docker : how to share ssh-keys between containers? - docker

Question

I've 4 containers configured like the following (docker-compose.yml):
version: '3'
networks:
my-ntwk:
ipam:
config:
- subnet: 172.20.0.0/24
services:
f-app:
image: f-app
tty: true
container_name: f-app
hostname: f-app.info.my
ports:
- "22:22"
networks:
my-ntwk:
ipv4_address: 172.20.0.5
extra_hosts:
- "f-db.info.my:172.20.0.6"
- "p-app.info.my:172.20.0.7"
- "p-db.info.my:172.20.0.8"
depends_on:
- f-db
- p-app
- p-db
f-db:
image: f-db
tty: true
container_name: f-db
hostname: f-db.info.my
networks:
my-ntwk:
ipv4_address: 172.20.0.6
p-app:
image: p-app
tty: true
container_name: p-app
hostname: p-app.info.my
networks:
my-ntwk:
ipv4_address: 172.20.0.7
p-db:
image: p-db
tty: true
container_name: prod-db
hostname: p-db.info.my
networks:
my-ntwk:
ipv4_address: 172.20.0.8
Each image is build by the same Dockerfile :
FROM openjdk:8
RUN apt-get update && \
apt-get install -y openssh-server
EXPOSE 22
RUN useradd -s /bin/bash -p $(openssl passwd -1 myuser) -d /home/nf2/ -m myuser
ENTRYPOINT service ssh start && bash
Now I want to be able to connect from f-app to any other machine without typing the password when running this line : ssh myuser#f-db.info.my.
I know that I need to exchange ssh-keys between the servers (thats not a problem). My problem is how to do it with docker containers and when (build or runtime)!

Answer 1

For doing ssh without password you to need to create passwordless user along with configuring SSH keys in the container, plus you will also need to add ssh keys in the sources container plus public key should be added in the authorized of the destination container.
Here is the working Dockerfile
FROM openjdk:7
RUN apt-get update && \
apt-get install -y openssh-server vim
EXPOSE 22
RUN useradd -rm -d /home/nf2/ -s /bin/bash -g root -G sudo -u 1001 ubuntu
USER ubuntu
WORKDIR /home/ubuntu
RUN mkdir -p /home/nf2/.ssh/ && \
chmod 0700 /home/nf2/.ssh && \
touch /home/nf2/.ssh/authorized_keys && \
chmod 600 /home/nf2/.ssh/authorized_keys
COPY ssh-keys/ /keys/
RUN cat /keys/ssh_test.pub >> /home/nf2/.ssh/authorized_keys
USER root
ENTRYPOINT service ssh start && bash
docker-compose will remain same, here is the testing script that you can try.
#!/bin/bash
set -e
echo "start docker-compose"
docker-compose up -d
echo "list of containers"
docker-compose ps
echo "starting ssh test from f-db to f-app"
docker exec -it f-db sh -c "ssh -i /keys/ssh_test ubuntu#f-app"
For further detail, you can try the above working example docker-container-ssh
git clone git#github.com:Adiii717/docker-container-ssh.git
cd docker-container-ssh;
./test.sh
You can replace the keys as these were used for testing purpose only.

Answer 2

If you are using docker compose an easy choice is to forward SSH agent like that:
something:
container_name: something
volumes:
- $SSH_AUTH_SOCK:/ssh-agent # Forward local machine SSH key to docker
environment:
SSH_AUTH_SOCK: /ssh-agent
ssh-forwarding on macOS hosts - instead of mounting the path of $SSH_AUTH_SOCK, you have to mount this path - /run/host-services/ssh-auth.sock
or you can do it like:
It's a harder problem if you need to use SSH at build time. For example if you're using git clone, or in my case pip and npm to download from a private repository.
The solution I found is to add your keys using the --build-arg flag. Then you can use the new experimental --squash command (added 1.13) to merge the layers so that the keys are no longer available after removal. Here's my solution:
Build command
$ docker build -t example --build-arg ssh_prv_key="$(cat ~/.ssh/id_rsa)" --build-arg ssh_pub_key="$(cat ~/.ssh/id_rsa.pub)" --squash .
Dockerfile
FROM openjdk:8
ARG ssh_prv_key
ARG ssh_pub_key
RUN apt-get update && \
apt-get install -y \
git \
openssh-server \
libmysqlclient-dev
# Authorize SSH Host
RUN mkdir -p /root/.ssh && \
chmod 0700 /root/.ssh && \
ssh-keyscan github.com > /root/.ssh/known_hosts
# Add the keys and set permissions
RUN echo "$ssh_prv_key" > /root/.ssh/id_rsa && \
echo "$ssh_pub_key" > /root/.ssh/id_rsa.pub && \
chmod 600 /root/.ssh/id_rsa && \
chmod 600 /root/.ssh/id_rsa.pub
RUN apt-get update && \
apt-get install -y openssh-server && \
apt-get install -y openssh-client
EXPOSE 22
RUN useradd -s /bin/bash -p $(openssl passwd -1 myuser) -d /home/nf2/ -m myuser
ENTRYPOINT service ssh start && bash
If you're using Docker 1.13+ and/or have experimental features on you can append --squash to the build command which will merge the layers, removing the SSH keys and hiding them from docker history.