Capabilities and privilege control in Docker - Mini Project: Build & Apply

Docker Capabilities and Privilege Control

📖 Scenario: You are managing Docker containers for a small web application. You want to control what system capabilities the containers have to improve security. This means allowing only the minimum permissions needed for the container to run.

🎯 Goal: Learn how to run a Docker container with specific Linux capabilities removed and understand how to run a container without full root privileges.

📋 What You'll Learn

Create a Docker container running the alpine image

Remove the NET_RAW capability from the container

Run the container with the --cap-drop option

Run the container without root privileges using the --user option

Verify the non-root user and dropped NET_RAW capability inside the container

💡 Why This Matters

🌍 Real World

Limiting container capabilities reduces the risk of security breaches by restricting what the container can do on the host system.

💼 Career

DevOps engineers and system administrators use capability and privilege controls to harden containerized applications in production environments.

1

Create a Docker container using the alpine image

Write a Docker command to run a container named secure_container using the alpine image in interactive mode with a shell.

Docker

# Write the docker run command to start an alpine container named secure_container interactively
# Your code here

Hint

Use docker run -it --name secure_container alpine sh to start an interactive shell in the alpine container.

2

Remove the NET_RAW capability from the container

Modify the Docker run command to drop the NET_RAW capability by adding the --cap-drop=NET_RAW option. Keep the container name secure_container and run interactively with shell.

Docker

docker run -it --name secure_container alpine sh
# Add --cap-drop=NET_RAW option to the docker run command above
# Your code here

Hint

Add --cap-drop=NET_RAW to the docker run command to remove the NET_RAW capability.

3

Run the container without root privileges

Modify the Docker run command to run the container as a non-root user by adding the option --user 1000. Keep the container name secure_container, drop NET_RAW capability, and run interactively with shell.

Docker

docker run -it --name secure_container --cap-drop=NET_RAW alpine sh
# Add --user 1000 option to run container as non-root user
# Your code here

Hint

Add --user 1000 to run the container as a non-root user with UID 1000.

4

Verify capabilities inside the container

Inside the running container, run id to confirm the non-root user and ping -c 1 127.0.0.1 to verify that NET_RAW is dropped (ping should fail with permission denied).

Docker

docker run -it --name secure_container --cap-drop=NET_RAW --user 1000 alpine sh
# Inside the container, run id and ping -c 1 127.0.0.1 to verify
# Your code here

Hint

Use id to see the user (should show uid=1000) and ping -c 1 127.0.0.1 to test NET_RAW (should fail).

Practice

1. What does the --cap-drop option do when running a Docker container?

easy

A. Removes specific Linux capabilities from the container

B. Adds new capabilities to the container

C. Runs the container with full root privileges

D. Limits the container's CPU usage

Solution

1. Step 1: Understand Docker capabilities

   Capabilities are Linux kernel permissions that control what a container can do on the host.

2. Step 2: Role of --cap-drop

   This option removes specific capabilities from the container, reducing its privileges.

3. Final Answer:

   Removes specific Linux capabilities from the container -> Option A

4. Quick Check:

   --cap-drop means remove capabilities [OK]

Hint: Drop caps to reduce container privileges quickly [OK]

Common Mistakes:

• Confusing --cap-drop with --cap-add
• Thinking it controls CPU or memory
• Assuming it grants root access

2. Which of the following is the correct syntax to add the NET_ADMIN capability to a Docker container?

easy

A. docker run --cap NET_ADMIN myimage

B. docker run --add-cap NET_ADMIN myimage

C. docker run --capability-add NET_ADMIN myimage

D. docker run --cap-add NET_ADMIN myimage

Solution

1. Step 1: Recall Docker capability syntax

   The correct flag to add capabilities is --cap-add.

2. Step 2: Match the correct command

   docker run --cap-add NET_ADMIN myimage uses --cap-add NET_ADMIN, which is the exact syntax Docker expects.

3. Final Answer:

   docker run --cap-add NET_ADMIN myimage -> Option D

4. Quick Check:

   Use --cap-add to add capabilities [OK]

Hint: Remember: add = --cap-add, drop = --cap-drop [OK]

Common Mistakes:

• Using incorrect flags like --add-cap
• Omitting the capability name
• Using --cap or --capability-add which are invalid

3. What will be the effect of running this command?

docker run --cap-drop ALL --cap-add CHOWN alpine id

medium

A. The container has only the CHOWN capability enabled

B. The container has all capabilities except CHOWN

C. The container runs with full root privileges

D. The container will fail to start due to conflicting options

Solution

1. Step 1: Analyze --cap-drop ALL

   This removes all Linux capabilities from the container, making it very restricted.

2. Step 2: Analyze --cap-add CHOWN

   This adds back the CHOWN capability, allowing the container to change file ownership.

3. Final Answer:

   The container has only the CHOWN capability enabled -> Option A

4. Quick Check:

   Drop all then add one = only that one capability [OK]

Hint: Drop ALL then add one = only that one capability [OK]

Common Mistakes:

• Thinking --cap-add adds on top without drop effect
• Assuming container runs with full privileges
• Believing conflicting options cause failure

4. You want to run a container with the SYS_TIME capability but accidentally use this command:

docker run --cap-drop SYS_TIME myimage

What is the problem and how to fix it?

medium

A. Problem: Syntax error in command. Fix: Remove --cap-drop

B. Problem: SYS_TIME is dropped, so container can't change time. Fix: Use --cap-add SYS_TIME

C. Problem: Container runs with full privileges. Fix: Add --cap-drop ALL

D. Problem: SYS_TIME is not a valid capability. Fix: Use a valid capability name

Solution

1. Step 1: Understand the effect of --cap-drop SYS_TIME

   This removes the SYS_TIME capability, preventing the container from changing system time.

2. Step 2: Correct the command to add the capability

   To allow changing time, use --cap-add SYS_TIME instead of dropping it.

3. Final Answer:

   Problem: SYS_TIME is dropped, so container can't change time. Fix: Use --cap-add SYS_TIME -> Option B

4. Quick Check:

   Drop removes, add grants capabilities [OK]

Hint: Add needed caps, don't drop them by mistake [OK]

Common Mistakes:

• Confusing --cap-drop with --cap-add
• Assuming dropping caps grants them
• Thinking SYS_TIME is invalid capability

5. You want to run a container that can modify network settings but nothing else. Which command best limits privileges to only what is needed?

hard

A. docker run --cap-drop NET_ADMIN myimage

B. docker run --privileged myimage

C. docker run --cap-drop ALL --cap-add NET_ADMIN myimage

D. docker run --cap-add ALL myimage

Solution

1. Step 1: Understand the goal

   The container should only have network admin rights, no other privileges.

2. Step 2: Evaluate options

   docker run --cap-drop ALL --cap-add NET_ADMIN myimage drops all capabilities then adds only NET_ADMIN, perfectly limiting privileges.

3. Step 3: Why others are wrong

   --privileged grants full privileges, --cap-add ALL adds all capabilities (too much), --cap-drop NET_ADMIN removes NET_ADMIN (opposite of goal).

4. Final Answer:

   docker run --cap-drop ALL --cap-add NET_ADMIN myimage -> Option C

5. Quick Check:

   Drop all then add needed cap = minimal privileges [OK]

Hint: Drop all caps, then add only needed ones [OK]

Common Mistakes:

• Using --privileged which grants too many rights
• Adding all capabilities instead of limiting
• Dropping the needed capability by mistake