Docker login and authentication - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the docker login command?

The docker login command is used to authenticate a user to a Docker registry by providing credentials. This allows the user to push or pull private images securely.

beginner

Which file stores Docker login credentials on your local machine?

Docker stores login credentials in a file called config.json located in the ~/.docker/ directory on your local machine.

beginner

How do you log in to a private Docker registry named myregistry.example.com?

Use the command:
docker login myregistry.example.com
Then enter your username and password when prompted.

beginner

What happens if you try to pull a private image without logging in first?

Docker will return an authentication error because it cannot access private images without valid credentials.

beginner

How can you log out from a Docker registry?

Use the command docker logout [registry]. If no registry is specified, it logs out from Docker Hub by default.

What command do you use to authenticate to Docker Hub?

A docker login

B docker push

C docker pull

D docker logout

The docker login command is used to authenticate to Docker Hub or any Docker registry.

Where does Docker store your login credentials locally?

A ~/.docker/config.json

B /etc/docker/credentials

C ~/.docker/login.txt

D /var/lib/docker/auth

Docker stores credentials in the config.json file inside the ~/.docker/ directory.

What will happen if you try to pull a private image without logging in?

A Docker automatically logs you in

B You get an authentication error

C The image downloads successfully

D The image is deleted

Pulling private images requires authentication; otherwise, Docker returns an error.

How do you log out from Docker Hub?

A docker disconnect

B docker login --logout

C docker remove login

D docker logout

The docker logout command logs you out from Docker Hub or a specified registry.

Which information do you need to provide when running docker login?

A Image name

B Container ID

C Username and password

D Dockerfile path

You must provide your username and password to authenticate with the registry.

Explain the steps to log in to a private Docker registry and why it is necessary.

Think about how you prove your identity to access private content.

Describe where Docker stores your login credentials and how you can log out from a registry.

Consider the local files and commands managing your login state.

Practice

1. What is the main purpose of the docker login command?

easy

A. To authenticate your computer with a Docker registry

B. To start a Docker container

C. To build a Docker image

D. To remove unused Docker images

Solution

1. Step 1: Understand the role of docker login

   The docker login command is used to provide your credentials to a Docker registry so you can access private images.

2. Step 2: Differentiate from other Docker commands

   Starting containers, building images, or removing images are done by other commands like docker run, docker build, and docker image prune.

3. Final Answer:

   To authenticate your computer with a Docker registry -> Option A

4. Quick Check:

   docker login = authenticate registry [OK]

Hint: Login connects you to registry for private image access [OK]

Common Mistakes:

• Confusing login with image build or container start
• Thinking login removes images
• Assuming login runs containers

2. Which of the following is the correct syntax to login to Docker Hub using the command line?

easy

A. docker login username password

B. docker login -u username

C. docker login --user username

D. docker login --user=username password

Solution

1. Step 1: Recall the docker login syntax

   The correct syntax to login is docker login -u username or docker login --username username. The password is prompted securely after running the command.

2. Step 2: Identify the correct option

   docker login -u username uses the short flag -u correctly without exposing the password inline, which is the recommended way.

3. Final Answer:

   docker login -u username -> Option B

4. Quick Check:

   docker login -u username = correct syntax [OK]

Hint: Use -u or --username, password entered after prompt [OK]

Common Mistakes:

• Passing password directly in command
• Using incorrect flags like --user
• Omitting username flag

3. What will be the output of the following command if the login is successful?

docker login -u user123

medium

A. Error: Invalid username or password

B. Cannot connect to the Docker daemon

C. Usage: docker login [OPTIONS]

D. Login Succeeded

Solution

1. Step 1: Understand the expected output of successful login

   When docker login succeeds, it prints Login Succeeded to confirm authentication.

2. Step 2: Differentiate from error messages

   Other options represent errors or usage info, which appear only if login fails or command is incorrect.

3. Final Answer:

   Login Succeeded -> Option D

4. Quick Check:

   Successful login message = Login Succeeded [OK]

Hint: Success message is 'Login Succeeded' exactly [OK]

Common Mistakes:

• Confusing error messages with success
• Expecting no output on success
• Misreading usage info as success

4. You run docker login -u user123 but get the error: Cannot connect to the Docker daemon. What is the most likely cause?

medium

A. Docker daemon is not running on your machine

B. Incorrect username or password

C. You forgot to specify the registry URL

D. Docker CLI version is outdated

Solution

1. Step 1: Analyze the error message

   The error Cannot connect to the Docker daemon means the Docker service (daemon) is not running or accessible.

2. Step 2: Rule out other causes

   Incorrect credentials or missing registry URL cause authentication errors, not daemon connection errors. CLI version issues usually show different messages.

3. Final Answer:

   Docker daemon is not running on your machine -> Option A

4. Quick Check:

   Daemon connection error = daemon not running [OK]

Hint: Daemon errors mean Docker service is off or unreachable [OK]

Common Mistakes:

• Assuming wrong password causes daemon error
• Thinking registry URL is mandatory for Docker Hub login
• Blaming CLI version for connection issues

5. You want to automate Docker login in a CI/CD pipeline without exposing your password in the command line. Which approach is best?

hard

A. Use docker login -u user -p password directly in the script

B. Write your password in a plain text file and read it in the script

C. Store credentials in environment variables and use echo $PASSWORD | docker login -u user --password-stdin

D. Skip login and pull images anonymously

Solution

1. Step 1: Understand secure login methods for automation

   Using --password-stdin with environment variables avoids exposing passwords in command history or process lists.

2. Step 2: Evaluate other options for security

   Passing password directly in command or storing in plain text files risks exposure. Skipping login may fail for private images.

3. Final Answer:

   Store credentials in environment variables and use echo $PASSWORD | docker login -u user --password-stdin -> Option C

4. Quick Check:

   Use --password-stdin with env vars for secure automation [OK]

Hint: Use --password-stdin and env vars to keep passwords safe [OK]

Common Mistakes:

• Passing password directly in command line
• Storing passwords in plain text files
• Skipping login for private image pulls