Content trust and image signing in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Docker Content Trust?

Docker Content Trust is a feature that uses digital signatures to verify the integrity and publisher of Docker images before they are pulled or run.

beginner

How does image signing improve security in Docker?

Image signing ensures that the image you use is exactly what the publisher created, preventing tampering or use of malicious images.

beginner

Which command enables Docker Content Trust for your session?

You enable Docker Content Trust by setting the environment variable: export DOCKER_CONTENT_TRUST=1.

intermediate

What is the role of Notary in Docker Content Trust?

Notary is the tool Docker uses to create and manage trusted collections of signed images, ensuring image authenticity.

beginner

What happens if you try to pull an unsigned image with Docker Content Trust enabled?

Docker will refuse to pull the image and show an error because it cannot verify the image's signature.

What environment variable do you set to enable Docker Content Trust?

A DOCKER_SIGN_IMAGES=yes

B DOCKER_CONTENT_TRUST=1

C CONTENT_TRUST=enabled

D DOCKER_TRUST_ENABLE=true

Setting DOCKER_CONTENT_TRUST=1 enables Docker Content Trust for the session.

What does Docker Content Trust verify before pulling an image?

A Image size

B Image download speed

C Image creation date

D Image signature and publisher

Docker Content Trust verifies the image's digital signature and publisher identity.

Which tool does Docker use to manage image signing keys?

A Kubernetes

B Helm

C Notary

D Docker Compose

Notary manages signing keys and trusted collections for Docker Content Trust.

If Docker Content Trust is enabled, what happens when pulling an unsigned image?

A Docker refuses to pull the image

B Docker pulls a warning but continues

C Image is pulled without verification

D Docker signs the image automatically

Docker refuses to pull unsigned images when Content Trust is enabled.

Why is image signing important in Docker workflows?

A To verify image authenticity and prevent tampering

B To speed up image downloads

C To reduce image size

D To enable multi-platform builds

Image signing ensures the image is authentic and has not been tampered with.

Explain how Docker Content Trust helps secure your container images.

Think about how you can trust the source and content of images before using them.

Describe the steps to enable and use Docker Content Trust when pulling images.

Consider what environment variable you set and what happens during image pull.

Practice

1. What is the main purpose of Docker content trust?

easy

A. To speed up Docker image downloads

B. To verify the authenticity and integrity of Docker images

C. To compress Docker images for storage

D. To automatically update Docker images

Solution

1. Step 1: Understand content trust concept

   Content trust in Docker is designed to ensure images are verified and safe to use by checking their signatures.

2. Step 2: Identify the purpose from options

   Only To verify the authenticity and integrity of Docker images mentions verifying authenticity and integrity, which matches content trust's goal.

3. Final Answer:

   To verify the authenticity and integrity of Docker images -> Option B

4. Quick Check:

   Content trust = verify image authenticity [OK]

Hint: Content trust means verifying image signatures before use [OK]

Common Mistakes:

• Confusing content trust with image compression
• Thinking content trust speeds up downloads
• Assuming content trust auto-updates images

2. Which command correctly enables Docker content trust before pulling an image?

easy

A. set DOCKER_CONTENT_TRUST=1 docker pull alpine

B. docker pull --trust alpine

C. export DOCKER_CONTENT_TRUST=1 && docker pull alpine

D. docker pull alpine --enable-content-trust

Solution

1. Step 1: Recall how to enable content trust

   Content trust is enabled by setting the environment variable DOCKER_CONTENT_TRUST=1 before running docker pull.

2. Step 2: Match correct syntax

   export DOCKER_CONTENT_TRUST=1 && docker pull alpine uses the correct shell syntax to export the variable and then pull the image.

3. Final Answer:

   export DOCKER_CONTENT_TRUST=1 && docker pull alpine -> Option C

4. Quick Check:

   Set env var DOCKER_CONTENT_TRUST=1 before pull [OK]

Hint: Use export DOCKER_CONTENT_TRUST=1 before docker pull [OK]

Common Mistakes:

• Using incorrect flags like --trust or --enable-content-trust
• Trying to set env var inline without export
• Confusing Windows and Linux environment variable syntax

3. What will happen if you run docker pull alpine with DOCKER_CONTENT_TRUST=1 set but the image is unsigned?

medium

A. The image will be pulled without verification

B. Docker will automatically sign the image locally

C. Docker will pull the image but warn about missing signature

D. Docker will refuse to pull the image and show an error

Solution

1. Step 1: Understand content trust behavior on unsigned images

   When content trust is enabled, Docker requires a valid signature to pull an image.

2. Step 2: Identify expected Docker response

   If the image is unsigned, Docker refuses to pull it and shows an error to prevent unverified usage.

3. Final Answer:

   Docker will refuse to pull the image and show an error -> Option D

4. Quick Check:

   Content trust blocks unsigned image pulls [OK]

Hint: Content trust blocks unsigned images from pulling [OK]

Common Mistakes:

• Thinking Docker pulls unsigned images anyway
• Assuming Docker signs images automatically
• Believing Docker only warns but allows pull

4. You enabled content trust but get an error: no trust data available when pulling an image. What is the likely cause?

medium

A. The image has no signed tags in the registry

B. Docker daemon is not running

C. You forgot to set DOCKER_CONTENT_TRUST=0

D. Your internet connection is down

Solution

1. Step 1: Analyze the error message meaning

   'No trust data available' means Docker cannot find signed metadata for the image tags in the registry.

2. Step 2: Match cause to options

   The image has no signed tags in the registry correctly states the image lacks signed tags, causing this error when content trust is enabled.

3. Final Answer:

   The image has no signed tags in the registry -> Option A

4. Quick Check:

   No trust data = no signed tags in registry [OK]

Hint: No trust data means image tags are unsigned [OK]

Common Mistakes:

• Assuming Docker daemon stopped causes this error
• Thinking DOCKER_CONTENT_TRUST=0 fixes error when trust is needed
• Blaming internet connection without checking image signing

5. You want to ensure only signed images are deployed in your production environment. Which approach best achieves this?

hard

A. Set DOCKER_CONTENT_TRUST=1 on all production servers and pull images normally

B. Manually verify image signatures after pulling unsigned images

C. Disable content trust and rely on image tags naming conventions

D. Use docker pull --no-trust to skip signature checks

Solution

1. Step 1: Identify how to enforce signed image usage

   Setting DOCKER_CONTENT_TRUST=1 forces Docker to only pull images with valid signatures, blocking unsigned ones.

2. Step 2: Evaluate options for production safety

   Set DOCKER_CONTENT_TRUST=1 on all production servers and pull images normally correctly enforces signature verification automatically on production servers.

3. Final Answer:

   Set DOCKER_CONTENT_TRUST=1 on all production servers and pull images normally -> Option A

4. Quick Check:

   Enable content trust to enforce signed images [OK]

Hint: Enable DOCKER_CONTENT_TRUST=1 in production for safety [OK]

Common Mistakes:

• Trying to verify signatures manually after pull
• Disabling content trust and trusting tags only
• Using flags to skip trust checks in production