Capabilities and privilege control in Docker - Commands & Configuration

Introduction

Sometimes containers need special permissions to do certain tasks, but giving full access can be risky. Capabilities and privilege control let you give containers only the permissions they need, keeping your system safer.

When you want a container to access hardware features like networking or devices without full root access.

When you need to limit what a container can do to reduce security risks.

When running containers that require specific Linux capabilities like changing network settings.

When you want to avoid running containers as fully privileged but still allow some extra permissions.

When debugging or testing container permissions by adding or removing capabilities.

Commands

This command runs an Alpine container and adds the NET_ADMIN capability so it can create a dummy network interface. It shows how to add a specific capability to a container.

Terminal

docker run --rm --cap-add=NET_ADMIN alpine ip link add dummy0 type dummy

Expected Output

No output (command runs silently)

→

--cap-add=NET_ADMIN - Adds the NET_ADMIN capability to the container to allow network administration tasks.

→

--rm - Automatically removes the container after it exits.

This command runs the same Alpine container but with full privileged mode, allowing all capabilities. It shows how privileged mode grants all permissions.

Terminal

docker run --rm --privileged alpine ip link add dummy0 type dummy

Expected Output

No output (command runs silently)

→

--privileged - Gives the container all capabilities and access to devices.

→

--rm - Automatically removes the container after it exits.

This command drops all capabilities and then adds only CHOWN capability. It creates a file, changes its owner, and lists it to show limited capability usage.

Terminal

docker run --rm --cap-drop=ALL --cap-add=CHOWN alpine sh -c 'touch testfile && chown 1000 testfile && ls -l testfile'

Expected Output

-rw-r--r--    1 1000     0             0 Apr 27 00:00 testfile

→

--cap-drop=ALL - Removes all capabilities from the container.

→

--cap-add=CHOWN - Adds only the CHOWN capability to allow changing file ownership.

→

--rm - Automatically removes the container after it exits.

Key Concept

If you remember nothing else from this pattern, remember: give containers only the capabilities they need to reduce security risks.

Common Mistakes

Running containers with --privileged when only a few capabilities are needed.

This gives the container full access, increasing security risks unnecessarily.

Use --cap-add to add only the specific capabilities the container requires.

Not adding required capabilities, causing container commands to fail with permission errors.

The container lacks permissions to perform needed actions.

Identify and add the minimal capabilities needed using --cap-add.

Dropping all capabilities without adding back needed ones.

The container cannot perform even basic tasks it needs.

Use --cap-drop=ALL with --cap-add to precisely control permissions.

Summary

Use --cap-add to give containers specific Linux capabilities they need.

Use --cap-drop to remove unnecessary capabilities and reduce risk.

Use --privileged only when full access is absolutely required.

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