Use trusted images for continuous integration

The document provides a minimal example on setting up Docker Content Trust (DCT) in Universal Control Plane (UCP) for use with a Continuous Integration (CI) system. It covers setting up the necessary accounts and trust delegations to restrict only those images built by your CI system to be deployed to your UCP managed cluster.

Set up UCP accounts and teams

The first step is to create a user account for your CI system. For the purposes of this document we will assume you are using Jenkins as your CI system and will therefore name the account “jenkins”. As an admin user logged in to UCP, navigate to “User Management” and select “Add User”. Create a user with the name “jenkins” and set a strong password.

Next, create a team called “CI” and add the “jenkins” user to this team. All signing policy is team based, so if we want only a single user to be able to sign images destined to be deployed on the cluster, we must create a team for this one user.

Set up the signing policy

While still logged in as an admin, navigate to “Admin Settings” and select the “Content Trust” subsection. Select the checkbox to enable content trust and in the select box that appears, select the “CI” team we have just created. Save the settings.

This policy will require that every image that referenced in a docker pull, docker run, or docker service create must be signed by a key corresponding to a member of the “CI” team. In this case, the only member is the “jenkins” user.

Create keys for the Jenkins user

The signing policy implementation uses the certificates issued in user client bundles to connect a signature to a user. Using an incognito browser window (or otherwise), log in to the “jenkins” user account you created earlier. Download a client bundle for this user. It is also recommeneded to change the description associated with the public key stored in UCP such that you can identify in the future which key is being used for signing.

N.B. each time a user retrieves a new client bundle, a new keypair is generated. It is therefore necessary to keep track of a specific bundle that a user chooses to designate as their signing bundle.

Once you have decompressed the client bundle, the only two files you need for the purposes of signing are cert.pem and key.pem. These represent the public and private parts of the user’s signing identity respectively. We will load the key.pem file onto the Jenkins servers, and user cert.pem to create delegations for the “jenkins” user in our Trusted Collection.

Prepare the Jenkins server

Load key.pem on Jenkins

You will need to use the notary client to load keys onto your Jenkins server. Simply run notary -d /path/to/.docker/trust key import /path/to/key.pem. You will be asked to set a password to encrypt the key on disk. For automated signing, this password can be configured into the environment under the variable name DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE. The -d flag to the command specifies the path to the trust subdirectory within the server’s docker configuration directory. Typically this is found at ~/.docker/trust.

Enable content trust

There are two ways to enable content trust: globally, and per operation. To enabled content trust globally, set the environment variable DOCKER_CONTENT_TRUST=1. To enable on a per operation basis, wherever you run docker push in your Jenkins scripts, add the flag --disable-content-trust=false. You may wish to use this second option if you only want to sign some images.

The Jenkins server is now prepared to sign images, but we need to create delegations referencing the key to give it the necessary permissions.

Initialize a repository

Any commands displayed in this section should not be run from the Jenkins server. You will most likely want to run them from your local system.

If this is a new repository, create it in Docker Trusted Registry (DTR) or Docker Hub, depending on which you use to store your images, before proceeding further.

We will now initialize the trust data and create the delegation that provides the Jenkins key with permissions to sign content. The following commands initialize the trust data and rotate snapshotting responsibilities to the server. This is necessary to ensure human involvement it not required to publish new content.

notary -s https://my_notary_server.com -d ~/.docker/trust init my_repository
notary -s https://my_notary_server.com -d ~/.docker/trust key rotate my_repository snapshot -r
notary -s https://my_notary_server.com -d ~/.docker/trust publish my_repository

N.B. the -s flag provides the server hosting a notary service. If you are operating against Docker Hub, this will be https://notary.docker.io. If you are operating against your own DTR instance, this will be the same hostname you use in image names when running docker commands preceded by the https:// scheme. For example, if you would run docker push my_dtr:4443/me/an_image the value of the -s flag would be expected to be https://my_dtr:4443.

N.B. if you are using DTR, the name of the repository should be identical to the full name you use in a docker push command. If however you use Docker Hub, the name you use in a docker push must be preceded by docker.io/. i.e. if you ran docker push me/alpine, you would notary init docker.io/me/alpine.

For brevity, we will exclude the -s and -d flags from subsequent command, but be aware you will still need to provide them for the commands to work correctly.

Now that the repository is initialized, we need to create the delegations for Jenkins. Docker Content Trust treats a delegation role called targets/releases specially. It considers this delegation to contain the canonical list of published images for the repository. It is therefore generally desiable to add all users to this delegation with the following command:

notary delegation add my_repository targets/releases --all-paths /path/to/cert.pem

This solves a number of prioritization problems that would result from needing to determine which delegation should ultimately be trusted for a specific image. However, because it is anticipated that any user will be able to sign the targets/releases role it is not trusted in determining if a signing policy has been met. Therefore it is also necessary to create a delegation specifically for Jenkins:

notary delegation add my_repository targets/jenkins --all-paths /path/to/cert.pem

We will then publish both these updates (remember to add the correct -s and -d flags):

notary publish my_repository

Informational (Advanced): If we included the targets/releases role in determining if a signing policy had been met, we would run into the situation of images being opportunistically deployed when an appropriate user signs. In the scenario we have described so far, only images signed by the “CI” team (containing only the “jenkins” user) should be deployable. If a user “Moby” could also sign images but was not part of the “CI” team, they might sign and publish a new targets/releases that contained their image. UCP would refuse to deploy this image because it was not signed by the “CI” team. However, the next time Jenkins published an image, it would update and sign the targets/releases role as whole, enabling “Moby” to deploy their image.

Conclusion

With the Trusted Collection initialized, and delegations created, the Jenkins server will now use the key we imported to sign any images we push to this repository.

Through either the Docker CLI, or the UCP browser interface, we will find that any images that do not meet our signing policy cannot be used. The signing policy we set up requires that the “CI” team must have signed any image we attempt to docker pull, docker run, or docker service create, and the only member of that team is the “jenkins” user. This restricts us to only running images that were published by our Jenkins CI system.

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