Using SSH keys

GitLab currently doesn't have built-in support for managing SSH keys in a build environment.

The SSH keys can be useful when:

1. You want to checkout internal submodules
2. You want to download private packages using your package manager (eg. bundler)
3. You want to deploy your application to eg. Heroku or your own server
4. You want to execute SSH commands from the build server to the remote server
5. You want to rsync files from your build server to the remote server

If anything of the above rings a bell, then you most likely need an SSH key.

Inject keys in your build server

The most widely supported method is to inject an SSH key into your build environment by extending your .gitlab-ci.yml.

This is the universal solution which works with any type of executor (docker, shell, etc.).

How it works

1. Create a new SSH key pair with ssh-keygen
2. Add the private key as a Secret Variable to the project
3. Run the ssh-agent during job to load the private key.

SSH keys when using the Docker executor

You will first need to create an SSH key pair. For more information, follow the instructions to generate an SSH key. Do not add a passphrase to the SSH key, or the before_script will prompt for it.

Then, create a new Secret Variable in your project settings on GitLab following Settings > Pipelines and look for the "Secret Variables" section. As Key add the name SSH_PRIVATE_KEY and in the Value field paste the content of your private key that you created earlier.

It is also good practice to check the server's own public key to make sure you are not being targeted by a man-in-the-middle attack. To do this, add another variable named SSH_SERVER_HOSTKEYS. To find out the hostkeys of your server, run the ssh-keyscan YOUR_SERVER command from a trusted network (ideally, from the server itself), and paste its output into the SSH_SERVER_HOSTKEY variable. If you need to connect to multiple servers, concatenate all the server public keys that you collected into the Value of the variable. There must be one key per line.

Next you need to modify your .gitlab-ci.yml with a before_script action. Add it to the top:

before_script:
  # Install ssh-agent if not already installed, it is required by Docker.
  # (change apt-get to yum if you use a CentOS-based image)
  - 'which ssh-agent || ( apt-get update -y && apt-get install openssh-client -y )'

  # Run ssh-agent (inside the build environment)
  - eval $(ssh-agent -s)

  # Add the SSH key stored in SSH_PRIVATE_KEY variable to the agent store
  - ssh-add <(echo "$SSH_PRIVATE_KEY")

  # For Docker builds disable host key checking. Be aware that by adding that
  # you are suspectible to man-in-the-middle attacks.
  # WARNING: Use this only with the Docker executor, if you use it with shell
  # you will overwrite your user's SSH config.
  - mkdir -p ~/.ssh
  - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
  # In order to properly check the server's host key, assuming you created the
  # SSH_SERVER_HOSTKEYS variable previously, uncomment the following two lines
  # instead.
  # - mkdir -p ~/.ssh
  # - '[[ -f /.dockerenv ]] && echo "$SSH_SERVER_HOSTKEYS" > ~/.ssh/known_hosts'

As a final step, add the public key from the one you created earlier to the services that you want to have an access to from within the build environment. If you are accessing a private GitLab repository you need to add it as a deploy key.

That's it! You can now have access to private servers or repositories in your build environment.

SSH keys when using the Shell executor

If you are using the Shell executor and not Docker, it is easier to set up an SSH key.

You can generate the SSH key from the machine that GitLab Runner is installed on, and use that key for all projects that are run on this machine.

First, you need to login to the server that runs your jobs.

Then from the terminal login as the gitlab-runner user and generate the SSH key pair as described in the SSH keys documentation.

As a final step, add the public key from the one you created earlier to the services that you want to have an access to from within the build environment. If you are accessing a private GitLab repository you need to add it as a deploy key.

Once done, try to login to the remote server in order to accept the fingerprint:

ssh <address-of-my-server>

For accessing repositories on GitLab.com, the <address-of-my-server> would be git@gitlab.com.

Example project

We have set up an Example SSH Project for your convenience that runs on GitLab.com using our publicly available shared runners.

Want to hack on it? Simply fork it, commit and push your changes. Within a few moments the changes will be picked by a public runner and the job will begin.