Manage sensitive data with Docker secrets

Estimated reading time: 35 minutes

About secrets

In terms of Docker Swarm services, a secret is a blob of data, such as a password, SSH private key, SSL certificate, or another piece of data that should not be transmitted over a network or stored unencrypted in a Dockerfile or in your application’s source code. In Docker 1.13 and higher, you can use Docker secrets to centrally manage this data and securely transmit it to only those containers that need access to it. Secrets are encrypted during transit and at rest in a Docker swarm. A given secret is only accessible to those services which have been granted explicit access to it, and only while those service tasks are running.

You can use secrets to manage any sensitive data which a container needs at runtime but you don’t want to store in the image or in source control, such as:

  • Usernames and passwords
  • TLS certificates and keys
  • SSH keys
  • Other important data such as the name of a database or internal server
  • Generic strings or binary content (up to 500 kb in size)

Note: Docker secrets are only available to swarm services, not to standalone containers. To use this feature, consider adapting your container to run as a service with a scale of 1.

Another use case for using secrets is to provide a layer of abstraction between the container and a set of credentials. Consider a scenario where you have separate development, test, and production environments for your application. Each of these environments can have different credentials, stored in the development, test, and production swarms with the same secret name. Your containers only need to know the name of the secret in order to function in all three environments.

You can also use secrets to manage non-sensitive data, such as configuration files. However, Docker 17.06 and higher support the use of configs for storing non-sensitive data. Configs are mounted into the container’s filesystem directly, without the use of a RAM disk.

Windows support

Docker 17.06 and higher include support for secrets on Windows containers. Where there are differences in the implementations, they are called out in the examples below. Keep the following notable differences in mind:

  • Microsoft Windows has no built-in driver for managing RAM disks, so within running Windows containers, secrets are persisted in clear text to the container’s root disk. However, the secrets are explicitly removed when a container stops. In addition, Windows does not support persisting a running container as an image using docker commit or similar commands.

  • On Windows, we recommend enabling BitLocker on the volume containing the Docker root directory on the host machine to ensure that secrets for running containers are encrypted at rest.

  • Secret files with custom targets are not directly bind-mounted into Windows containers, since Windows does not support non-directory file bind-mounts. Instead, secrets for a container are all mounted in C:\ProgramData\Docker\internal\secrets (an implementation detail which should not be relied upon by applications) within the container. Symbolic links are used to point from there to the desired target of the secret within the container. The default target is C:\ProgramData\Docker\secrets.

  • When creating a service which uses Windows containers, the options to specify UID, GID, and mode are not supported for secrets. Secrets are currently only accessible by administrators and users with system access within the container.

How Docker manages secrets

When you add a secret to the swarm, Docker sends the secret to the swarm manager over a mutual TLS connection. The secret is stored in the Raft log, which is encrypted. The entire Raft log is replicated across the other managers, ensuring the same high availability guarantees for secrets as for the rest of the swarm management data.

Warning: Raft data is encrypted in Docker 1.13 and higher. If any of your Swarm managers run an earlier version, and one of those managers becomes the manager of the swarm, the secrets will be stored unencrypted in that node’s Raft logs. Before adding any secrets, update all of your manager nodes to Docker 1.13 or higher to prevent secrets from being written to plain-text Raft logs.

When you grant a newly-created or running service access to a secret, the decrypted secret is mounted into the container in an in-memory filesystem. The location of the mount point within the container defaults to /run/secrets/<secret_name> in Linux containers, or C:\ProgramData\Docker\secrets in Windows containers. You can specify a custom location in Docker 17.06 and higher.

You can update a service to grant it access to additional secrets or revoke its access to a given secret at any time.

A node only has access to (encrypted) secrets if the node is a swarm manager or if it is running service tasks which have been granted access to the secret. When a container task stops running, the decrypted secrets shared to it are unmounted from the in-memory filesystem for that container and flushed from the node’s memory.

If a node loses connectivity to the swarm while it is running a task container with access to a secret, the task container still has access to its secrets, but cannot receive updates until the node reconnects to the swarm.

You can add or inspect an individual secret at any time, or list all secrets. You cannot remove a secret that a running service is using. See Rotate a secret for a way to remove a secret without disrupting running services.

