Detailed steps: Create and manage SSH keys for authentication to a Linux VM in Azure

With a secure shell (SSH) key pair, you can create a Linux virtual machine on Azure that defaults to using SSH keys for authentication, eliminating the need for passwords to log in. VMs created with the Azure portal, Azure CLI, Resource Manager templates, or other tools can include your SSH public key as part of the deployment, which sets up SSH key authentication for SSH connections.

This article provides detailed background and steps to create and manage an SSH RSA public-private key file pair for SSH client connections. If you want quick commands, see How to create an SSH public-private key pair for Linux VMs in Azure.

For additional ways to generate and use SSH keys on a Windows computer, see How to use SSH keys with Windows on Azure.

Overview of SSH and keys

SSH is an encrypted connection protocol that allows secure sign-ins over unsecured connections. SSH is the default connection protocol for Linux VMs hosted in Azure. Although SSH itself provides an encrypted connection, using passwords with SSH connections still leaves the VM vulnerable to brute-force attacks or guessing of passwords. A more secure and preferred method of connecting to a VM using SSH is by using a public-private key pair, also known as SSH keys.

  • The public key is placed on your Linux VM, or any other service that you wish to use with public-key cryptography.

  • The private key is what you present to your Linux VM when you make an SSH connection, to verify your identity. Protect this private key. Do not share it.

Depending on your organization's security policies, you can reuse a single public-private key pair to access multiple Azure VMs and services. You do not need a separate pair of keys for each VM or service you wish to access.

Your public key can be shared with anyone, but only you (or your local security infrastructure) should possess your private key.

Private key passphrase

The SSH private key should have a very secure passphrase to safeguard it. This passphrase is just to access the private SSH key file and is not the user account password. When you add a passphrase to your SSH key, it encrypts the private key using 128-bit AES, so that the private key is useless without the passphrase to decrypt it. If an attacker stole your private key and that key did not have a passphrase, they would be able to use that private key to log in to any servers that have the corresponding public key. If a private key is protected by a passphrase, it cannot be used by that attacker, providing an additional layer of security for your infrastructure on Azure.

Supported SSH key formats

Azure currently supports SSH protocol 2 (SSH-2) RSA public-private key pairs with a minimum length of 2048 bits. Other key formats such as ED25519 and ECDSA are not supported.

SSH keys use and benefits

When you create an Azure VM by specifying the public key, Azure copies the public key (in the .pub format) to the ~/.ssh/authorized_keys folder on the VM. SSH keys in ~/.ssh/authorized_keys are used to challenge the client to match the corresponding private key on an SSH login connection. In an Azure Linux VM that uses SSH keys for authentication, Azure configures the SSHD server to not allow password logins, only SSH keys. Therefore, by creating an Azure Linux VM with SSH keys, you can help secure the VM deployment and save yourself the typical post-deployment configuration step of disabling passwords in the sshd_config file.

If you do not wish to use SSH keys, you can set up your Linux VM to use password authentication. If your VM is not exposed to the Internet, using passwords may be sufficient. However, you still need to manage your passwords for each Linux VM and maintain healthy password policies and practices, such as minimum password length and regular updates. Using SSH keys reduces the complexity of managing individual credentials across multiple VMs.

Generate keys with ssh-keygen

To create the keys, a preferred command is ssh-keygen, which is available with OpenSSH utilities in the Azure Cloud Shell, a macOS or Linux host, the Windows Subsystem for Linux, and other tools. ssh-keygen asks a series of questions and then writes a private key and a matching public key.

SSH keys are by default kept in the ~/.ssh directory. If you do not have a ~/.ssh directory, the ssh-keygen command creates it for you with the correct permissions.

Basic example

The following ssh-keygen command generates 2048-bit SSH RSA public and private key files by default in the ~/.ssh directory. If an SSH key pair exists in the current location, those files are overwritten.

ssh-keygen -t rsa -b 2048

Detailed example

The following example shows additional command options to create an SSH RSA key pair. If an SSH key pair exists in the current location, those files are overwritten.

ssh-keygen \
    -t rsa \
    -b 4096 \
    -C "azureuser@myserver" \
    -f ~/.ssh/mykeys/myprivatekey \
    -N mypassphrase

Command explained

ssh-keygen = the program used to create the keys

-t rsa = type of key to create, in this case in the RSA format

-b 4096 = the number of bits in the key, in this case 4096

-C "azureuser@myserver" = a comment appended to the end of the public key file to easily identify it. Normally an email address is used as the comment, but use whatever works best for your infrastructure.

-f ~/.ssh/mykeys/myprivatekey = the filename of the private key file, if you choose not to use the default name. A corresponding public key file appended with .pub is generated in the same directory. The directory must exist.

-N mypassphrase = an additional passphrase used to access the private key file.

Example of ssh-keygen

ssh-keygen -t rsa -b 2048 -C "azureuser@myserver"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/azureuser/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/azureuser/.ssh/id_rsa.
Your public key has been saved in /home/azureuser/.ssh/id_rsa.pub.
The key fingerprint is:
14:a3:cb:3e:78:ad:25:cc:55:e9:0c:08:e5:d1:a9:08 azureuser@myserver
The keys randomart image is:
+--[ RSA 2048]----+
|        o o. .   |
|      E. = .o    |
|      ..o...     |
|     . o....     |
|      o S =      |
|     . + O       |
|      + = =      |
|       o +       |
|        .        |
+-----------------+

Saved key files

Enter file in which to save the key (/home/azureuser/.ssh/id_rsa): ~/.ssh/id_rsa

The key pair name for this article. Having a key pair named id_rsa is the default; some tools might expect the id_rsa private key file name, so having one is a good idea. The directory ~/.ssh/ is the default location for SSH key pairs and the SSH config file. If not specified with a full path, ssh-keygen creates the keys in the current working directory, not the default ~/.ssh.

