Summary
I. Introduction
Docker is a popular containerization technology that allows you to package and run applications in a portable and isolated environment. When running Docker containers, it’s important to consider the permissions and security of the container’s users. By default, Docker containers run as the root user, which can pose security risks and limit the portability of your applications.
In this guide, we’ll explore how to manage Docker user permissions and run containers as different users. We’ll answer common questions such as “with which user your Docker container runs in by default?”, “how do I run a container as a non-root user?”, and “how do I run a Docker container as a root user?”.
We’ll also explain what user 1000 means in Docker, how to see a container’s user ID, and provide examples of working code for running containers as different users.
By the end of this guide, you’ll have a better understanding of Docker user permissions and best practices for managing user permissions in your Docker containers.
II. With which user your docker container runs in by default
By default, Docker containers run as the root user. This means that any processes running within the container have root-level permissions, which can pose security risks if the container is compromised.
To illustrate this, let’s look at an example Dockerfile:
FROM alpine RUN touch /root/testfile
This Dockerfile uses the Alpine Linux base image to create a new image that creates a file named testfile in the root directory. When you run a container using this image, the testfile will be created in the root directory of the container. However, because the container is running as the root user, the testfile will be created with root ownership, even though the container itself may be running with limited privileges.
This can be a security risk because an attacker who gains access to the container can potentially escalate their privileges to gain root access on the host system.
This can be a security risk because an attacker who gains access to the container can potentially escalate their privileges to gain root access on the host system.
To mitigate this risk, it’s recommended to run containers as non-root users whenever possible. In the next section, we’ll explore how to run containers as different users.
*Disclaimer:* Please note that the code example provided in this section is not meant to be executed as it may create a file in the root directory of the container which is not a recommended practice. Instead, it is meant to illustrate the potential security risks associated with running containers as the root user.
III. How do I run a container as a different user?
To run a container as a different user, you can use the --user flag when starting the container. This flag allows you to specify the username or UID/GID of the user that the container should run as.
Here’s an example of running a container as a non-root user:
docker run --user 1000 nginx
In this example, the nginx container is started with the --user flag set to 1000, which is the UID of a non-root user. When the container starts, any processes running within the container will have the same permissions as the 1000 user.
You can also specify the username of the user to run the container as:
docker run --user username nginx
Or you can specify the UID and GID of the user:
docker run --user 1001:1001 nginx
In this example, the nginx container is started with the --user flag set to 1001:1001, which specifies the UID and GID of a specific user.
Important note: It’s important to note that not all images may have the specified user or group configured. In such cases, you may need to create a custom Docker image with the desired user or group.
Here’s a working example of running a container as a non-root user with a custom Docker image:
FROM nginx
RUN adduser --disabled-password --gecos "" myuser
USER myuser
In this example, we’re using the official Nginx Docker image as the base image, and then creating a new image that adds a non-root user named myuser with no password and no additional information. Finally, the USER instruction is used to switch to the myuser user before starting any processes.
When you run a container using this custom image, it will be run as the myuser user instead of the default root user. This can help mitigate potential security risks associated with running containers as the root user.
IV. How do I run a container as a non-root user?
Running containers as a non-root user can help mitigate potential security risks associated with running containers as the root user. When a container is run as a non-root user, any processes running within the container have reduced permissions and cannot perform actions outside of the container’s scope.
To run a container as a non-root user, you can create a new user in a Dockerfile and then use the USER instruction to switch to that user before starting any processes.
To run a container as a non-root user, you can create a new user in a Dockerfile and then use the
USERinstruction to switch to that user before starting any processes.
Here’s an example Dockerfile that creates a new user named myuser with no password and no additional information:
FROM nginx
RUN adduser --disabled-password --gecos "" myuser
USER myuser
In this Dockerfile, we’re using the official Nginx Docker image as the base image, and then creating a new image that adds a non-root user named myuser with no password and no additional information. Finally, the USER instruction is used to switch to the myuser user before starting any processes.
To build the image, you can use the following command:
docker build -t mynginx .
This will build a new Docker image named mynginx that includes the myuser user.
To run a container using this custom image, you can use the docker run command with the --user flag to specify the UID of the myuser user:
docker run --user 1000 mynginx
In this example, the mynginx container is started with the --user flag set to 1000, which is the UID of the myuser user we created in the Dockerfile.
By default, Docker containers run as the root user, which can potentially pose security risks. Running containers as a non-root user can help mitigate these risks by limiting the permissions of any processes running within the container. Creating a new user in a Dockerfile and running containers as that user is a simple way to achieve this.
V. Running Containers as the privileged User
By default, Docker containers run as the root user, which can potentially pose security risks. Running containers as the root user can allow processes running within the container to perform actions outside of the container’s scope and potentially compromise the host system.
However, there may be certain scenarios where running a container as the root user is necessary. In such cases, Docker provides the --privileged flag to run a container as the root user with full capabilities.
To run a container as the root user using the --privileged flag, you can use the following command:
docker run --privileged myimage
In this example, the myimage container is started with the --privileged flag, which gives the container full access to the host system and runs as the root user.
It’s important to exercise caution when running containers as the root user, as this can potentially compromise the host system. Only use the --privileged flag when absolutely necessary.
While it’s generally not recommended to run containers as the root user, there may be certain scenarios where this is necessary. In such cases, Docker provides the --privileged flag to run a container with full capabilities and as the root user. However, it’s important to exercise caution when using this flag, as it can potentially compromise the host system.
VI. Understanding User IDs in Docker
In Docker, user IDs are assigned to users and groups within a container, and can be used to manage permissions and access control.
By default, the first user in a Docker container is assigned user ID 1000. This user is created when the container is built and is often used as the default user for running processes within the container.
In addition to user 1000, Docker also assigns other user IDs for specific purposes. For example, user ID 1001 is typically used for the nobody user, which is a special user account that has no privileges and is used for security purposes.
To see the user ID of a running container, you can use the docker exec command with the id command:
docker exec mycontainer id
In this example, the id command is run within the mycontainer container using the docker exec command, which displays the user ID of the container.
Understanding user IDs in Docker is important for managing permissions and access control within containers. By default, the first user in a container is assigned user ID 1000, and Docker also assigns other user IDs for specific purposes such as the nobody user. You can use the docker exec command with the id command to see the user ID of a running container.
VII. Conclusion
In this article, we’ve explored various aspects of Docker user permissions and how to manage them. We’ve learned that by default, Docker containers run as the first user with ID 1000, which can pose security risks. We’ve also learned how to run containers as different users using the --user flag, and how to create and run containers as non-root users.
In addition, we’ve discussed the risks of running containers as the root user, and how to run containers with elevated privileges using the --privileged flag.
Finally, we’ve explored the significance of user IDs in Docker and how to see the user ID of a running container using the docker exec command.
By following best practices for Docker user permissions, you can ensure that your containers are secure and run with appropriate access controls.
For further learning about Docker user permissions, here are some additional resources:
- Docker documentation on security: https://docs.docker.com/engine/security/
- Docker best practices for security: https://www.docker.com/blog/security-best-practices-for-building-a-container-image/
- Docker tutorial on user namespaces: https://docs.docker.com/engine/security/userns-remap/
- Blog post on securing Docker containers with user namespaces: https://www.redhat.com/sysadmin/user-namespaces-docker
We hope this article has been helpful in understanding Docker user permissions and how to manage them.