In order to update or roll back secrets more easily, consider adding a version number or date to the secret name. This is made easier by the ability to control the mount point of the secret within a given container.

Read more about docker secret commands

Use these links to read about specific commands, or continue to the example about using secrets with a service.

Examples

This section includes three graduated examples which illustrate how to use Docker secrets. The images used in these examples have been updated to make it easier to use Docker secrets. To find out how to modify your own images in a similar way, see Build support for Docker Secrets into your images.

Note: These examples use a single-Engine swarm and unscaled services for simplicity. The examples use Linux containers, but Windows containers also support secrets in Docker 17.06 and higher. See Windows support.

Defining and using secrets in compose files

Both the docker compose and docker stack commands support defining secrets in a compose file. See the Compose file reference for details.

Simple example: Get started with secrets

This simple example shows how secrets work in just a few commands. For a real-world example, continue to Intermediate example: Use secrets with a Nginx service.

  1. Add a secret to Docker. The docker secret create command reads standard input because the last argument, which represents the file to read the secret from, is set to -.

    $ echo "This is a secret" | docker secret create my_secret_data -
    
  2. Create a redis service and grant it access to the secret. By default, the container can access the secret at /run/secrets/<secret_name>, but you can customize the file name on the container using the target option.

    $ docker service  create --name redis --secret my_secret_data redis:alpine
    
  3. Verify that the task is running without issues using docker service ps. If everything is working, the output looks similar to this:

    $ docker service ps redis
    
    ID            NAME     IMAGE         NODE              DESIRED STATE  CURRENT STATE          ERROR  PORTS
    bkna6bpn8r1a  redis.1  redis:alpine  ip-172-31-46-109  Running        Running 8 seconds ago  
    

    If there were an error, and the task were failing and repeatedly restarting, you would see something like this:

    $ docker service ps redis
    
    NAME                      IMAGE         NODE  DESIRED STATE  CURRENT STATE          ERROR                      PORTS
    redis.1.siftice35gla      redis:alpine  moby  Running        Running 4 seconds ago                             
     \_ redis.1.whum5b7gu13e  redis:alpine  moby  Shutdown       Failed 20 seconds ago      "task: non-zero exit (1)"  
     \_ redis.1.2s6yorvd9zow  redis:alpine  moby  Shutdown       Failed 56 seconds ago      "task: non-zero exit (1)"  
     \_ redis.1.ulfzrcyaf6pg  redis:alpine  moby  Shutdown       Failed about a minute ago  "task: non-zero exit (1)"  
     \_ redis.1.wrny5v4xyps6  redis:alpine  moby  Shutdown       Failed 2 minutes ago       "task: non-zero exit (1)"
    
  4. Get the ID of the redis service task container using docker ps , so that you can use docker exec to connect to the container and read the contents of the secret data file, which defaults to being readable by all and has the same name as the name of the secret. The first command below illustrates how to find the container ID, and the second and third commands use shell completion to do this automatically.

    $ docker ps --filter name=redis -q
    
    5cb1c2348a59
    
    $ docker exec $(docker ps --filter name=redis -q) ls -l /run/secrets
    
    total 4
    -r--r--r--    1 root     root            17 Dec 13 22:48 my_secret_data
    
    $ docker exec $(docker ps --filter name=redis -q) cat /run/secrets/my_secret_data
    
    This is a secret
    
  5. Verify that the secret is not available if you commit the container.

    $ docker commit $(docker ps --filter name=redis -q) committed_redis
    
    $ docker run --rm -it committed_redis cat /run/secrets/my_secret_data
    
    cat: can't open '/run/secrets/my_secret_data': No such file or directory
    
  6. Try removing the secret. The removal fails because the redis service is running and has access to the secret.

    
    $ docker secret ls
    
    ID                          NAME                CREATED             UPDATED
    wwwrxza8sxy025bas86593fqs   my_secret_data      4 hours ago         4 hours ago
    
    
    $ docker secret rm my_secret_data
    
    Error response from daemon: rpc error: code = 3 desc = secret
    'my_secret_data' is in use by the following service: redis
    
  7. Remove access to the secret from the running redis service by updating the service.

    $ docker service update --secret-rm my_secret_data redis
    
  8. Repeat steps 3 and 4 again, verifying that the service no longer has access to the secret. The container ID will be different, because the service update command redeploys the service.

