User namespace remapping in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is user namespace remapping in Docker?

User namespace remapping is a Docker feature that maps container user IDs to different user IDs on the host. This improves security by isolating container users from host users.

beginner

Why use user namespace remapping in Docker?

It reduces the risk of privilege escalation by ensuring container root users do not have root privileges on the host system.

intermediate

Which Docker daemon configuration file is used to enable user namespace remapping?

The Docker daemon configuration file is /etc/docker/daemon.json where you can set the "userns-remap" option.

intermediate

What is the effect of setting "userns-remap": "default" in Docker daemon.json?

It enables user namespace remapping using a default user and group created by Docker to map container users to host users.

intermediate

How can you verify if user namespace remapping is active on a running Docker daemon?

Run the command `docker info` and check the "Security Options" section for "name=userns" entries.

What does user namespace remapping do in Docker?

A Removes all user IDs from containers

B Maps container user IDs to different host user IDs

C Disables user permissions inside containers

D Shares the same user IDs between container and host

User namespace remapping maps container user IDs to different host user IDs to improve security.

Where do you configure user namespace remapping in Docker?

A /etc/docker/daemon.json

B /etc/docker/config.yaml

C ~/.docker/config.json

D /usr/bin/docker

User namespace remapping is configured in the Docker daemon configuration file /etc/docker/daemon.json.

What is the default value to enable user namespace remapping in Docker daemon.json?

A "userns-remap": "root"

B "userns-remap": "none"

C "userns-remap": "default"

D "userns-remap": "disabled"

Setting "userns-remap": "default" enables user namespace remapping with Docker's default user.

How can you check if user namespace remapping is active?

A Run `docker info` and look for userns in Security Options

B Run `docker ps` and check container IDs

C Run `docker version` and check client info

D Run `docker logs` on any container

The `docker info` command shows if user namespace remapping is active under Security Options.

What security benefit does user namespace remapping provide?

A Shares host user permissions with containers

B Allows containers to run as host root

C Disables all container users

D Prevents container root from being host root

User namespace remapping prevents container root users from having root privileges on the host.

Explain how user namespace remapping improves Docker container security.

Think about how user IDs inside containers relate to host system users.

Describe the steps to enable user namespace remapping on a Docker host.

Focus on configuration file and verification commands.

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