Unable to ssh localhost within a running Docker container - linux

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.

Answer 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

Answer 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

Answer 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.

Answer 4

I also faced this error today, here's how to fix it:
If(and only if) you are facing this error inside a running container that isn't in production.
Do this:
docker exec -it -u 0 [your container id here] /bin/bash
then when you entered the container in god mode, run this:
service ssh start
then you can run your ssh based commands.
Of course it is best practice to do it in your Dockerfile before all these, but no need to sweat if you are not done with your image built process just yet.