    $ docker exec -it $(docker ps --filter name=redis -q) cat /run/secrets/my_secret_data
    
    cat: can't open '/run/secrets/my_secret_data': No such file or directory
    
  9. Stop and remove the service, and remove the secret from Docker.

    $ docker service rm redis
    
    $ docker secret rm my_secret_data
    

Simple example: Use secrets in a Windows service

This is a very simple example which shows how to use secrets with a Microsoft IIS service running on Docker 17.06 EE on Microsoft Windows Server 2016 or Docker for Mac 17.06 on Microsoft Windows 10. It is a naive example that stores the webpage in a secret.

This example assumes that you have PowerShell installed.

  1. Save the following into a new file index.html.

    <html>
      <head><title>Hello Docker</title></head>
      <body>
        <p>Hello Docker! You have deployed a HTML page.</p>
      </body>
    </html>
    
  2. If you have not already done so, initialize or join the swarm.

    PS> docker swarm init
    
  3. Save the index.html file as a swarm secret named homepage.

    PS> docker secret create homepage index.html
    
  4. Create an IIS service and grant it access to the homepage secret.

    PS> docker service create
        --name my-iis
        --publish published=8000,target=8000
        --secret src=homepage,target="\inetpub\wwwroot\index.html"
        microsoft/iis:nanoserver  
    

    Note: There is technically no reason to use secrets for this example. With Docker 17.06 and higher, configs are a better fit. This example is for illustration only.

  5. Access the IIS service at http://localhost:8000/. It should serve the HTML content from the first step.

  6. Remove the service and the secret.

    PS> docker service rm my-iis
    
    PS> docker secret rm homepage
    PS> docker image remove secret-test
    

Intermediate example: Use secrets with a Nginx service

This example is divided into two parts. The first part is all about generating the site certificate and does not directly involve Docker secrets at all, but it sets up the second part, where you store and use the site certificate and Nginx configuration as secrets.

Generate the site certificate

Generate a root CA and TLS certificate and key for your site. For production sites, you may want to use a service such as Let’s Encrypt to generate the TLS certificate and key, but this example uses command-line tools. This step is a little complicated, but is only a set-up step so that you have something to store as a Docker secret. If you want to skip these sub-steps, you can use Let’s Encrypt to generate the site key and certificate, name the files site.key and site.crt, and skip to Configure the Nginx container.

  1. Generate a root key.

    $ openssl genrsa -out "root-ca.key" 4096
    
  2. Generate a CSR using the root key.

    $ openssl req \
              -new -key "root-ca.key" \
              -out "root-ca.csr" -sha256 \
              -subj '/C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA'
    
  3. Configure the root CA. Edit a new file called root-ca.cnf and paste the following contents into it. This constrains the root CA to only be able to sign leaf certificates and not intermediate CAs.

    [root_ca]
    basicConstraints = critical,CA:TRUE,pathlen:1
    keyUsage = critical, nonRepudiation, cRLSign, keyCertSign
    subjectKeyIdentifier=hash
    
  4. Sign the certificate.

    $ openssl x509 -req  -days 3650  -in "root-ca.csr" \
                   -signkey "root-ca.key" -sha256 -out "root-ca.crt" \
                   -extfile "root-ca.cnf" -extensions \
                   root_ca
    
  5. Generate the site key.

    $ openssl genrsa -out "site.key" 4096
    
  6. Generate the site certificate and sign it with the site key.

    $ openssl req -new -key "site.key" -out "site.csr" -sha256 \
              -subj '/C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost'
    
  7. Configure the site certificate. Edit a new file called site.cnf and paste the following contents into it. This constrains the site certificate so that it can only be used to authenticate a server and can’t be used to sign certificates.

