Using SSH keys with GitLab CI/CD

GitLab currently doesn't have built-in support for managing SSH keys in a build environment (where the GitLab Runner runs).

The SSH keys can be useful when:

You want to checkout internal submodules
You want to download private packages using your package manager (e.g., Bundler)
You want to deploy your application to your own server, or, for example, Heroku
You want to execute SSH commands from the build environment to a remote server
You want to rsync files from the build environment to a remote server

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

The most widely supported method is to inject an SSH key into your build environment by extending your .gitlab-ci.yml, and it's a solution which works with any type of executor (Docker, shell, etc.).

How it works

Create a new SSH key pair locally with ssh-keygen
Add the private key as a variable to your project
Run the ssh-agent during job to load the private key.
Copy the public key to the servers you want to have access to (usually in ~/.ssh/authorized_keys) or add it as a deploy key if you are accessing a private GitLab repository.

In the following example, the ssh-add - command does not display the value of $SSH_PRIVATE_KEY in the job log, though it could be exposed if you enable debug logging. You might also want to check the visibility of your pipelines.

SSH keys when using the Docker executor

When your CI/CD jobs run inside Docker containers (meaning the environment is contained) and you want to deploy your code in a private server, you need a way to access it. This is where an SSH key pair comes in handy.

You 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.
Create a new CI/CD variable. As Key enter the name SSH_PRIVATE_KEY and in the Value field paste the content of your private key that you created earlier.

Modify your .gitlab-ci.yml with a before_script action. In the following example, a Debian based image is assumed. Edit to your needs:

before_script:
  ##
  ## Install ssh-agent if not already installed, it is required by Docker.
  ## (change apt-get to yum if you use an RPM-based image)
  ##
  - 'command -v ssh-agent >/dev/null || ( 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
  ## We're using tr to fix line endings which makes ed25519 keys work
  ## without extra base64 encoding.
  ## https://gitlab.com/gitlab-examples/ssh-private-key/issues/1#note_48526556
  ##
  - echo "$SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add -

  ##
  ## Create the SSH directory and give it the right permissions
  ##
  - mkdir -p ~/.ssh
  - chmod 700 ~/.ssh

  ##
  ## Optionally, if you will be using any Git commands, set the user name and
  ## and email.
  ##
  # - git config --global user.email "user@example.com"
  # - git config --global user.name "User name"

The before_script can be set globally or per-job.

Make sure the private server's SSH host keys are verified.
As a final step, add the public key from the one you created in the first step 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, log in to the server that runs your jobs.
Then, from the terminal, log in as the gitlab-runner user:
```
sudo su - gitlab-runner
```
Generate the SSH key pair as described in the instructions to generate an SSH key. Do not add a passphrase to the SSH key, or the before_script will prompt for it.
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.

After generating the key, try to sign in to the remote server to accept the fingerprint:

ssh example.com

For accessing repositories on GitLab.com, you would use git@gitlab.com.

Verifying the SSH host keys

It is a good practice to check the private server's own public key to make sure you are not being targeted by a man-in-the-middle attack. If anything suspicious happens, you notice it because the job fails (the SSH connection fails when the public keys don't match).

To find out the host keys of your server, run the ssh-keyscan command from a trusted network (ideally, from the private server itself):

## Use the domain name
ssh-keyscan example.com

## Or use an IP
ssh-keyscan 1.2.3.4

Create a new CI/CD variable with SSH_KNOWN_HOSTS as "Key", and as a "Value" add the output of ssh-keyscan.

If you need to connect to multiple servers, all the server host keys need to be collected in the Value of the variable, one key per line.

NOTE: By using a variable instead of ssh-keyscan directly inside .gitlab-ci.yml, it has the benefit that you don't have to change .gitlab-ci.yml if the host domain name changes for some reason. Also, the values are predefined by you, meaning that if the host keys suddenly change, the CI/CD job doesn't fail, so there's something wrong with the server or the network.

Now that the SSH_KNOWN_HOSTS variable is created, in addition to the content of .gitlab-ci.yml above, here's what more you need to add:

before_script:
  ##
  ## Assuming you created the SSH_KNOWN_HOSTS variable, uncomment the
  ## following two lines.
  ##
  - echo "$SSH_KNOWN_HOSTS" >> ~/.ssh/known_hosts
  - chmod 644 ~/.ssh/known_hosts

  ##
  ## Alternatively, use ssh-keyscan to scan the keys of your private server.
  ## Replace example.com with your private server's domain name. Repeat that
  ## command if you have more than one server to connect to.
  ##
  # - ssh-keyscan example.com >> ~/.ssh/known_hosts
  # - chmod 644 ~/.ssh/known_hosts

  ##
  ## You can optionally disable host key checking. Be aware that by adding that
  ## you are susceptible 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.
  ##
  # - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" >> ~/.ssh/config'

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 is picked by a public runner and the job starts.