Using ssh-agent with docker on macOS

Question

I would like to use ssh-agent to forward my keys into the docker image and pull from a private github repo.

I am using a slightly modified version of https://github.com/phusion/passenger-docker with boot2docker on Yosemite.

ssh-add -l
...key details
boot2docker up

Then I use the command which I have seen in a number of places (i.e. https://gist.github.com/d11wtq/8699521):

docker run --rm -t -i -v $SSH_AUTH_SOCK:/ssh-agent -e SSH_AUTH_SOCK=/ssh-agent my_image /bin/bash

However it doesn't seem to work:

root@299212f6fee3:/# ssh-add -l
Could not open a connection to your authentication agent.

root@299212f6fee3:/# eval `ssh-agent -s`
Agent pid 19

root@299212f6fee3:/# ssh-add -l
The agent has no identities.

root@299212f6fee3:/# ssh [email protected]
Warning: Permanently added the RSA host key for IP address '192.30.252.128' to the list of known hosts.
Permission denied (publickey).

Berkman

What are the permissions on the file $SSH_AUTH_SOCK? – Ardith

Answer 1

Since version 2.2.0.0, docker for macOS allows users to access the host’s SSH agent inside containers.

Here's an example command that let's you do it:

docker run --rm -it \
-v /run/host-services/ssh-auth.sock:/ssh-agent \
-e SSH_AUTH_SOCK="/ssh-agent" \
my_image

Note that you have to mount the specific path (/run/host-services/ssh-auth.sock) instead of the path contained in $SSH_AUTH_SOCK environment variable, like you would do on linux hosts.

Fusible

After more digging : Works only if the user inside the container is root I think (otherwise he cannot access the socket in the VM) – Ununa

On my mac OS Big Sur there's no such path /run/host-services. Why? Should I install something? – Diu

@Diu I also don't have this path myself. This is a "magic" mount - docker knows how to handle this path when it sees it. Did you try it out? – Fusible

@JakubKukul yep, and it worked. Why isn't it documented anywhere? Or at least my google isn't able to find the documentation where this is mentioned. – Diu

@Diu Ideally this should be documented somewhere, yes. I found out about it by following this Github issue's thread: github.com/docker/for-mac/issues/410 – Fusible

Here's the doc as of today docs.docker.com/desktop/networking/#ssh-agent-forwarding – Footless

Answer 2

A one-liner:

Here’s how to set it up on Ubuntu 16 running a Debian Jessie image:

docker run --rm -it --name container_name \
-v $(dirname $SSH_AUTH_SOCK):$(dirname $SSH_AUTH_SOCK) \
-e SSH_AUTH_SOCK=$SSH_AUTH_SOCK my_image

https://techtip.tech.blog/2016/12/04/using-ssh-agent-forwarding-with-a-docker-container/

Selfdevotion

This looks very promising, but on OS X I'm getting a Permission denied (publickey). error. Perhaps it's because on non-linux hosts Docker is running in it's own WM? – Bron

Answer 3

I expanded on @wilwilson's answer, and created a script that will setup agent forwarding in an OSX boot2docker environment.

https://gist.github.com/rcoup/53e8dee9f5ea27a51855

#!/bin/bash

# Use a unique ssh socket name per-invocation of this script
SSH_SOCK=boot2docker.$$.ssh.socket

# ssh into boot2docker with agent forwarding
ssh -i ~/.ssh/id_boot2docker \
    -o StrictHostKeyChecking=no \
    -o IdentitiesOnly=yes \
    -o UserKnownHostsFile=/dev/null \
    -o LogLevel=quiet \
    -p 2022 docker@localhost \
    -A -M -S $SSH_SOCK -f -n \
    tail -f /dev/null

# get the agent socket path from the boot2docker vm
B2D_AGENT_SOCK=$(ssh -S $SSH_SOCK docker@localhost echo \$SSH_AUTH_SOCK)

# mount the socket (from the boot2docker vm) onto the docker container
# and set the ssh agent environment variable so ssh tools pick it up
docker run \
    -v $B2D_AGENT_SOCK:/ssh-agent \
    -e "SSH_AUTH_SOCK=/ssh-agent" \
    "$@"

# we're done; kill off the boot2docker ssh agent
ssh -S $SSH_SOCK -O exit docker@localhost

Stick it in ~/bin/docker-run-ssh, chmod +x it, and use docker-run-ssh instead of docker run.

Benue

I created a version that works with docker-machine, now that Docker is recommending against using boot2docker. gist.github.com/leedm777/923706741c8296869e7d – Dichroscope

dave - have you tried using this with docker-compose? Not sure how to modify your example to work with statically defined environment variables in docker-compose.yml. – Tsimshian

Answer 4

For me accessing ssh-agent to forward keys worked on OSX Mavericks and docker 1.5 as follows:

1. ssh into the boot2docker VM with boot2docker ssh -A. Don't forget to use option -A which enables forwarding of the authentication agent connection.