    [server]
    authorityKeyIdentifier=keyid,issuer
    basicConstraints = critical,CA:FALSE
    extendedKeyUsage=serverAuth
    keyUsage = critical, digitalSignature, keyEncipherment
    subjectAltName = DNS:localhost, IP:127.0.0.1
    subjectKeyIdentifier=hash
    
  8. Sign the site certificate.

    $ openssl x509 -req -days 750 -in "site.csr" -sha256 \
        -CA "root-ca.crt" -CAkey "root-ca.key"  -CAcreateserial \
        -out "site.crt" -extfile "site.cnf" -extensions server
    
  9. The site.csr and site.cnf files are not needed by the Nginx service, but you will need them if you want to generate a new site certificate. Protect the root-ca.key file.

Configure the Nginx container

  1. Produce a very basic Nginx configuration that serves static files over HTTPS. The TLS certificate and key will be stored as Docker secrets so that they can be rotated easily.

    In the current directory, create a new file called site.conf with the following contents:

    server {
        listen                443 ssl;
        server_name           localhost;
        ssl_certificate       /run/secrets/site.crt;
        ssl_certificate_key   /run/secrets/site.key;
    
        location / {
            root   /usr/share/nginx/html;
            index  index.html index.htm;
        }
    }
    
  2. Create three secrets, representing the key, the certificate, and the site.conf. You can store any file as a secret as long as it is smaller than 500 KB. This allows you to decouple the key, certificate, and configuration from the services that will use them. In each of these commands, the last argument represents the path to the file to read the secret from on the host machine’s filesystem. In these examples, the secret name and the file name are the same.

    $ docker secret create site.key site.key
    
    $ docker secret create site.crt site.crt
    
    $ docker secret create site.conf site.conf
    
    $ docker secret ls
    
    ID                          NAME                  CREATED             UPDATED
    2hvoi9mnnaof7olr3z5g3g7fp   site.key       58 seconds ago      58 seconds ago
    aya1dh363719pkiuoldpter4b   site.crt       24 seconds ago      24 seconds ago
    zoa5df26f7vpcoz42qf2csth8   site.conf      11 seconds ago      11 seconds ago
    
  3. Create a service that runs Nginx and has access to the three secrets. The last part of the docker service create command creates a symbolic link from the location of the site.conf secret to /etc/nginx.conf.d/, where Nginx looks for extra configuration files. This step happens before Nginx actually starts, so you don’t need to rebuild your image if you change the Nginx configuration.

    Note: Normally you would create a Dockerfile which copies the site.conf into place, build the image, and run a container using your custom image. This example does not require a custom image. It puts the site.conf into place and runs the container all in one step.

    In Docker 17.05 and earlier, secrets are always located within the /run/secrets/ directory. Docker 17.06 and higher allow you to specify a custom location for a secret within the container. The two examples below illustrate the difference. The older version of this command requires you to create a symbolic link to the true location of the site.conf file so that Nginx can read it, but the newer version does not require this. The older example is preserved so that you can see the difference.

    • Docker 17.06 and higher:

      $ docker service create \
           --name nginx \
           --secret site.key \
           --secret site.crt \
           --secret source=site.conf,target=/etc/nginx/conf.d/site.conf \
           --publish published=3000,target=443 \
           nginx:latest \
           sh -c "exec nginx -g 'daemon off;'"
      
    • Docker 17.05 and earlier:

      $ docker service create \
           --name nginx \
           --secret site.key \
           --secret site.crt \
           --secret site.conf \
           --publish published=3000,target=443 \
           nginx:latest \
           sh -c "ln -s /run/secrets/site.conf /etc/nginx/conf.d/site.conf && exec nginx -g 'daemon off;'"
      

    The first example shows both the short and long syntax for secrets, and the second example shows only the short syntax. The short syntax creates files in /run/secrets/ with the same name as the secret. Within the running containers, the following three files now exist:

    • /run/secrets/site.key
    • /run/secrets/site.crt
    • /etc/nginx/conf.d/site.conf (or /run/secrets/site.conf if you used the second example)
  4. Verify that the Nginx service is running.

