aidanhs/sshagent-socket repository overview

⁠docker-sshagent-socket

When using the secret server⁠ container it can be tiresome to manually manage SSH keys (or impossible if they're password protected).

As of 1.8, names of containers are automatically inserted into /etc/hosts, so if you run this container (assuming you have an SSH agent running) with

docker run -d -v $(dirname $SSH_AUTH_SOCK):/s$(dirname $SSH_AUTH_SOCK) --name=dsshagent aidanhs/sshagent-socket

(it may ask you to add $SSH_AUTH_SOCK at the end of the line)

you can add the following lines to a Dockerfile for an image with socat installed

FROM myimage

ENV SSH_AUTH_SOCK /tmp/ssh/auth.sock
RUN dir=$(dirname $SSH_AUTH_SOCK) && mkdir -p $dir && chmod 777 $dir
RUN mkdir -p ~/.ssh && printf "Host *\n\
  StrictHostKeyChecking no\n\
  ProxyCommand setsid socat UNIX-LISTEN:$SSH_AUTH_SOCK,unlink-early,mode=777 TCP:dsshagent:5522 >/dev/null 2>&1 & \
    sleep 0.5 && socat - TCP:%%h:%%p\n\
" > ~/.ssh/config

RUN scp user@mysecretserver:~/data /data

And (in theory) you have flawless access to any ssh servers your host user has access to! You should probably remove StrictHostKeyChecking no and add known hosts into your image though.

This works by the SSH agent server exposing a single port (5522). Whenever the container being built attempts to make an SSH connection, it will run socat to connect to the SSH agent container and expose the port as a Unix socket (as required by SSH).

Unfortunately there are some issues with the current approach:

• you have to sleep a small amount of time to let socat start before you let SSH begin - assuming you're being good and not doing too much SSH, this shouldn't really impact your build times
• executing a number of SSH connections in parallel may break as the socat instances step on the same socket - try adding ,fork to the end of the UNIX-LISTEN argument to keep socat listening for all connections, which may help with race conditions (though you end up with one socat process per ssh connection during the layer)
• different host/container combinations may break things in strange ways

Raise an issue if you hit any of these problems - they're easy to solve by writing a small program, this is currently just intended as a PoC.

Boot2Docker users and people with more exotic setups may wish to read some of the later comments on this gist⁠.

⁠Security

Running this gives anyone on your machine access to your SSH agent (and therefore access to all servers your keys give access to).

If they have the ability to run arbitrary containers, they already had this.