Dockerize an SSH service

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Build an eg_sshd image

Generate a secure root password for your image

Using a static password for root access is dangerous. Create a random password before proceeding.

Build the image

The following Dockerfile sets up an SSHd service in a container that you can use to connect to and inspect other container’s volumes, or to get quick access to a test container. Make the following substitutions:

  • With RUN echo 'root:THEPASSWORDYOUCREATED' | chpasswd, replace “THEPASSWORDYOUCREATED” with the password you’ve previously generated.
  • With RUN sed -i 's/PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config, use without-password instead of prohibit-password for Ubuntu 14.04.
FROM ubuntu:20.04

RUN apt-get update && apt-get install -y openssh-server
RUN mkdir /var/run/sshd
RUN echo 'root:THEPASSWORDYOUCREATED' | chpasswd
RUN sed -i 's/#*PermitRootLogin prohibit-password/PermitRootLogin yes/g' /etc/ssh/sshd_config

# SSH login fix. Otherwise user is kicked off after login
RUN sed -i 's@session\s*required\s* optional' /etc/pam.d/sshd

ENV NOTVISIBLE "in users profile"
RUN echo "export VISIBLE=now" >> /etc/profile

CMD ["/usr/sbin/sshd", "-D"]

Build the image using:

$ docker build -t eg_sshd .

Run a test_sshd container

Then run it. You can then use docker port to find out what host port the container’s port 22 is mapped to:

$ docker run -d -P --name test_sshd eg_sshd
$ docker port test_sshd 22

And now you can ssh as root on the container’s IP address (you can find it with docker inspect) or on port 49154 of the Docker daemon’s host IP address (ip address or ifconfig can tell you that) or localhost if on the Docker daemon host:

$ ssh root@ -p 49154
# or
$ ssh root@localhost -p 49154
# The password is ``screencast``.

Environment variables

Using the sshd daemon to spawn shells makes it complicated to pass environment variables to the user’s shell via the normal Docker mechanisms, as sshd scrubs the environment before it starts the shell.

If you’re setting values in the Dockerfile using ENV, you need to push them to a shell initialization file like the /etc/profile example in the Dockerfile above.

If you need to passdocker run -e ENV=value values, you need to write a short script to do the same before you start sshd -D and then replace the CMD with that script.

Clean up

Finally, clean up after your test by stopping and removing the container, and then removing the image.

$ docker container stop test_sshd
$ docker container rm test_sshd
$ docker image rm eg_sshd
docker, example, package installation, networking