User namespace remapping in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to enable user namespace remapping in Docker daemon.

Docker

dockerd --userns-[1]

Drag options to blanks, or click blank then click option'

Auser

Benable

Cmap

Dremap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'enable' or 'map' instead of 'remap' causes the command to fail.

Omitting the flag entirely disables user namespace remapping.

The correct flag to enable user namespace remapping is --userns-remap.

2fill in blank

medium

Complete the daemon.json snippet to set user namespace remapping to the default user.

Docker

{
  "userns-remap": "[1]"
}

Drag options to blanks, or click blank then click option'

Aroot

Bdefault

Cnobody

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'root' disables remapping.

Using 'nobody' is not a valid remapping user.

Setting "userns-remap": "default" enables remapping to the default user created by Docker.

3fill in blank

hard

Fix the error in this Docker run command to use user namespace remapping correctly.

Docker

docker run --userns [1] alpine id

Drag options to blanks, or click blank then click option'

Aremap

Bmap

Cdefault

Duserns-remap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remap' causes an unknown mode error.

Using 'userns-remap' is invalid for this flag.

The correct option for the --userns flag is default to enable remapping.

4fill in blank

hard

Fill both blanks to create a user namespace remapping entry in /etc/subuid and /etc/subgid.

Docker

dockremap:[1]:[2]

Drag options to blanks, or click blank then click option'

A100000

B65536

C65535

D100001

Attempts:

3 left

💡 Hint

Common Mistakes

Using 65535 instead of 65536 causes off-by-one errors.

Using 100001 as start UID is incorrect for default remapping.

The user namespace remapping uses UID 100000 and a range of 65536 IDs by default.

5fill in blank

hard

Fill all three blanks to complete the Docker daemon configuration for user namespace remapping with a custom user.

Docker

{
  "userns-remap": "[1]",
  "subuid": "[2]",
  "subgid": "[3]"
}

Drag options to blanks, or click blank then click option'

Acustomuser

B100000-165535

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' as user disables custom remapping.

Mismatching UID and GID ranges causes errors.

To use a custom user for remapping, set userns-remap to the username, and specify the UID and GID ranges in subuid and subgid respectively.

Practice

1.

What is the main purpose of user namespace remapping in Docker?

easy

A. To allow containers to use more CPU cores

B. To increase container startup speed

C. To enable automatic container backups

D. To improve security by isolating container user IDs from host user IDs

Solution

1. Step 1: Understand user namespace remapping concept

   User namespace remapping changes container user IDs so they do not match host user IDs.

2. Step 2: Identify the security benefit

   This isolation reduces risk if a container is compromised, protecting the host system.

3. Final Answer:

   To improve security by isolating container user IDs from host user IDs -> Option D

4. Quick Check:

   User namespace remapping = security isolation [OK]

Hint: User namespace remapping isolates users for security [OK]

Common Mistakes:

• Thinking it improves performance
• Confusing with resource limits
• Assuming it enables backups

2.

Which of the following is the correct way to enable user namespace remapping in Docker's daemon.json file?

{
  "userns-remap": "default"
}

easy

A. { "userns-remap": true }

B. { "userns-remap": "enabled" }

C. { "userns-remap": "default" }

D. { "userns-remap": 1 }

Solution

1. Step 1: Check Docker daemon configuration syntax

   The correct key is "userns-remap" and it expects a string value like "default" or a username.

2. Step 2: Validate options

   Only { "userns-remap": "default" } uses the correct key and a valid string value "default".

3. Final Answer:

   { "userns-remap": "default" } -> Option C

4. Quick Check:

   Correct key and string value = { "userns-remap": "default" } [OK]

Hint: Use string "default" for userns-remap in daemon.json [OK]

Common Mistakes:

• Using boolean true instead of string
• Using numeric values
• Using invalid strings like "enabled"

3.

Given Docker daemon configured with "userns-remap": "default", what will be the output of id -u inside a container run as root?

medium

A. 100000

B. 0

C. 65534

D. 1

Solution

1. Step 1: Understand user ID remapping

   When user namespace remapping is enabled, the container runs in its own user namespace where root is UID 0 inside the container.

2. Step 2: Identify the output of id -u inside container

   Inside the container, id -u for the root user reports 0, as it views UIDs from the container's namespace perspective. The host sees the remapped UID (e.g., 100000).

3. Final Answer:

   0 -> Option B

4. Quick Check:

   id -u inside container shows 0 for root [OK]

Hint: Inside container, root UID is 0 despite host remapping [OK]

Common Mistakes:

• Confusing container-internal UID (0) with host-remapped UID (100000)
• Thinking remapping changes UID view inside container
• Choosing 65534 which is nobody user

4.

You enabled user namespace remapping in Docker but containers fail to start with permission errors. What is the most likely cause?

medium

A. The remapped user IDs do not have permission on container files

B. Docker daemon.json syntax is incorrect

C. Containers are using too much CPU

D. Docker version does not support user namespaces

Solution

1. Step 1: Analyze permission errors after enabling userns-remap

   User namespace remapping changes container user IDs, so files owned by root on host may not be accessible by remapped IDs.

2. Step 2: Identify cause of permission failure

   If the remapped user IDs lack permissions on container files or volumes, containers fail to start with permission denied errors.

3. Final Answer:

   The remapped user IDs do not have permission on container files -> Option A

4. Quick Check:

   Permission errors = remapped UID lacks file access [OK]

Hint: Check file permissions for remapped user IDs [OK]

Common Mistakes:

• Blaming daemon.json syntax without checking logs
• Assuming CPU limits cause permission errors
• Ignoring Docker version compatibility

5.

You want to run a container with user namespace remapping but need the container's root user to have access to a host directory /data. What is the best way to configure this?

hard

A. Change ownership of /data on host to the remapped UID range

B. Disable user namespace remapping for this container

C. Run the container as a non-root user

D. Mount /data as read-only volume

Solution

1. Step 1: Understand user namespace remapping effect on file access

   With remapping, container root maps to a high UID on host, so host files must be owned or accessible by that UID.

2. Step 2: Adjust host directory ownership

   Changing /data ownership to the remapped UID range allows container root to access it safely without disabling remapping.

3. Final Answer:

   Change ownership of /data on host to the remapped UID range -> Option A

4. Quick Check:

   Host directory ownership matches remapped UID for access [OK]

Hint: Match host directory ownership to remapped UID [OK]

Common Mistakes:

• Disabling remapping loses security benefits
• Running as non-root may not solve access needs
• Mounting read-only prevents write access