Play in a content trust sandbox

Estimated reading time: 8 minutes

This page explains how to set up and use a sandbox for experimenting with trust. The sandbox allows you to configure and try trust operations locally without impacting your production images.

Before working through this sandbox, you should have read through the trust overview.


These instructions assume you are running in Linux or macOS. You can run this sandbox on a local machine or on a virtual machine. You will need to have privileges to run docker commands on your local machine or in the VM.

This sandbox requires you to install two Docker tools: Docker Engine >= 1.10.0 and Docker Compose >= 1.6.0. To install the Docker Engine, choose from the list of supported platforms. To install Docker Compose, see the detailed instructions here.

Finally, you’ll need to have a text editor installed on your local system or VM.

What is in the sandbox?

If you are just using trust out-of-the-box you only need your Docker Engine client and access to the Docker Hub. The sandbox mimics a production trust environment, and sets up these additional components.

Container Description
trustsandbox A container with the latest version of Docker Engine and with some preconfigured certificates. This is your sandbox where you can use the docker client to test trust operations.
Registry server A local registry service.
Notary server The service that does all the heavy-lifting of managing trust

This means you will be running your own content trust (Notary) server and registry. If you work exclusively with the Docker Hub, you would not need with these components. They are built into the Docker Hub for you. For the sandbox, however, you build your own entire, mock production environment.

Within the trustsandbox container, you interact with your local registry rather than the Docker Hub. This means your everyday image repositories are not used. They are protected while you play.

When you play in the sandbox, you’ll also create root and repository keys. The sandbox is configured to store all the keys and files inside the trustsandbox container. Since the keys you create in the sandbox are for play only, destroying the container destroys them as well.

By using a docker-in-docker image for the trustsandbox container, you will also not pollute your real docker daemon cache with any images you push and pull. The images will instead be stored in an anonymous volume attached to this container, and can be destroyed after you destroy the container.

Build the sandbox

In this section, you’ll use Docker Compose to specify how to set up and link together the trustsandbox container, the Notary server, and the Registry server.

  1. Create a new trustsandbox directory and change into it.

     $ mkdir trustsandbox
     $ cd trustsandbox
  2. Create a file called docker-compose.yml with your favorite editor. For example, using vim:

     $ touch docker-compose.yml
     $ vim docker-compose.yml
  3. Add the following to the new file.

     version: "2"
         image: dockersecurity/notary_autobuilds:server-v0.4.2
           - notarycerts:/go/src/
           - sandbox
           - NOTARY_SERVER_STORAGE_TYPE=memory
         image: registry:2.4.1
           - sandbox
         container_name: sandboxregistry
         image: docker:dind
           - sandbox
           - notarycerts:/notarycerts
         privileged: true
         container_name: trustsandbox
         entrypoint: ""
         command: |-
             sh -c '
                 cp /notarycerts/root-ca.crt /usr/local/share/ca-certificates/root-ca.crt &&
                 update-ca-certificates &&
        --insecure-registry sandboxregistry:5000'
         external: false
         external: false
  4. Save and close the file.

  5. Run the containers on your local system.

     $ docker-compose up -d

    The first time you run this, the docker-in-docker, Notary server, and registry images will be first downloaded from Docker Hub.

Playing in the sandbox

Now that everything is setup, you can go into your trustsandbox container and start testing Docker content trust. From your host machine, obtain a shell in the trustsandbox container.

$ docker exec -it trustsandbox sh
/ #

Test some trust operations

Now, you’ll pull some images from within the trustsandbox container.

  1. Download a docker image to test with.

     / # docker pull docker/trusttest
     docker pull docker/trusttest
     Using default tag: latest
     latest: Pulling from docker/trusttest
     b3dbab3810fc: Pull complete
     a9539b34a6ab: Pull complete
     Digest: sha256:d149ab53f8718e987c3a3024bb8aa0e2caadf6c0328f1d9d850b2a2a67f2819a
     Status: Downloaded newer image for docker/trusttest:latest
  2. Tag it to be pushed to our sandbox registry:

     / # docker tag docker/trusttest sandboxregistry:5000/test/trusttest:latest
  3. Enable content trust.

     / # export DOCKER_CONTENT_TRUST=1
  4. Identify the trust server.

     / # export DOCKER_CONTENT_TRUST_SERVER=https://notaryserver:4443

    This step is only necessary because the sandbox is using its own server. Normally, if you are using the Docker Public Hub this step isn’t necessary.

  5. Pull the test image.

     / # docker pull sandboxregistry:5000/test/trusttest
     Using default tag: latest
     Error: remote trust data does not exist for sandboxregistry:5000/test/trusttest: notaryserver:4443 does not have trust data for sandboxregistry:5000/test/trusttest

    You see an error, because this content doesn’t exist on the notaryserver yet.

