How to SSH into Docker Containers With Examples

Sometimes, you need to access a running Docker container using SSH to troubleshoot issues or manage configurations. While Docker does not enable SSH by default, there are multiple ways to achieve this securely, including alternatives that may be preferable, which I'll cover below.

Why SSH into a Docker Container?

Debugging and troubleshooting running applications.
Managing configurations and making live updates.
Running administrative tasks inside the container.
Inspecting logs or files inside a containerized environment.

Methods to Access a Docker Container

There are multiple ways to access a running Docker container, depending on the use case:

1. Using docker exec (Recommended Method)

Instead of SSH, Docker provides the docker exec command to access a running container’s shell.

Syntax:

docker exec -it container_name bash

Or, if the container uses sh instead of bash:

docker exec -it container_name sh

This command starts an interactive terminal session inside the container, allowing users to interact with the bash shell or another default shell available in the container.

2. Using docker attach

This method attaches your terminal to the running container’s process.

Syntax:

docker attach container_name

However, this method may not be ideal for interactive sessions, as it connects to the container’s main process.

3. SSH into a Container Running an SSH Server

If SSH is required, you need to install and configure an SSH server inside the container.

Steps to Enable SSH in a Container:

Modify the Dockerfile to Install SSH

FROM ubuntu:latest
RUN apt-get update && apt-get install -y openssh-server
RUN mkdir /var/run/sshd
CMD ["/usr/sbin/sshd", "-D"]

Build and Run the Container

docker build -t ssh-container .
docker run -d -p 2222:22 --name ssh-container ssh-container

Connect to the Container via SSH

ssh root@localhost -p 2222

You may need to set up an RSA key for authentication or use a root password for login.

Common Questions on Docker Container SSH

How to SSH in a Docker container?

Say you've finished your Docker course, but you still need to know how to SSH into a Docker container. Firstly, ensure that an SSH server is installed inside the container and the necessary port (typically 22) is exposed. Then, you can connect using:

ssh root@localhost -p 2222

Alternatively, use docker exec -it container_name bash for direct shell access without SSH.

How do I login into a Docker container?

You can log in to a running Docker container using:

docker exec -it container_name bash

If the container uses sh instead of bash, use:

docker exec -it container_name sh

This command allows you to access the container interactively. You can also find the container ID using:

docker ps

Then, login using:

docker exec -it container-id bash

How to expose port 22 in Docker container?

To expose port 22 for SSH access, you need to map it when running the container:

docker run -d -p 2222:22 --name my_container my_ssh_image

This maps port 22 inside the container to port 2222 on the host machine, allowing SSH connections. You can then find the container's IP address using:

docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' container_name

Once you have the IP address, connect using:

ssh username@container-ip-address -p 2222

How to add SSL to Docker container?

To add SSL to a Docker container, install an SSL certificate inside the container. For example, to set up SSL in an Nginx container:

Copy your SSL certificate and key into the container:

docker cp cert.pem my_container:/etc/ssl/certs/
docker cp key.pem my_container:/etc/ssl/private/

Modify the Nginx config file inside the container:

server {
    listen 443 ssl;
    ssl_certificate /etc/ssl/certs/cert.pem;
    ssl_certificate_key /etc/ssl/private/key.pem;
    location / {
        proxy_pass http://localhost:80;
    }
}

Restart the Nginx service:

docker restart my_container

This enables SSL inside the container.

Best Practices for Secure Container Access

Use docker exec instead of SSH unless absolutely necessary.
Restrict SSH access by limiting exposed ports and using firewalls.
Use SSH keys instead of passwords for authentication.
Run containers with minimal privileges to reduce security risks.
Store sensitive data such as passwords and keys securely, avoiding direct inclusion in a Docker image.

Key Takeaways

Docker does not require SSH by default, as docker exec provides direct shell access.
SSH can be installed in a container if remote access is needed.
Ports must be exposed to enable SSH connections.
SSL can be configured inside a Docker container to secure communications.
Use docker ps and docker inspect to find running containers and retrieve IP addresses.
Security best practices should always be followed to protect containerized environments.

Wrapping Up

Accessing a running Docker container can be done efficiently using docker exec, docker attach, or SSH if necessary. While SSH should be used cautiously, it can be a useful tool for debugging and administration.

Understanding the best approach to access containers ensures better security and management of your Docker environments. By using SSH keys, RSA authentication, and proper config settings, you can enhance security while managing your Docker containers effectively.