Capabilities and privilege control in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Effect of Dropping a Capability in Docker

You run a Docker container with the command:

docker run --rm --cap-drop=NET_RAW alpine ping -c 1 8.8.8.8

What will be the output or result of this command?

Docker

docker run --rm --cap-drop=NET_RAW alpine ping -c 1 8.8.8.8

A The ping command fails with 'ping: socket: Operation not permitted' error.

B The ping command succeeds and shows the ping statistics.

C Docker throws an error: 'unknown capability: NET_RAW'.

D The container starts but ping command is not found.

💡 Hint

Dropping NET_RAW capability disables raw socket creation needed by ping.

The NET_RAW capability allows creating raw sockets. Dropping it prevents ping from working, causing a permission error.

🧠 Conceptual

intermediate

Understanding Privileged Mode in Docker

What is the main effect of running a Docker container with the --privileged flag?

A It limits the container to only the default capabilities for security.

B It grants the container all Linux capabilities and lifts most security restrictions.

C It disables networking inside the container.

D It runs the container with read-only filesystem.

💡 Hint

Think about what 'privileged' means in terms of access rights.

The --privileged flag gives the container almost all capabilities and disables many security restrictions, allowing full access to host resources.

❓ Configuration

advanced

Configuring a Docker Container to Add a Capability

You want to run a container that needs the ability to change system time. Which Docker run option correctly adds the required capability?

A docker run --privileged=false alpine date

B docker run --cap-drop=SYS_TIME alpine date

C docker run --cap-add=SYS_TIME alpine date

D docker run --cap-add=NET_ADMIN alpine date

💡 Hint

Changing system time requires SYS_TIME capability.

The SYS_TIME capability allows changing the system clock. Adding it with --cap-add=SYS_TIME enables this inside the container.

❓ Troubleshoot

advanced

Diagnosing Permission Issues with Docker Capabilities

A containerized application fails with 'Operation not permitted' when trying to modify network interfaces. The container was started with:

docker run --rm --cap-drop=NET_ADMIN myapp

What is the cause of the failure?

A The image 'myapp' does not have ping installed.

B The container lacks SYS_ADMIN capability needed for network changes.

C The container must be run with --privileged to modify network.

D Dropping NET_ADMIN capability prevents network interface modifications.

💡 Hint

NET_ADMIN controls network administration tasks.

NET_ADMIN capability is required to modify network interfaces. Dropping it causes permission errors for such operations.

✅ Best Practice

expert

Security Best Practice for Running Containers with Capabilities

Which practice best improves container security regarding Linux capabilities?

A Start containers with no capabilities and add only those strictly required.

B Use default capabilities and never modify them.

C Always run containers with --privileged for full functionality.

D Start containers with all capabilities and drop them as needed.

💡 Hint

Think about the principle of least privilege.

Starting containers with no capabilities and adding only those needed minimizes attack surface and improves security.

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