  6. Push and sign the trusted image.

     / # docker push sandboxregistry:5000/test/trusttest:latest
     The push refers to a repository [sandboxregistry:5000/test/trusttest]
     5f70bf18a086: Pushed
     c22f7bc058a9: Pushed
     latest: digest: sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926 size: 734
     Signing and pushing trust metadata
     You are about to create a new root signing key passphrase. This passphrase
     will be used to protect the most sensitive key in your signing system. Please
     choose a long, complex passphrase and be careful to keep the password and the
     key file itself secure and backed up. It is highly recommended that you use a
     password manager to generate the passphrase and keep it safe. There will be no
     way to recover this key. You can find the key in your config directory.
     Enter passphrase for new root key with ID 27ec255:
     Repeat passphrase for new root key with ID 27ec255:
     Enter passphrase for new repository key with ID 58233f9 (sandboxregistry:5000/test/trusttest):
     Repeat passphrase for new repository key with ID 58233f9 (sandboxregistry:5000/test/trusttest):
     Finished initializing "sandboxregistry:5000/test/trusttest"
     Successfully signed "sandboxregistry:5000/test/trusttest":latest

    Because you are pushing this repository for the first time, docker creates new root and repository keys and asks you for passphrases with which to encrypt them. If you push again after this, it will only ask you for repository passphrase so it can decrypt the key and sign again.

  7. Try pulling the image you just pushed:

     / # docker pull sandboxregistry:5000/test/trusttest
     Using default tag: latest
     Pull (1 of 1): sandboxregistry:5000/test/trusttest:latest@sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926
     sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926: Pulling from test/trusttest
     Digest: sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926
     Status: Downloaded newer image for sandboxregistry:5000/test/trusttest@sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926
     Tagging sandboxregistry:5000/test/trusttest@sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926 as sandboxregistry:5000/test/trusttest:latest

Test with malicious images

What happens when data is corrupted and you try to pull it when trust is enabled? In this section, you go into the sandboxregistry and tamper with some data. Then, you try and pull it.

  1. Leave the trustsandbox shell and container running.

  2. Open a new interactive terminal from your host, and obtain a shell into the sandboxregistry container.

     $ docker exec -it sandboxregistry bash
  3. List the layers for the test/trusttest image you pushed:

     root@65084fc6f047:/# ls -l /var/lib/registry/docker/registry/v2/repositories/test/trusttest/_layers/sha256
     total 12
     drwxr-xr-x 2 root root 4096 Jun 10 17:26 a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
     drwxr-xr-x 2 root root 4096 Jun 10 17:26 aac0c133338db2b18ff054943cee3267fe50c75cdee969aed88b1992539ed042
     drwxr-xr-x 2 root root 4096 Jun 10 17:26 cc7629d1331a7362b5e5126beb5bf15ca0bf67eb41eab994c719a45de53255cd
  4. Change into the registry storage for one of those layers (note that this is in a different directory):

     root@65084fc6f047:/# cd /var/lib/registry/docker/registry/v2/blobs/sha256/aa/aac0c133338db2b18ff054943cee3267fe50c75cdee969aed88b1992539ed042
  5. Add malicious data to one of the trusttest layers:

     root@65084fc6f047:/# echo "Malicious data" > data
  6. Go back to your trustsandbox terminal.

  7. List the trusttest image.

     / # docker images | grep trusttest
     REPOSITORY                            TAG                 IMAGE ID            CREATED             SIZE
     docker/trusttest                      latest              cc7629d1331a        11 months ago       5.025 MB
     sandboxregistry:5000/test/trusttest   latest              cc7629d1331a        11 months ago       5.025 MB
     sandboxregistry:5000/test/trusttest   <none>              cc7629d1331a        11 months ago       5.025 MB
  8. Remove the trusttest:latest image from our local cache.

     / # docker rmi -f cc7629d1331a
     Untagged: docker/trusttest:latest
     Untagged: sandboxregistry:5000/test/trusttest:latest
     Untagged: sandboxregistry:5000/test/trusttest@sha256:ebf59c538accdf160ef435f1a19938ab8c0d6bd96aef8d4ddd1b379edf15a926
     Deleted: sha256:cc7629d1331a7362b5e5126beb5bf15ca0bf67eb41eab994c719a45de53255cd
     Deleted: sha256:2a1f6535dc6816ffadcdbe20590045e6cbf048d63fd4cc753a684c9bc01abeea
     Deleted: sha256:c22f7bc058a9a8ffeb32989b5d3338787e73855bf224af7aa162823da015d44c

    Docker does not re-download images that it already has cached, but we want Docker to attempt to download the tampered image from the registry and reject it because it is invalid.

  9. Pull the image again. This will download the image from the registry, because we don’t have it cached.

     / # docker pull sandboxregistry:5000/test/trusttest
     Using default tag: latest
     Pull (1 of 1): sandboxregistry:5000/test/trusttest:latest@sha256:35d5bc26fd358da8320c137784fe590d8fcf9417263ef261653e8e1c7f15672e
     sha256:35d5bc26fd358da8320c137784fe590d8fcf9417263ef261653e8e1c7f15672e: Pulling from test/trusttest
     aac0c133338d: Retrying in 5 seconds
     a3ed95caeb02: Download complete
     error pulling image configuration: unexpected EOF

    You’ll see the pull did not complete because the trust system was unable to verify the image.

More play in the sandbox

Now, you have a full Docker content trust sandbox on your local system, feel free to play with it and see how it behaves. If you find any security issues with Docker, feel free to send us an email at

Cleaning up your sandbox

When you are done, and want to clean up all the services you’ve started and any anonymous volumes that have been created, just run the following command in the directory where you’ve created your Docker Compose file:

    $ docker-compose down -v
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