2. Inside the boot2docker ssh session:

   docker@boot2docker:~$ echo $SSH_AUTH_SOCK
   /tmp/ssh-BRLb99Y69U/agent.7750
   

This session must be left open. Take note of the value of the SSH_AUTH_SOCK environmental variable.

3. In another OS X terminal issue the docker run command with the SSH_AUTH_SOCK value from step 2 as follows:

   docker run --rm -t -i \
     -v /tmp/ssh-BRLb99Y69U/agent.7750:/ssh-agent \
     -e SSH_AUTH_SOCK=/ssh-agent my_image /bin/bash
   root@600d0e9b443d:/# ssh-add -l
   2048 6c:8e:82:08:74:33:78:61:f9:9a:74:1b:65:46:be:eb         
   /Users/dev/.ssh/id_rsa (RSA)
   

I don't really like the fact that I have to keep a boot2docker ssh session open to make this work, but until a better solution is found, this at least worked for me.

Chartography

Answer 5

I ran into a similar issue, and was able to make things pretty seamless by using ssh in master mode with a control socket and wrapping it all in a script like this:

#!/bin/sh   

ssh -i ~/.vagrant.d/insecure_private_key -p 2222 -A -M -S ssh.socket -f [email protected] tail -f /dev/null

HOST_SSH_AUTH_SOCK=$(ssh -S ssh.socket [email protected] env | grep "SSH_AUTH_SOCK" | cut -f 2 -d =)

docker run -v $HOST_SSH_AUTH_SOCK:/ssh-agent \
       -e "SSH_AUTH_SOCK=/ssh-agent" \
       -t hello-world "$@"

ssh -S ssh.socket -O exit [email protected]

Not the prettiest thing in the universe, but much better than manually keeping an SSH session open IMO.

Supervisor

Meanwhile newes docker complains: docker: Error response from daemon: invalid volume spec ":/ssh-agent": invalid volume specification: ':/ssh-agent'. – Irving

Answer 6

Socket forwarding doesn't work on OS X yet. Here is a variation of @henrjk answer brought into 2019 using Docker for Mac instead of boot2docker which is now obsolete.

1. First run a ssh server in the container, with /tmp being on the exportable volume. Like this

    docker run -v tmp:/tmp -v \
    ${HOME}/.ssh/id_rsa.pub:/root/.ssh/authorized_keys:ro \
    -d -p 2222:22 arvindr226/alpine-ssh
   

2. Then ssh into this container with agent forwarding

    ssh -A -p 2222 root@localhost
   

3. Inside of that ssh session find out the current socket for ssh-agent

    3f53fa1f5452:~# echo $SSH_AUTH_SOCK
    /tmp/ssh-9zjJcSa3DM/agent.7
   

4. Now you can run your real container. Just make sure to replace the value of SSH_AUTH_SOCK below, with the value you got in the step above

    docker run -it -v tmp:/tmp  \
    -e SSH_AUTH_SOCK=/tmp/ssh-9zjJcSa3DM/agent.7 \
    vladistan/ansible
   

Indohittite

Still useful and working on Docker Desktop for Mac in Aug,2022 for deploying rails 7 app using capistrano agent forwarding. – Babushka

Answer 7

By default, boot2docker shares only files under /Users. SSH_AUTH_SOCK is probably under /tmp so the -v mounts the agent of the VM, not the one from your mac.

If you setup your VirtualBox to share /tmp, it should be working.

Ligature

This sounds promising, could you provide some more information on how to do this? I found this: github.com/boot2docker/boot2docker#virtualbox-guest-additions but I'm relatively new to docker – Berkman

Open your virtualbox, right click on the boo2docker vm, settings, shared directires, click the add button, enter /tmp, tick automount and permanent. However, after testing, it appears that shared directory does not allow you to share the socket, so it will not work. – Ligature

I don't think this is possible. SSH_AUTH_SOCK is a Unix socket, not a file, so it can't be shared with VirtualBox the way files are. – Sherillsherilyn

Answer 8

Could not open a connection to your authentication agent.

This error occurs when $SSH_AUTH_SOCK env var is set incorrectly on the host or not set at all. There are various workarounds you could try. My suggestion, however, is to dual-boot Linux and macOS.

Additional resources:

• Using SSH keys inside docker container - Related Question
• SSH and docker-compose - Blog post
• Build secrets and SSH forwarding in Docker 18.09 - Blog post

Platelayer