ssh-agent-inject

Forwards the host's ssh-agent into a Docker container. This is especially useful when working with the Visual Studio Code Remote - Containers extension and Git repos cloned via SSH.

Why this is needed

While you can bind-mount the SSH_AUTH_SOCK from a Linux host, this is not possible from a macOS or Windows host. Also, none of the existing solutions is cross-platform and easy. The recommended solution is to copy the SSH key from the host to the container, but then you have to manually add the key (assuming you've setup ssh-agent within the container) and enter the password within the container.

With ssh-agent-inject you can skip those annoyances and simply reuse your host's ssh-agent.

Usage

Download ssh-agent-inject for your platform. Make sure ssh-agent-inject runs in the background or just launch it on-demand.

Add the following to your Dockerfile:

ENV SSH_AUTH_SOCK=/tmp/.ssh-auth-sock
LABEL com.ensody.ssh-agent-inject=

Alternatively, you can run an arbitrary container directly:

docker run -e SSH_AUTH_SOCK=/tmp/.ssh-auth-sock -l com.ensody.ssh-agent-inject ...

Note that this project is itself using ssh-agent-inject with VS Code (see .devcontainer/).

How it works

This project consists of two applications that communicate through stdio: ssh-agent-inject and ssh-agent-pipe which is embedded within the ssh-agent-inject binary (that's why you don't see it in the release archive).

The ssh-agent-inject command runs on the host and

• watches Docker for containers having the com.ensody.ssh-agent-inject label
• copies the embedded ssh-agent-pipe binary into those containers
• runs ssh-agent-pipe within each container via docker exec
• connects to the host's ssh-agent (one connection per container)
• forwards the host's ssh-agent to ssh-agent-pipe via stdio

The ssh-agent-pipe command runs in the container and

• listens on a UNIX socket at $SSH_AUTH_SOCK
• handles parallel connections on that UNIX socket
• serializes all socket<->stdio communication (handles one request-response pair at a time)

The apps communicate via stdio because this keeps the attack surface small and makes it easier to ensure that nobody else can connect to your ssh-agent (assuming you can trust the Docker container, of course).

Building

All required dependencies are contained in a Docker image defined in .devcontainer/, which can be automatically used with Visual Studio Code (or manually via Docker build & run). That way your host system stays clean and the whole environment is automated, exactly defined, isolated from the host, and easily reproducible. This saves time and prevents mistakes (wrong version, interference with other software installed on host, etc.).

Run ./build.sh to build binaries for all platforms.

Releasing

• Update CHANGELOG.md.
• Add a tag (e.g. git tag v1.2.3) and push it.
• The CI system will deploy a draft release to GitHub.
• Edit the release description and publish it.

Note: Only tags that look like a version number and start with "v" will be deployed to GitHub.