Unable to ssh localhost within a running Docker container

Question

I'm building a Docker image for an application which requires to ssh into localhost (i.e ssh user@localhost)

I'm working on a Ubuntu desktop machine and started with a basic ubuntu:16.04 container. Following is the content of my Dockerfile:

FROM ubuntu:16.04

RUN apt-get update && apt-get install -y \
        openjdk-8-jdk \
        ssh && \
        groupadd -r custom_group && useradd -r -g custom_group -m user1

USER user1

RUN ssh-keygen -b 2048 -t rsa -f ~/.ssh/id_rsa -q -N "" && \
        cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

Then I build this container using the command:

docker build -t test-container .

And run it using:

docker run -it test-container

The container opens with the following prompt and the keys are generated correctly to enable ssh into localhost:

user1@0531c0f71e0a:/$ 
user1@0531c0f71e0a:/$ cd ~/.ssh/
user1@0531c0f71e0a:~/.ssh$ ls
authorized_keys  id_rsa  id_rsa.pub

Then ssh into localhost and greeted by the error:

user1@0531c0f71e0a:~$ ssh user1@localhost
ssh: connect to host localhost port 22: Cannot assign requested address

Is there anything I'm doing wrong or any additional network settings that needs to be configured? I just want to ssh into localhost within the running container.

Solution 1

First you need to install the ssh server in the image building script:

• RUN sudo apt-get install -y openssh-server

Then you need to start the ssh server:

• RUN sudo /etc/init.d/ssh start

or probably even in the last lines of the Dockerfile ( you must have one binary instantiated to keep the container running ... )

 USER root
 CMD [ "sh", "/etc/init.d/ssh", "start"]

on the host than

# init a container from an the image
run -d --name my-ssh-container-name-01 \
    -v /opt/local/dir:/opt/container/dir my-image-01

Solution 2

As @user2915097 stated in the OP comments, this was due to the ssh instance in the container was attempting to connect to the host using IPv6. Forcing connection over IPv4 using -4 solved the issue.

$ docker run -it ubuntu ssh -4 user@hostname

Solution 3

For Docker Compose I was able to add the following to my .yml file:

network_mode: "host"

I believe the equivalent in Docker is:

--net=host

Documentation:

https://docs.docker.com/compose/compose-file/compose-file-v3/#network_mode

https://docs.docker.com/network/#network-drivers

host: For standalone containers, remove network isolation between the container and the Docker host, and use the host’s networking directly. See use the host network.