    $ docker service ls
    
    ID            NAME   MODE        REPLICAS  IMAGE
    zeskcec62q24  nginx  replicated  1/1       nginx:latest
    
    $ docker service ps nginx
    
    NAME                  IMAGE         NODE  DESIRED STATE  CURRENT STATE          ERROR  PORTS
    nginx.1.9ls3yo9ugcls  nginx:latest  moby  Running        Running 3 minutes ago
    
  5. Verify that the service is operational: you can reach the Nginx server, and that the correct TLS certificate is being used.

    $ curl --cacert root-ca.crt https://localhost:3000
    
    <!DOCTYPE html>
    <html>
    <head>
    <title>Welcome to nginx!</title>
    <style>
        body {
            width: 35em;
            margin: 0 auto;
            font-family: Tahoma, Verdana, Arial, sans-serif;
        }
    </style>
    </head>
    <body>
    <h1>Welcome to nginx!</h1>
    <p>If you see this page, the nginx web server is successfully installed and
    working. Further configuration is required.</p>
    
    <p>For online documentation and support please refer to
    <a href="http://nginx.org/">nginx.org</a>.<br/>
    Commercial support is available at
    <a href="http://nginx.com/">nginx.com</a>.</p>
    
    <p><em>Thank you for using nginx.</em></p>
    </body>
    </html>
    
    $ openssl s_client -connect localhost:3000 -CAfile root-ca.crt
    
    CONNECTED(00000003)
    depth=1 /C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA
    verify return:1
    depth=0 /C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost
    verify return:1
    ---
    Certificate chain
     0 s:/C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost
       i:/C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA
    ---
    Server certificate
    -----BEGIN CERTIFICATE-----
    …
    -----END CERTIFICATE-----
    subject=/C=US/ST=CA/L=San Francisco/O=Docker/CN=localhost
    issuer=/C=US/ST=CA/L=San Francisco/O=Docker/CN=Swarm Secret Example CA
    ---
    No client certificate CA names sent
    ---
    SSL handshake has read 1663 bytes and written 712 bytes
    ---
    New, TLSv1/SSLv3, Cipher is AES256-SHA
    Server public key is 4096 bit
    Secure Renegotiation IS supported
    Compression: NONE
    Expansion: NONE
    SSL-Session:
        Protocol  : TLSv1
        Cipher    : AES256-SHA
        Session-ID: A1A8BF35549C5715648A12FD7B7E3D861539316B03440187D9DA6C2E48822853
        Session-ID-ctx:
        Master-Key: F39D1B12274BA16D3A906F390A61438221E381952E9E1E05D3DD784F0135FB81353DA38C6D5C021CB926E844DFC49FC4
        Key-Arg   : None
        Start Time: 1481685096
        Timeout   : 300 (sec)
        Verify return code: 0 (ok)
    
  6. To clean up after running this example, remove the nginx service and the stored secrets.

    $ docker service rm nginx
    
    $ docker secret rm site.crt site.key site.conf
    

Advanced example: Use secrets with a WordPress service

In this example, you create a single-node MySQL service with a custom root password, add the credentials as secrets, and create a single-node WordPress service which uses these credentials to connect to MySQL. The next example builds on this one and shows you how to rotate the MySQL password and update the services so that the WordPress service can still connect to MySQL.

This example illustrates some techniques to use Docker secrets to avoid saving sensitive credentials within your image or passing them directly on the command line.

Note: This example uses a single-Engine swarm for simplicity, and uses a single-node MySQL service because a single MySQL server instance cannot be scaled by simply using a replicated service, and setting up a MySQL cluster is beyond the scope of this example.

Also, changing a MySQL root passphrase isn’t as simple as changing a file on disk. You must use a query or a mysqladmin command to change the password in MySQL.

  1. Generate a random alphanumeric password for MySQL and store it as a Docker secret with the name mysql_password using the docker secret create command. To make the password shorter or longer, adjust the last argument of the openssl command. This is just one way to create a relatively random password. You can use another command to generate the password if you choose.

    Note: After you create a secret, you cannot update it. You can only remove and re-create it, and you cannot remove a secret that a service is using. However, you can grant or revoke a running service’s access to secrets using docker service update. If you need the ability to update a secret, consider adding a version component to the secret name, so that you can later add a new version, update the service to use it, then remove the old version.

    The last argument is set to -, which indicates that the input is read from standard input.

