How to Enable SSH Inside a Docker Container: A Comprehensive Guide and Best Practices

Enabling SSH inside a Docker container involves several steps. Here's a detailed guide on how to do it, along with best practices and considerations to ensure secure and efficient operations.

1. Create a Dockerfile

The first step in enabling SSH within a Docker container is to create a Dockerfile. This file will include instructions to install and configure the SSH server. Here’s an example based on Ubuntu:

Create a Dockerfile:

FROM ubuntu:20.04
ENV DEBIAN_FRONTENDnoninteractive
RUN apt-get update  apt-get install -y openssh-server  mkdir /var/run/sshd
RUN useradd -ms /bin/bash user
RUN echo user:password | chpasswd
EXPOSE 22
CMD ["/usr/sbin/sshd", "-D"]

2. Build the Docker Image

Next, build the Docker image using the created Dockerfile. Navigate to the directory containing the Dockerfile and execute the following command:

docker build -t my-ssh-container .

3. Run the Docker Container

Run the Docker container with the necessary options:

docker run -d -p 2222:22 --name my-running-ssh-container my-ssh-container

This command maps port 2222 on your host to port 22 on the container.

4. Connect via SSH

Now, you can connect to your Docker container using SSH:

ssh -p 2222 

Important Considerations

1. Security

Running an SSH server inside a container may expose your environment to security risks. Consider using Docker’s built-in features for remote management instead of SSH. Securely manage your containers using Docker Compose, Kubernetes, or Docker Swarm for better security.

2. Persisting Changes

If you need to persist data, use Docker volumes or secrets for secure and persistent storage. Avoid storing sensitive information like passwords in the Dockerfile or container configurations.

3. Docker-in-Docker

If you are using Docker-in-Docker, ensure that the SSH server is correctly configured to handle nested containers. This setup can increase complexity and potential security risks.

You Should Not Enable SSH for Your Container

Unless you’re running one of the very few applications that use SSH as their communication protocol, you’re likely using containers incorrectly. SSH is primarily for maintaining machines, not applications.

SSH can get you inside a container to debug, but it also exposes a network port that hackers will target. A major tenet of security is to assume that hackers know your system better than you do. Using docker to run commands inside the container is a far better solution for debugging and management.

Scale Up: Servers Are Like Pets. Scale Out: Servers Are Like Cattle.

Containers should not be treated like pets.

Scale Up: Servers Are Like Pets

You name them and when they get sick, you nurse them back to health.

Scale Out: Servers Are Like Cattle

You number them and when they get sick, you shoot them.

Bill Baker described this analogy to illustrate the philosophy behind container orchestration. In the context of containerized applications, treating containers as "pets" means special care is required for each container's health, which can add significant complexity and cost.

On the other hand, treating containers as "cattle" means using automation and orchestration tools to scale out quickly. If a container goes down, a new one can replace it instantly without special maintenance. This approach ensures immutability and scalability.

Think hard about why you really need SSH in your containers. Losing immutability and scalability is a very costly tradeoff.