List of the ~/.ssh directory

ls -al ~/.ssh
-rw------- 1 azureuser staff  1675 Aug 25 18:04 id_rsa
-rw-r--r-- 1 azureuser staff   410 Aug 25 18:04 id_rsa.pub

Key passphrase

Enter passphrase (empty for no passphrase):

It is strongly recommended to add a passphrase to your private key. Without a passphrase to protect the key file, anyone with the file can use it to log in to any server that has the corresponding public key. Adding a passphrase offers more protection in case someone is able to gain access to your private key file, giving you time to change the keys.

Generate keys automatically during deployment

If you use the Azure CLI to create your VM, you can optionally generate SSH public and private key files by running the az vm create command with the --generate-ssh-keys option. The keys are stored in the ~/.ssh directory. Note that this command option does not overwrite keys if they already exist in that location.

Provide SSH public key when deploying a VM

To create a Linux VM that uses SSH keys for authentication, provide your SSH public key when creating the VM using the Azure portal, CLI, Resource Manager templates, or other methods. When using the portal, you enter the public key itself. If you use the Azure CLI to create your VM with an existing public key, specify the value or location of this public key by running the az vm create command with the --ssh-key-value option.

If you're not familiar with the format of an SSH public key, you can see your public key by running cat as follows, replacing ~/.ssh/id_rsa.pub with your own public key file location:

cat ~/.ssh/id_rsa.pub

Output is similar to the following (here redacted):

ssh-rsa XXXXXXXXXXc2EAAAADAXABAAABAXC5Am7+fGZ+5zXBGgXS6GUvmsXCLGc7tX7/rViXk3+eShZzaXnt75gUmT1I2f75zFn2hlAIDGKWf4g12KWcZxy81TniUOTjUsVlwPymXUXxESL/UfJKfbdstBhTOdy5EG9rYWA0K43SJmwPhH28BpoLfXXXXXG+/ilsXXXXXKgRLiJ2W19MzXHp8z3Lxw7r9wx3HaVlP4XiFv9U4hGcp8RMI1MP1nNesFlOBpG4pV2bJRBTXNXeY4l6F8WZ3C4kuf8XxOo08mXaTpvZ3T1841altmNTZCcPkXuMrBjYSJbA8npoXAXNwiivyoe3X2KMXXXXXdXXXXXXXXXXCXXXXX/ azureuser@myserver

If you copy and paste the contents of the public key file into the Azure portal or a Resource Manager template, make sure you don't copy any additional whitespace or introduce additional linebreaks. For example, if you use macOS, you can pipe the public key file (by default, ~/.ssh/id_rsa.pub) to pbcopy to copy the contents (there are other Linux programs that do the same thing, such as xclip).

If you prefer to use a public key that is in a multiline format, you can generate an RFC4716 formatted key in a pem container from the public key you previously created.

To create a RFC4716 formatted key from an existing SSH public key:

ssh-keygen \
-f ~/.ssh/id_rsa.pub \
-e \
-m RFC4716 > ~/.ssh/id_ssh2.pem

SSH to your VM with an SSH client

With the public key deployed on your Azure VM, and the private key on your local system, SSH to your VM using the IP address or DNS name of your VM. Replace azureuser and myvm.westus.cloudapp.azure.com in the following command with the administrator user name and the fully qualified domain name (or IP address):

ssh azureuser@myvm.westus.cloudapp.azure.com

If you provided a passphrase when you created your key pair, enter the passphrase when prompted during the login process. (The server is added to your ~/.ssh/known_hosts folder, and you won't be asked to connect again until the public key on your Azure VM changes or the server name is removed from ~/.ssh/known_hosts.)

Use ssh-agent to store your private key passphrase

To avoid typing your private key file passphrase with every SSH login, you can use ssh-agent to cache your private key file passphrase. If you are using a Mac, the macOS Keychain securely stores the private key passphrase when you invoke ssh-agent.

Verify and use ssh-agent and ssh-add to inform the SSH system about the key files so that you do not need to use the passphrase interactively.

eval "$(ssh-agent -s)"

Now add the private key to ssh-agent using the command ssh-add.

ssh-add ~/.ssh/id_rsa

The private key passphrase is now stored in ssh-agent.

Use ssh-copy-id to copy the key to an existing VM

If you have already created a VM, you can install the new SSH public key to your Linux VM with a command similar to the following:

ssh-copy-id -i ~/.ssh/id_rsa.pub azureuser@myserver

Create and configure an SSH config file

You can create and configure an SSH config file (~/.ssh/config) to speed up log-ins and to optimize your SSH client behavior.

The following example shows a simple configuration that you can use to quickly log in as a user to a specific VM using the default SSH private key.

Create the file

touch ~/.ssh/config

Edit the file to add the new SSH configuration

vim ~/.ssh/config

Example configuration

Add configuration settings appropriate for your host VM.

# Azure Keys
Host myvm
  Hostname 102.160.203.241
  User azureuser
# ./Azure Keys

You can add configurations for additional hosts to enable each to use its own dedicated key pair. See SSH config file for more advanced configuration options.

Now that you have an SSH key pair and a configured SSH config file, you are able to log in to your Linux VM quickly and securely. When you run the following command, SSH locates and loads any settings from the Host myvm block in the SSH config file.

ssh myvm

The first time you log in to a server using an SSH key, the command prompts you for the passphrase for that key file.

Next steps

Next up is to create Azure Linux VMs using the new SSH public key. Azure VMs that are created with an SSH public key as the login are better secured than VMs created with the default login method, passwords.