    $ openssl rand -base64 20 | docker secret create mysql_password -
    
    l1vinzevzhj4goakjap5ya409
    

    The value returned is not the password, but the ID of the secret. In the remainder of this tutorial, the ID output is omitted.

    Generate a second secret for the MySQL root user. This secret won’t be shared with the WordPress service created later. It’s only needed to bootstrap the mysql service.

    $ openssl rand -base64 20 | docker secret create mysql_root_password -
    

    List the secrets managed by Docker using docker secret ls:

    $ docker secret ls
    
    ID                          NAME                  CREATED             UPDATED
    l1vinzevzhj4goakjap5ya409   mysql_password        41 seconds ago      41 seconds ago
    yvsczlx9votfw3l0nz5rlidig   mysql_root_password   12 seconds ago      12 seconds ago
    

    The secrets are stored in the encrypted Raft logs for the swarm.

  2. Create a user-defined overlay network which will be used for communication between the MySQL and WordPress services. There is no need to expose the MySQL service to any external host or container.

    $ docker network create -d overlay mysql_private
    
  3. Create the MySQL service. The MySQL service will have the following characteristics:

    • Because the scale is set to 1, only a single MySQL task runs. Load-balancing MySQL is left as an exercise to the reader and involves more than just scaling the service.
    • Only reachable by other containers on the mysql_private network.
    • Uses the volume mydata to store the MySQL data, so that it persists across restarts to the mysql service.
    • The secrets are each mounted in a tmpfs filesystem at /run/secrets/mysql_password and /run/secrets/mysql_root_password. They are never exposed as environment variables, nor can they be committed to an image if the docker commit command is run. The mysql_password secret is the one used by the non-privileged WordPress container to connect to MySQL.
    • Sets the environment variables MYSQL_PASSWORD_FILE and MYSQL_ROOT_PASSWORD_FILE to point to the files /run/secrets/mysql_password and /run/secrets/mysql_root_password. The mysql image reads the password strings from those files when initializing the system database for the first time. Afterward, the passwords are stored in the MySQL system database itself.
    • Sets environment variables MYSQL_USER and MYSQL_DATABASE. A new database called wordpress is created when the container starts, and the wordpress user will have full permissions for this database only. This user will not be able to create or drop databases or change the MySQL configuration.

      $ docker service create \
           --name mysql \
           --replicas 1 \
           --network mysql_private \
           --mount type=volume,source=mydata,destination=/var/lib/mysql \
           --secret source=mysql_root_password,target=mysql_root_password \
           --secret source=mysql_password,target=mysql_password \
           -e MYSQL_ROOT_PASSWORD_FILE="/run/secrets/mysql_root_password" \
           -e MYSQL_PASSWORD_FILE="/run/secrets/mysql_password" \
           -e MYSQL_USER="wordpress" \
           -e MYSQL_DATABASE="wordpress" \
           mysql:latest
      
  4. Verify that the mysql container is running using the docker service ls command.

    $ docker service ls
    
    ID            NAME   MODE        REPLICAS  IMAGE
    wvnh0siktqr3  mysql  replicated  1/1       mysql:latest
    

    At this point, you could actually revoke the mysql service’s access to the mysql_password and mysql_root_password secrets because the passwords have been saved in the MySQL system database. Don’t do that for now, because we will use them later to facilitate rotating the MySQL password.

  5. Now that MySQL is set up, create a WordPress service that connects to the MySQL service. The WordPress service has the following characteristics:

    • Because the scale is set to 1, only a single WordPress task runs. Load-balancing WordPress is left as an exercise to the reader, because of limitations with storing WordPress session data on the container filesystem.
    • Exposes WordPress on port 30000 of the host machine, so that you can access it from external hosts. You can expose port 80 instead if you do not have a web server running on port 80 of the host machine.
    • Connects to the mysql_private network so it can communicate with the mysql container, and also publishes port 80 to port 30000 on all swarm nodes.
    • Has access to the mysql_password secret, but specifies a different target file name within the container. The WordPress container will use the mount point /run/secrets/wp_db_password. Also specifies that the secret is not group-or-world-readable, by setting the mode to 0400.
    • Sets the environment variable WORDPRESS_DB_PASSWORD_FILE to the file path where the secret is mounted. The WordPress service will read the MySQL password string from that file and add it to the wp-config.php configuration file.
    • Connects to the MySQL container using the username wordpress and the password in /run/secrets/wp_db_password and creates the wordpress database if it does not yet exist.
    • Stores its data, such as themes and plugins, in a volume called wpdata so these files persist when the service restarts.
    $ docker service create \
         --name wordpress \
         --replicas 1 \
         --network mysql_private \
         --publish published=30000,target=80 \
         --mount type=volume,source=wpdata,destination=/var/www/html \
         --secret source=mysql_password,target=wp_db_password,mode=0400 \
         -e WORDPRESS_DB_USER="wordpress" \
         -e WORDPRESS_DB_PASSWORD_FILE="/run/secrets/wp_db_password" \
         -e WORDPRESS_DB_HOST="mysql:3306" \
         -e WORDPRESS_DB_NAME="wordpress" \
         wordpress:latest
    
  6. Verify the service is running using docker service ls and docker service ps commands.

    $ docker service ls
    
    ID            NAME       MODE        REPLICAS  IMAGE
    wvnh0siktqr3  mysql      replicated  1/1       mysql:latest
    nzt5xzae4n62  wordpress  replicated  1/1       wordpress:latest
    
    $ docker service ps wordpress
    
    ID            NAME         IMAGE             NODE  DESIRED STATE  CURRENT STATE           ERROR  PORTS
    aukx6hgs9gwc  wordpress.1  wordpress:latest  moby  Running        Running 52 seconds ago   
    

    At this point, you could actually revoke the WordPress service’s access to the mysql_password secret, because WordPress has copied the secret to its configuration file wp-config.php. Don’t do that for now, because we will use it later to facilitate rotating the MySQL password.

  7. Access http://localhost:30000/ from any swarm node and set up WordPress using the web-based wizard. All of these settings are stored in the MySQL wordpress database. WordPress automatically generates a password for your WordPress user, which is completely different from the password WordPress uses to access MySQL. Store this password securely, such as in a password manager. You will need it to log into WordPress after rotating the secret.

    Go ahead and write a blog post or two and install a WordPress plugin or theme to verify that WordPress is fully operational and its state is saved across service restarts.

  8. Do not clean up any services or secrets if you intend to proceed to the next example, which demonstrates how to rotate the MySQL root password.

Example: Rotate a secret

This example builds upon the previous one. In this scenario, you create a new secret with a new MySQL password, update the mysql and wordpress services to use it, then remove the old secret.

Note: Changing the password on a MySQL database involves running extra queries or commands, as opposed to just changing a single environment variable or a file, since the image only sets the MySQL password if the database doesn’t already exist, and MySQL stores the password within a MySQL database by default. Rotating passwords or other secrets may involve additional steps outside of Docker.

  1. Create the new password and store it as a secret named mysql_password_v2.

    $ openssl rand -base64 20 | docker secret create mysql_password_v2 -
    
  2. Update the MySQL service to give it access to both the old and new secrets. Remember that you cannot update or rename a secret, but you can revoke a secret and grant access to it using a new target filename.

    $ docker service update \
         --secret-rm mysql_password mysql
    
    $ docker service update \
         --secret-add source=mysql_password,target=old_mysql_password \
         --secret-add source=mysql_password_v2,target=mysql_password \
         mysql
    

    Updating a service causes it to restart, and when the MySQL service restarts the second time, it has access to the old secret under /run/secrets/old_mysql_password and the new secret under /run/secrets/mysql_password.

    Even though the MySQL service has access to both the old and new secrets now, the MySQL password for the WordPress user has not yet been changed.

    Note: This example does not rotate the MySQL root password.

  3. Now, change the MySQL password for the wordpress user using the mysqladmin CLI. This command reads the old and new password from the files in /run/secrets but does not expose them on the command line or save them in the shell history.

    Do this quickly and move on to the next step, because WordPress will lose the ability to connect to MySQL.

    First, find the ID of the mysql container task.

