Easily Test SSH Connections with Alpine Docker

Sometimes you need a quick and disposable SSH server for testing. An Alpine-based Docker container is the perfect solution: lightweight, fast to build, and easy to tear down.

Building the Docker Image

The Docker image is constructed using Alpine Linux, a lightweight and security-oriented Linux distribution.

FROM alpine:latest
RUN apk update && apk add openssh
RUN adduser -D testuser
RUN echo "testuser:password" | chpasswd
RUN echo "root:rootpassword" | chpasswd
# Generate SSH host keys
RUN ssh-keygen -A
# Configure SSH to allow root login
RUN sed -i 's/#PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config
EXPOSE 22
# Start the SSH service
CMD ["/usr/sbin/sshd", "-D"]

Build the image with:

docker build -t ssh-test .

Keep the image current by using:

docker build --pull -t ssh-test .

Running the Container

Two primary deployment methods are available:

Using the Bridge Network

This approach maps a specific port on the host machine to port 22 within the container, preventing conflicts with existing SSH services:

docker run -d -p 2222:22 --name test-ssh-connexion ssh-test

Using the Host Network

This method binds the container directly to the host’s network stack:

docker run -d --net host --name test-ssh-connexion ssh-test

Testing the Connection

For the Bridge Network:

ssh testuser@<host_machine_ip_address> -p 2222

For the Host Network:

ssh testuser@<host_machine_ip_address>

Root Access

The container permits root SSH connections by default. Test root access using:

ssh root@<host_machine_ip_address>

Default Credentials

The container includes predefined credentials for testing:

testuser / password
root / rootpassword

Modify these lines in the Dockerfile to change credentials:

RUN echo "testuser:password" | chpasswd
RUN echo "root:rootpassword" | chpasswd

Warning: Never use weak passwords in production environments.

Improve the Security

To enhance container security:

Keep container images regularly updated
Check for vulnerabilities frequently
Use Podman in rootless mode
Run containers with minimum privilege user accounts

This approach addresses the numerous attacks targeting the SSH protocol.