How to use docker login command to authenticate to registries

Introduction

In this lab, you will learn how to authenticate to Docker registries using the docker login command. We will cover authenticating to Docker Hub using the standard web-based login, authenticating to a self-hosted registry with a username and password, and providing the password via STDIN for non-interactive scenarios. This hands-on experience will equip you with the essential skills to securely access and manage images in various Docker registry environments.

Skills Graph

Authenticate to Docker Hub using web-based login

In this step, you will learn how to authenticate to Docker Hub using the web-based login method. This is the most common way to log in to Docker Hub and is suitable for interactive sessions.

Docker Hub is a cloud-based registry service provided by Docker that allows you to store and share your Docker images. To push or pull private images, you need to authenticate with Docker Hub.

The docker login command is used to authenticate with a Docker registry. By default, it authenticates with Docker Hub.

Open your terminal in the LabEx environment. You are in the ~/project directory.

To log in to Docker Hub, run the following command:

docker login

When you run this command, you will be prompted to enter your Docker Hub username and password.

Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com/ to create one.
Username: your_docker_username
Password: your_docker_password

Enter your Docker Hub username and password when prompted. If the authentication is successful, you will see a message similar to this:

WARNING! Your password will be stored unencrypted in /home/labex/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credential-helpers

Login Succeeded

This message indicates that you have successfully logged in to Docker Hub. The warning about the unencrypted password is expected in this scenario and can be ignored for this lab.

Now that you are logged in, you can pull private images from Docker Hub or push your own images to your repository.

Let's try to pull a public image to confirm that Docker commands are working after login. We will pull the hello-world image, which is a small image used for testing.

docker pull hello-world

You should see output indicating that the image is being pulled and extracted:

Using default tag: latest
latest: Pulling from library/hello-world
...
Status: Downloaded newer image for hello-world:latest
docker.io/library/hello-world:latest

This confirms that you can interact with Docker Hub after authenticating.

Authenticate to a self-hosted registry with username and password

In this step, you will learn how to authenticate to a self-hosted Docker registry using a username and password. While Docker Hub is the default registry, you might need to interact with private registries hosted within your organization or on a cloud provider.

A self-hosted registry is a Docker registry that you set up and manage yourself, rather than using a public service like Docker Hub. This is often done for security, compliance, or performance reasons.

To authenticate to a self-hosted registry, you use the same docker login command, but you need to specify the address of the registry. The format is docker login <registry_address>.

For this lab, we will simulate a self-hosted registry. We will use a placeholder address myregistry.local. In a real-world scenario, this would be the actual domain name or IP address of your registry.

Open your terminal in the ~/project directory.

To log in to our simulated self-hosted registry, run the following command, replacing your_registry_username and your_registry_password with hypothetical credentials you would use for that registry:

docker login myregistry.local

You will be prompted to enter the username and password for the registry myregistry.local:

Login with your Docker ID to push and pull images from myregistry.local. If you don't have a Docker ID, head over to https://hub.docker.com/ to create one.
Username: your_registry_username
Password: your_registry_password

Enter the hypothetical username and password. Since myregistry.local is not a real, running registry in this environment, the login will likely fail with an error message indicating that the registry is unreachable or authentication failed. This is expected for this simulation. The important part is understanding the command syntax and the process of specifying a different registry.

Error response from daemon: Get "http://myregistry.local/v2/": dial tcp: lookup myregistry.local on 127.0.0.53:53: no such host

Even though the login failed because the registry doesn't exist, the command syntax docker login <registry_address> is correct for attempting to authenticate to a self-hosted registry.

In a real scenario with a running self-hosted registry, a successful login would result in a "Login Succeeded" message, and your credentials for that specific registry would be stored in your Docker configuration file (~/.docker/config.json).

Let's examine the Docker configuration file to see how registry information is stored.

cat ~/.docker/config.json

You will see a JSON structure. After a successful login to Docker Hub in the previous step, you should see an entry for https://index.docker.io/v1/. If you had successfully logged in to myregistry.local, you would see an additional entry for that registry address.

{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "..."
    }
  },
  "HttpHeaders": {
    "User-Agent": "Docker-Client/..."
  },
  "credsStore": "desktop"
}

The auths section contains authentication information for different registries you have logged into. Each key in auths is the registry address, and the value contains the authentication details.

This step demonstrated how to specify a self-hosted registry address when using docker login.

Provide password using STDIN for non-interactive login

In this step, you will learn how to provide your Docker registry password using standard input (STDIN). This method is particularly useful for non-interactive scenarios, such as scripting or automated deployments, where you cannot manually enter the password when prompted.

When you use docker login in an interactive terminal, it prompts you for the password. However, in scripts, you need a way to pass the password without manual intervention. The --password-stdin flag allows you to read the password from STDIN.

Important Security Note: Passing passwords directly on the command line is generally insecure because the command and its arguments might be visible in process lists (ps) or shell history. Reading the password from STDIN is a more secure approach for scripting as it avoids exposing the password in these places.

Open your terminal in the ~/project directory.

To demonstrate this, we will simulate logging in to Docker Hub using STDIN. You will need your Docker Hub username and password.

The command structure is to echo the password and pipe it to docker login --username <your_docker_username> --password-stdin.

Replace your_docker_username with your actual Docker Hub username and your_docker_password with your actual Docker Hub password.

echo "your_docker_password" | docker login --username your_docker_username --password-stdin

After running this command, you should see output similar to this, indicating a successful login:

WARNING! Your password will be stored unencrypted in /home/labex/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credential-helpers

Login Succeeded

Again, the warning about the unencrypted password is expected in this lab environment.

This method is crucial for automating Docker operations that require authentication. For example, in a CI/CD pipeline, you might store your Docker Hub credentials as environment variables and use this method to log in before building and pushing images.

You can also use this method to log in to a self-hosted registry by adding the registry address at the end of the command:

echo "your_registry_password" | docker login myregistry.local --username your_registry_username --password-stdin

As in the previous step, this command will likely fail in this lab because myregistry.local is not a running registry. However, the syntax for providing the password via STDIN for a specific registry is correct.

This step has shown you how to perform a non-interactive login to a Docker registry by piping the password to the docker login command using the --password-stdin flag.

Summary

In this lab, you learned how to authenticate to Docker registries using the docker login command. You first practiced authenticating to Docker Hub using the standard interactive web-based login method, providing your username and password when prompted. This demonstrated the basic process of logging in to the default Docker registry.

You then explored authenticating to a self-hosted registry by specifying the registry URL along with a username and password. Finally, you learned how to provide the password using STDIN, which is a more secure method for non-interactive scenarios and scripting, preventing the password from being exposed in command history or logs. These steps covered different methods for securing access to Docker registries for pushing and pulling images.