    $ docker ps --filter name=mysql -q
    
    c7705cf6176f
    

    Substitute the ID in the command below, or use the second variant which uses shell expansion to do it all in a single step.

    $ docker exec <CONTAINER_ID> \
        bash -c 'mysqladmin --user=wordpress --password="$(< /run/secrets/old_mysql_password)" password "$(< /run/secrets/mysql_password)"'
    

    or:

    $ docker exec $(docker ps --filter name=mysql -q) \
        bash -c 'mysqladmin --user=wordpress --password="$(< /run/secrets/old_mysql_password)" password "$(< /run/secrets/mysql_password)"'
    
  4. Update the wordpress service to use the new password, keeping the target path at /run/secrets/wp_db_secret and keeping the file permissions at 0400. This will trigger a rolling restart of the WordPress service and the new secret will be used.

    $ docker service update \
         --secret-rm mysql_password \
         --secret-add source=mysql_password_v2,target=wp_db_password,mode=0400 \
         wordpress    
    
  5. Verify that WordPress works by browsing to http://localhost:30000/ on any swarm node again. You’ll need to use the WordPress username and password from when you ran through the WordPress wizard in the previous task.

    Verify that the blog post you wrote still exists, and if you changed any configuration values, verify that they are still changed.

  6. Revoke access to the old secret from the MySQL service and remove the old secret from Docker.

    
    $ docker service update \
         --secret-rm mysql_password \
         mysql
    
    $ docker secret rm mysql_password
    
  7. If you want to try the running all of these examples again or just want to clean up after running through them, use these commands to remove the WordPress service, the MySQL container, the mydata and wpdata volumes, and the Docker secrets.

    $ docker service rm wordpress mysql
    
    $ docker volume rm mydata wpdata
    
    $ docker secret rm mysql_password_v2 mysql_root_password
    

Build support for Docker Secrets into your images

If you develop a container that can be deployed as a service and requires sensitive data, such as a credential, as an environment variable, consider adapting your image to take advantage of Docker secrets. One way to do this is to ensure that each parameter you pass to the image when creating the container can also be read from a file.

Many of the official images in the Docker library, such as the wordpress image used in the above examples, have been updated in this way.

When you start a WordPress container, you provide it with the parameters it needs by setting them as environment variables. The WordPress image has been updated so that the environment variables which contain important data for WordPress, such as WORDPRESS_DB_PASSWORD, also have variants which can read their values from a file (WORDPRESS_DB_PASSWORD_FILE). This strategy ensures that backward compatibility is preserved, while allowing your container to read the information from a Docker-managed secret instead of being passed directly.

Note: Docker secrets do not set environment variables directly. This was a conscious decision, because environment variables can unintentionally be leaked between containers (for instance, if you use --link).

Use Secrets in Compose

version: '3.1'

services:
   db:
     image: mysql:latest
     volumes:
       - db_data:/var/lib/mysql
     environment:
       MYSQL_ROOT_PASSWORD_FILE: /run/secrets/db_root_password
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD_FILE: /run/secrets/db_password
     secrets:
       - db_root_password
       - db_password

   wordpress:
     depends_on:
       - db
     image: wordpress:latest
     ports:
       - "8000:80"
     environment:
       WORDPRESS_DB_HOST: db:3306
       WORDPRESS_DB_USER: wordpress
       WORDPRESS_DB_PASSWORD_FILE: /run/secrets/db_password
     secrets:
       - db_password


secrets:
   db_password:
     file: db_password.txt
   db_root_password:
     file: db_root_password.txt

volumes:
    db_data:

This example creates a simple WordPress site using two secrets in a compose file.

The keyword secrets: defines two secrets db_password: and db_root_password:.

When deploying, Docker will create these two secrets and populate them with the content from the file specified in the compose file.

The db service uses both secrets, and the wordpress is using one.

When you deploy, Docker will mount a file under /run/secrets/<secret_name> in the services. These files are never persisted in disk, but are managed in memory.

Each service uses environment variables to specify where the service should look for that secret data.

More information on short and long syntax for secrets can be found at Compose file version 3 reference.

swarm, secrets, credentials, sensitive strings, sensitive data, security, encryption, encryption at rest