GitLab and SSH keys (FREE)

Git is a distributed version control system, which means you can work locally, then share or "push" your changes to a server. In this case, the server is GitLab.

GitLab uses the SSH protocol to securely communicate with Git. When you use SSH keys to authenticate to the GitLab remote server, you don't need to supply your username and password each time.

Prerequisites

To use SSH to communicate with GitLab, you need:

The OpenSSH client, which comes pre-installed on GNU/Linux, macOS, and Windows 10.
SSH version 6.5 or later. Earlier versions used an MD5 signature, which is not secure.

To view the version of SSH installed on your system, run ssh -V.

Supported SSH key types

To communicate with GitLab, you can use the following SSH key types:

ED25519
RSA
DSA (Deprecated in GitLab 11.0.)
ECDSA (As noted in Practical Cryptography With Go, the security issues related to DSA also apply to ECDSA.)

Administrators can restrict which keys are permitted and their minimum lengths.

ED25519 SSH keys

The book Practical Cryptography With Go suggests that ED25519 keys are more secure and performant than RSA keys.

OpenSSH 6.5 introduced ED25519 SSH keys in 2014 and they should be available on most operating systems.

RSA SSH keys

Available documentation suggests that ED25519 is more secure than RSA.

If you use an RSA key, the US National Institute of Science and Technology in Publication 800-57 Part 3 (PDF) recommends a key size of at least 2048 bits. The default key size depends on your version of ssh-keygen. Review the man page for your installed ssh-keygen command for details.

See if you have an existing SSH key pair

Before you create a key pair, see if a key pair already exists.

On Windows, Linux, or macOS, go to your home directory.
Go to the .ssh/ subdirectory. If the .ssh/ subdirectory doesn't exist, you are either not in the home directory, or you haven't used ssh before. In the latter case, you need to generate an SSH key pair.

See if a file with one of the following formats exists:

Algorithm	Public key	Private key
ED25519 (preferred)	`id_ed25519.pub`	`id_ed25519`
RSA (at least 2048-bit key size)	`id_rsa.pub`	`id_rsa`
DSA (deprecated)	`id_dsa.pub`	`id_dsa`
ECDSA	`id_ecdsa.pub`	`id_ecdsa`

Generate an SSH key pair

If you do not have an existing SSH key pair, generate a new one.

Open a terminal.
Type ssh-keygen -t followed by the key type and an optional comment. This comment is included in the .pub file that's created. You may want to use an email address for the comment.

For example, for ED25519:
```
ssh-keygen -t ed25519 -C "<comment>"
```
For 2048-bit RSA:
```
ssh-keygen -t rsa -b 2048 -C "<comment>"
```

Press Enter. Output similar to the following is displayed:

Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/user/.ssh/id_ed25519):

Accept the suggested filename and directory, unless you are generating a deploy key or want to save in a specific directory where you store other keys.

You can also dedicate the SSH key pair to a specific host.

Specify a passphrase:

Enter passphrase (empty for no passphrase):
Enter same passphrase again:

A confirmation is displayed, including information about where your files are stored.

A public and private key are generated. Add the public SSH key to your GitLab account and keep the private key secure.

Configure SSH to point to a different directory

If you did not save your SSH key pair in the default directory, configure your SSH client to point to the directory where the private key is stored.

Open a terminal and run this command:

eval $(ssh-agent -s)
ssh-add <directory to private SSH key>

Save these settings in the ~/.ssh/config file. For example:

# GitLab.com
Host gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/gitlab_com_rsa

# Private GitLab instance
Host gitlab.company.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/example_com_rsa

For more information on these settings, see the man ssh_config page in the SSH configuration manual.

Public SSH keys must be unique to GitLab because they bind to your account. Your SSH key is the only identifier you have when you push code with SSH. It must uniquely map to a single user.

Update your SSH key passphrase

You can update the passphrase for your SSH key.

Open a terminal and run this command:
```
ssh-keygen -p -f /path/to/ssh_key
```
At the prompts, type the passphrase and press Enter.

Upgrade your RSA key pair to a more secure format

If your version of OpenSSH is between 6.5 and 7.8, you can save your private RSA SSH keys in a more secure OpenSSH format.

Open a terminal and run this command:
```
ssh-keygen -o -f ~/.ssh/id_rsa
```
Alternatively, you can generate a new RSA key with the more secure encryption format with the following command:
```
ssh-keygen -o -t rsa -b 4096 -C "<comment>"
```

Add an SSH key to your GitLab account

To use SSH with GitLab, copy your public key to your GitLab account.

Copy the contents of your public key file. You can do this manually or use a script. For example, to copy an ED25519 key to the clipboard:

macOS:
```
tr -d '\n' < ~/.ssh/id_ed25519.pub | pbcopy
```
Linux (requires the xclip package):
```
xclip -sel clip < ~/.ssh/id_ed25519.pub
```
Git Bash on Windows:
```
cat ~/.ssh/id_ed25519.pub | clip
```
Replace id_ed25519.pub with your filename. For example, use id_rsa.pub for RSA.
Sign in to GitLab.
On the top bar, in the top right corner, select your avatar.
Select Preferences.
On the left sidebar, select SSH Keys.
In the Key box, paste the contents of your public key. If you manually copied the key, make sure you copy the entire key, which starts with ssh-ed25519 or ssh-rsa, and may end with a comment.
In the Title box, type a description, like Work Laptop or Home Workstation.
Optional. In the Expires at box, select an expiration date. (Introduced in GitLab 12.9.) In:
- GitLab 13.12 and earlier, the expiration date is informational only. It doesn't prevent you from using the key. Administrators can view expiration dates and use them for guidance when deleting keys.
- GitLab 14.0 and later, the expiration date is enforced. Administrators can allow expired keys to be used.
- GitLab checks all SSH keys at 02:00 AM UTC every day. It emails an expiration notice for all SSH keys that expire on the current date. (Introduced in GitLab 13.11.)
- GitLab checks all SSH keys at 01:00 AM UTC every day. It emails an expiration notice for all SSH keys that are scheduled to expire seven days from now. (Introduced in GitLab 13.11.)
Select Add key.

