In this post, we will be introducing docker secrets. Docker secrets offer a secure way to store sensitive information such as username, passwords, and even files like self-signed certificates.
Docker – See how to install docker here.
Swarm mode enabled – Ensure that swarm mode is enabled before starting this tutorial. If it is not, open your terminal window and type the docker swarm init command.
No Secrets Are No Good
Before we start using secrets lets look at the disadvantages of not using it. Below is a compose file with the definition of a postgres and adminer (a database client) service:
version: '3.1' services: db: image: postgres environment: POSTGRES_USER: myuser POSTGRES_PASSWORD: mysupersecretpassword POSTGRES_DB: mydatabase adminer: image: adminer ports: - 8080:8080
We have supplied the username, password and database name for the postgres service by setting the POSTGRES_USER, POSTGRES_PASSWORD and POSTGRES_DB environment variables (have a look a postgres docker image documentation to see other environment variables which can be used).
You may have noticed that the sensitive details for our database are in plain text for all the world to see. This is poor practice and should only be done for local deployments of containers. In the next section, we will be modifying this example to use secrets.
Securing Our Swarm With Secrets
Let’s dive right in and see how to create secrets.
Open a terminal window and create a secret for the username by typing the following command:
echo "myuser" | docker secret create pg_user -
We have used the docker secret create command above to create a secret called pg_user. The dash “-” at the end of the command is important, it lets docker know that the data for the secret is being taken from stdin.
To view the secret, type the following command:
docker secret ls
You should see a similar output displayed:
ID NAME CREATED UPDATED
dv82o89ngapgr9lrxm6987uqh pg_user 5 days ago 5 days ago
Let’s create the remaining secrets for password and database name:
echo "mysupersecretpassword" | docker secret create pg_password -
echo "mydatabase" | docker secret create pg_database -
Now we need to modify the compose file to use the secrets we created, see below:
version: '3.1' services: db: image: postgres restart: always environment: POSTGRES_USER_FILE: /run/secrets/pg_user POSTGRES_PASSWORD_FILE: /run/secrets/pg_password POSTGRES_DB_FILE: /run/secrets/pg_database secrets: - pg_password - pg_user - pg_database adminer: image: adminer ports: - 8080:8080 secrets: pg_user: external: true pg_password: external: true pg_database: external: true
Create a file called postgres-secrets.yml and copy the text above to it. This file will be used later in this chapter to deploy our services.
We’ve added a few things to make our secrets work. Firstly, environment variables we used previously have been suffixed with “_FILE”. The path to the secret is also specified, see below:
POSTGRES_USER_FILE: /run/secrets/pg_user POSTGRES_PASSWORD_FILE: /run/secrets/pg_password POSTGRES_DB_FILE: /run/secrets/pg_database
Docker secrets are stored in files under the /run/secrets folder of the container. This is why we have to specify new environment variables to read the secrets stored in these files.
Important Note: Not all images have environment variables which are compatible with secrets. In many cases you will have to modify the definition of images (Dockerfile) to read secrets.
Secondly, we specified the name of the secrets which are being used by the service:
secrets: - pg_password - pg_user - pg_database
And last but not least we indicated that the secrets are external:
secrets: pg_user: external: true pg_password: external: true pg_database: external: true
Deploying the service
Let’s deploy our service to determine if our secrets are working as expected, type the following command:
docker stack deploy -c postgres-secrets.yml postgres
Give the services about a minute to fully start, and navigate to http://127.0.0.1:8080/ to view the Adminer interface and enter the details as shown below:
Click the Login button and you should see the admin interface displayed, see below:
If you see the screen above then our secrets are being stored and read by our postgres service. Remember to always use secrets to improve the security of your docker services.