ssh-agent GitHub Action

This action

• starts the ssh-agent,
• exports the SSH_AUTH_SOCK environment variable,
• loads a private SSH key into the agent and
• configures known_hosts for GitHub.com.

Why?

When running a GitHub Action workflow to stage your project, run tests or build images, you might need to fetch additional libraries or vendors from private repositories.

GitHub Actions only have access to the repository they run for. So, in order to access additional private repositories, create an SSH key with sufficient access privileges. Then, use this action to make the key available with ssh-agent on the Action worker node. Once this has been set up, git clone commands using ssh URLs will just work. Also, running ssh commands to connect to other servers will be able to use the key.

Usage

1. Create an SSH key with sufficient access privileges. For security reasons, don't use your personal SSH key but set up a dedicated one for use in GitHub Actions. See below for a few hints if you are unsure about this step.
2. In your repository, go to the Settings > Secrets menu and create a new secret called SSH_PRIVATE_KEY. Put the unencrypted private SSH key in PEM format into the contents field.
   This key should start with -----BEGIN RSA PRIVATE KEY-----, consist of many lines and ends with -----END RSA PRIVATE KEY-----. You can just copy the key as-is from the private key file.
3. In your workflow definition file, add the following step. Preferably this would be rather on top, near the actions/checkout@v1 line.

# .github/workflows/my-workflow.yml
jobs:
    my_job:
        ...
        steps:
            - actions/checkout@v1
            # Make sure the @v0.1.1 matches the current version of the
            # action 
            - uses: webfactory/ssh-agent@v0.1.1
              with:
                  ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
            - ... other steps

4. If, for some reason, you need to change the location of the SSH agent socket, you can use the ssh-auth-sock input to provide a path.

Known issues and limitations

Currently OS X and Linux only

This action has not been tested for the Windows virtual environment. If you can provide the steps necessary to setup (even install?) OpenSSH on the Windows machine, please open an issue.

Works for the current job only

Since each job runs in a fresh instance of the virtual environment, the SSH key will only be available in the job where this action has been referenced. You can, of course, add the action in multiple jobs or even workflows. All instances can use the same SSH_PRIVATE_KEY secret.

SSH private key format

If the private key is not in the PEM format, you will see an Error loading key "(stdin)": invalid format message.

Use ssh-keygen -p -f path/to/your/key -m pem to convert your key file to PEM, but be sure to make a backup of the file first 😉.

What this Action cannot do for you

The following items are not issues, but beyond what this Action is supposed to do.

Work on remote machines

When using ssh to connect from the GitHub Action worker node to another machine, you can forward the SSH Agent socket and use your private key on the other (remote) machine. However, this Action will not configure known_hosts or other SSH settings on the remote machine for you.

Provide the SSH key as a file

This Action is designed to pass the SSH directly into ssh-agent; that is, the key is available in memory on the GitHub Action worker node, but never written to disk. As a consequence, you cannot pass the key as a build argument or a mounted file into Docker containers that you build or run on the worker node. You can, however, mount the ssh-agent Unix socket into a Docker container that you run, set up the SSH_AUTH_SOCK env var and then use SSH from within the container (see #11).

Run ssh-keyscan to add host keys for additional hosts

If you want to use ssh-keyscan to add additional hosts (that you own/know) to the known_hosts file, you can do so with a single shell line in your Action definition. You don't really need this Action to do this for you.

As a side note, using ssh-keyscan without proper key verification is susceptible to man-in-the-middle attacks. You might prefer putting your known SSH host key in your own Action files to add it to the known_hosts file. The SSH host key is not secret and can safely be committed into the repo.

Creating SSH keys

In order to create a new SSH key, run ssh-keygen -t rsa -b 4096 -m pem -f path/to/keyfile. This will prompt you for a key passphrase and save the key in path/to/keyfile.

Having a passphrase is a good thing, since it will keep the key encrypted on your disk. When configuring the secret SSH_PRIVATE_KEY value in your repository, however, you will need the private key unencrypted.

To show the private key unencrypted, run openssl rsa -in path/to/key -outform pem.

Authorizing a key

To actually grant the SSH key access, you can – on GitHub – use at least two ways:

• Deploy keys can be added to individual GitHub repositories. They can give read and/or write access to the particular repository. When pulling a lot of dependencies, however, you'll end up adding the key in many places. Rotating the key probably becomes difficult.

• A machine user can be used for more fine-grained permissions management and have access to multiple repositories with just one instance of the key being registered. It will, however, count against your number of users on paid GitHub plans.

Hacking

As a note to my future self, in order to work on this repo:

• Clone it
• Run npm install to fetch dependencies
• hack hack hack
• node index.js (inputs are passed through INPUT_ env vars, but how to set ssh-private-key?)
• Run ./node_modules/.bin/ncc build index.js to update dist/index.js, which is the file actually run
• Read https://help.github.com/en/articles/creating-a-javascript-action if unsure.
• Maybe update the README example when publishing a new version.

Credits, Copyright and License

This action was written by webfactory GmbH, Bonn, Germany. We're a software development agency with a focus on PHP (mostly Symfony). If you're a developer looking for new challenges, we'd like to hear from you!

• https://www.webfactory.de
• https://twitter.com/webfactory

Copyright 2019 webfactory GmbH, Bonn. Code released under the MIT license.