Verify that you can connect

Verify that your SSH key was added correctly.

For GitLab.com, to ensure you're connecting to the correct server, confirm the SSH host keys fingerprints.
Open a terminal and run this command, replacing gitlab.example.com with your GitLab instance URL:
```
ssh -T git@gitlab.example.com
```

If this is the first time you connect, you should verify the authenticity of the GitLab host. If you see a message like:

The authenticity of host 'gitlab.example.com (35.231.145.151)' can't be established.
ECDSA key fingerprint is SHA256:HbW3g8zUjNSksFbqTiUWPWg2Bq1x8xdGUrliXFzSnUw.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'gitlab.example.com' (ECDSA) to the list of known hosts.

Type yes and press Enter.

Run the ssh -T git@gitlab.example.com command again. You should receive a Welcome to GitLab, @username! message.

If the welcome message doesn't appear, you can troubleshoot by running ssh in verbose mode:

ssh -Tvvv git@gitlab.example.com

Use different keys for different repositories

You can use a different key for each repository.

Open a terminal and run this command:

git config core.sshCommand "ssh -o IdentitiesOnly=yes -i ~/.ssh/private-key-filename-for-this-repository -F /dev/null"

This command does not use the SSH Agent and requires Git 2.10 or later. For more information on ssh command options, see the man pages for both ssh and ssh_config.

Use different accounts on a single GitLab instance

You can use multiple accounts to connect to a single instance of GitLab. You can do this by using the command in the previous topic. However, even if you set IdentitiesOnly to yes, you cannot sign in if an IdentityFile exists outside of a Host block.

Instead, you can assign aliases to hosts in the ~.ssh/config file.

For the Host, use an alias like user_1.gitlab.com and user_2.gitlab.com. Advanced configurations are more difficult to maintain, and these strings are easier to understand when you use tools like git remote.
For the IdentityFile, use the path the private key.

# User1 Account Identity
Host <user_1.gitlab.com>
  Hostname gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/<example_ssh_key1>

# User2 Account Identity
Host <user_2.gitlab.com>
  Hostname gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/<example_ssh_key2>

Now, to clone a repository for user_1, use user_1.gitlab.com in the git clone command:

git clone git@<user_1.gitlab.com>:gitlab-org/gitlab.git

To update a previously-cloned repository that is aliased as origin:

git remote set-url origin git@<user_1.gitlab.com>:gitlab-org/gitlab.git

NOTE: Private and public keys contain sensitive data. Ensure the permissions on the files make them readable to you but not accessible to others.

Configure two-factor authentication (2FA)

You can set up two-factor authentication (2FA) for Git over SSH.

Use EGit on Eclipse

If you are using EGit, you can add your SSH key to Eclipse.

Use SSH on Microsoft Windows

If you're running Windows 10, you can either use the Windows Subsystem for Linux (WSL) with WSL 2 which has both git and ssh preinstalled, or install Git for Windows to use SSH through Powershell.

The SSH key generated in WSL is not directly available for Git for Windows, and vice versa, as both have a different home directory:

WSL: /home/<user>
Git for Windows: C:\Users\<user>

You can either copy over the .ssh/ directory to use the same key, or generate a key in each environment.

Alternative tools include:

Overriding SSH settings on the GitLab server

GitLab integrates with the system-installed SSH daemon and designates a user (typically named git) through which all access requests are handled. Users who connect to the GitLab server over SSH are identified by their SSH key instead of their username.

SSH client operations performed on the GitLab server are executed as this user. You can modify this SSH configuration. For example, you can specify a private SSH key for this user to use for authentication requests. However, this practice is not supported and is strongly discouraged as it presents significant security risks.

GitLab checks for this condition, and directs you to this section if your server is configured this way. For example:

$ gitlab-rake gitlab:check

Git user has default SSH configuration? ... no
  Try fixing it:
  mkdir ~/gitlab-check-backup-1504540051
  sudo mv /var/lib/git/.ssh/id_rsa ~/gitlab-check-backup-1504540051
  sudo mv /var/lib/git/.ssh/id_rsa.pub ~/gitlab-check-backup-1504540051
  For more information see:
  [Overriding SSH settings on the GitLab server](#overriding-ssh-settings-on-the-gitlab-server)
  Please fix the error above and rerun the checks.

Remove the custom configuration as soon as you can. These customizations are explicitly not supported and may stop working at any time.

Troubleshooting SSH connections

When you run git clone, you may be prompted for a password, like git@gitlab.example.com's password:. This indicates that something is wrong with your SSH setup.

Ensure that you generated your SSH key pair correctly and added the public SSH key to your GitLab profile.
Try to manually register your private SSH key by using ssh-agent.
Try to debug the connection by running ssh -Tv git@example.com. Replace example.com with your GitLab URL.