Run only the images you trust

With Docker Universal Control Plane you can enforce applications to only use Docker images signed by UCP users you trust. When a user tries to deploy an application to the cluster, UCP checks if the application uses a Docker image that is not trusted, and won’t continue with the deployment if that’s the case.

By signing and verifying the Docker images, you ensure that the images being used in your cluster are the ones you trust and haven’t been altered either in the image registry or on their way from the image registry to your UCP cluster.

Configure UCP

To configure UCP to only allow running applications that use Docker images you trust, go to the UCP web UI, navigate to the Settings page, and click the Content Trust tab.

Select the Run only signed images option to only allow deploying applications if they use images you trust.

Then, in the Require signature from field, you can specify all the teams that need sign the image, before it is trusted to run in the UCP cluster. If you specify multiple teams, the image needs to be signed by a member of each team, or someone that is a member of all those teams. If you don’t specify any team, the image will be trusted as long as it is signed by any UCP user whose keys are trusted in a Notary delegation role.

Set up the Docker Notary CLI client

Once you’ve configured UCP to only run applications that use Docker images you trust, you’ll need to specify which Docker images can be trusted. This is done by using the Docker Notary server that is built into Docker Trusted Registry. You’ll configure the Notary server to store signed metadata about the Docker images you trust.

To interact with the Notary server, you need to install the Notary CLI client.

Once you’ve installed the Notary client, you need to configure it to talk to the Notary server that is built into Docker Trusted Registry. This can be done using a Notary configuration file or by running:

# Create an alias to always have the notary client talking to the right server
$ alias notary="notary -s https://<dtr_url> -d ~/.docker/trust"

Where -s specifies the notary server to talk to, and -d specifies the directory to store private keys and cache data.

If your Docker Trusted Registry is not using certificates signed by a globally trusted certificate authority, you’ll also need to configure notary to use the certificate of the DTR CA:

$ alias notary="notary -s https://<dtr_url> -d ~/.docker/trust --tlscacert <dtr_ca.pem>"

Set up a trusted image repository

Once your Docker Notary CLI client is configured, you can check if Notary has information about a specific repository:

# <dtr_url>/<account>/<repository> is also known as a Globally Unique Name (GUN)
$ notary list <dtr_url>/<account>/<repository>

If notary has information about the repository it returns the list of image tags it knows about, their expected digests, and the role of the private key used to sign the metadata.

If Notary doesn’t know yet about an image repository, run:

$ notary init -p <dtr_url>/<account>/<repository>

The Notary CLI client generates public and private key pairs, prompts you for a passphrase to encrypt the private key, and stores the key pair in the directory you’ve specified with the notary -d flag. You should ensure you create backups for these keys, and that they are kept securely and offline. Learn more about the keys used by Docker Notary.

Sign and push an image

Now you can sign your images before pushing them to Docker Trusted Registry:

# Setting Docker content trust makes the Docker CLI client sign images before pushing them
$ export DOCKER_CONTENT_TRUST=1
# Push the image
$ docker push <dtr_url>/<account>/<repository>:<tag>

The Docker CLI client will prompt you for the passphrase you used to encrypt the private keys, sign the image, and push it to the registry.

Delegate image signing

Instead of signing the Docker images yourself, you can delegate that task to other users.

Delegation roles simplify collaborator workflows in Notary trusted collections, and also allow for fine-grained permissions within a collection’s contents across delegations. Delegation roles act as signers in Notary that are managed by the targets key and can be configured to use external signing keys. Keys can be dynamically added to or removed from delegation roles as collaborators join and leave trusted repositories. Learn more about Notary delegation roles.

Every change to the repository now needs to be signed by the snapshot key that was generated with the notary init command. To avoid having to distribute this key to other members so that they can also sign images with this key, you can rotate the key and make it be managed by the Notary server.

This operation only needs to be done once for the repository.

# This only needs to be done once for the repository
$ notary key rotate <dtr_url>/<account>/<repository> snapshot --server-managed

Then ask the users you want to delegate the image signing to share with you the cert.pem files that are included in their client bundles. These files should be shared using a trusted channel.

Then run the following command to create a new Notary delegation role, using the user certificates:

$ notary delegation add -p <dtr_url>/<account>/<repository> targets/releases --all-paths user1.pem user2.pem

The above command adds the the targets/releases delegation role to a trusted repository. This role is treated as an actual release branch for Docker Content Trust, since docker pull commands with trust enabled will pull directly from this role, if data exists. All users that can release images should be added to this role. Learn more about the targets/releases role.

Notary has no limit on how many delegation roles can exist, so you can add more delegation roles such as targets/qa_team or targets/security_team to the trusted repository.

Valid delegation roles take the form of targets/<delegation>, where <delegation> does not include further slashes.

You will need to add the key to at least one delegation in addition to the targets/releases delegation in order for UCP to honor the signed content:

$ notary delegation add -p <dtr_url>/<account>/<repository> targets/devops --all-paths user1.pem user2.pem

Before delegation role users can publish signed content with Notary or Docker Content Trust, they must import the private key associated with the user certificate:

$ notary key import key.pem

Where to go next

• Manage trusted repositories
• Get started with Notary

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