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 (ie 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

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.