker run --rm dhi.io/ansible:<tag>

Run a playbook

The working directory is /work. Mount a directory that contains your playbook there, and mount an SSH private key at /home/nonroot/.ssh/id_ecdsa. Arguments after the image name replace the default command.

The runtime image runs as UID 65532. The mounted directory must be traversable by that user, and the playbook must be readable. Make the directory mode 0755 and the playbook mode 0644 before a read-only mount. The private key must be mode 0600 and owned by UID 65532. OpenSSH will not use a key owned by another user.

Use an ECDSA or RSA key. Alpine FIPS variants reject Ed25519, and ssh-keygen creates an Ed25519 key unless you pass -t.

$ mkdir -p playbook
$ ssh-keygen -t ecdsa -b 256 -f id_ecdsa -N "" -q
$ chmod 600 id_ecdsa
$ sudo chown 65532:65532 id_ecdsa
$ cat > playbook/remote.yml <<'EOF'
- hosts: all
  gather_facts: true
  tasks:
    - ansible.builtin.copy:
        dest: /tmp/dhi-ansible.txt
        content: "hi {{ ansible_distribution }}"
    - ansible.builtin.command: cat /tmp/dhi-ansible.txt
      register: greeting
    - ansible.builtin.debug:
        msg: "{{ greeting.stdout }}"
EOF
$ chmod 755 playbook
$ chmod 644 playbook/remote.yml
$ cat > start-sshd.sh <<'EOF'
#!/bin/bash
set -eux
export DEBIAN_FRONTEND=noninteractive
apt-get update
apt-get install -y --no-install-recommends openssh-server python3
mkdir -p /root/.ssh /run/sshd
cp /authorized_keys /root/.ssh/authorized_keys
chmod 700 /root/.ssh
chmod 600 /root/.ssh/authorized_keys
ssh-keygen -A
exec /usr/sbin/sshd -D -e
EOF
$ chmod 755 start-sshd.sh
$ docker network create ansible-net
$ docker run -d --name ansible-target --network ansible-net --network-alias target \
    -v "$PWD/id_ecdsa.pub:/authorized_keys:ro" \
    -v "$PWD/start-sshd.sh:/start-sshd.sh:ro" \
    debian:trixie-slim bash /start-sshd.sh
$ until docker logs ansible-target 2>&1 | grep -q 'Server listening'; do sleep 2; done
$ docker run --rm --network ansible-net \
    -e ANSIBLE_HOST_KEY_CHECKING=False \
    -v "$PWD/playbook:/work:ro" \
    -v "$PWD/id_ecdsa:/home/nonroot/.ssh/id_ecdsa:ro" \
    dhi.io/ansible:<tag> \
    ansible-playbook -i target, -u root --private-key /home/nonroot/.ssh/id_ecdsa /work/remote.yml
$ docker rm -f ansible-target
$ docker network rm ansible-net

Image differences

There is no upstream end-user container image to migrate from.

• No entrypoint is set, so arguments after the image name replace the default command.
• Runtime variants run as UID 65532 and do not include a shell. A module that runs on localhost, including ping, copy, command, and fact gathering, needs /bin/sh and therefore a dev variant. ansible-pull needs a dev variant as well: its first task is a local git task, and the runtime image has no shell and no git.
• FIPS variants restrict the SSH client to AES-GCM and AES-CTR ciphers. Stock OpenSSH servers offer [email protected] first, and the FIPS provider rejects it.
• Alpine FIPS variants also reject Ed25519 host keys and client keys. OpenSSH on Alpine verifies those signatures through libcrypto, which the FIPS provider refuses. Use an ECDSA or RSA key. ssh-keygen defaults to Ed25519, so pass -t ecdsa or -t rsa. Debian FIPS variants can still use Ed25519.
• The working directory is /work.
• Dev variants include dev in the tag, run as root, and include a shell and git.

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.