Decide the SSH authentication method

Before we can create a Linux virtual machine in Azure, we will need to think about remote access. We want to be able to sign in to our Linux web server to configure the software and perform maintenance. The default approach to administering Linux VMs hosted in Azure is SSH.

What is SSH?

Secure Shell (SSH) is an encrypted connection protocol that allows secure sign-ins over unsecured connections. SSH allows you to connect to a terminal shell from a remote location using a network connection.

There are two approaches we can use to authenticate an SSH connection: username and password, or an SSH key pair.

Although SSH provides an encrypted connection, using passwords with SSH connections leaves the VM vulnerable to brute-force attacks of passwords. A more secure and preferred method of connecting to a Linux VM with SSH is a public-private key pair, also known as SSH keys.

With an SSH key pair, you can sign in to Linux-based Azure virtual machines without a password. This is a more secure approach if you only plan to sign in to the VM from a few computers. If you need to be able to access the Linux VM from a variety of locations, a username and password combination might be a better approach. There are two parts to an SSH key pair: a public key and a private key.

The public key is placed on your Linux VM or any other service that you wish to use with public-key cryptography. This can be shared with anyone.
The private key is what you present to verify your identity to your Linux VM when you make an SSH connection. Consider this confidential information and protect this like you would a password or any other private data.

You can use the same single public-private key pair to access multiple Azure VMs and services.

Create the SSH key pair

On Linux, Windows 10, and MacOS, you can use the built-in ssh-keygen command to generate the SSH public and private key files.

Windows 10 includes an SSH client with the Fall Creators Update. Earlier versions of Windows require additional software to use SSH. Alternatively, you can install the Linux subsystem for Windows and get the same functionality.

We will use Azure Cloud Shell, which will store the generated keys in Azure in your private storage account. You can also type these commands directly into your local shell if you prefer.

Here is the minimum command necessary to generate the key pair for an Azure VM. This will create an SSH protocol 2 (SSH-2) RSA public-private key pair. The minimum length is 2048, but for the sake of this learning we will use 4096.

Type this command into Cloud Shell:

ssh-keygen -t rsa -b 4096

The tool will prompt for file names and an optional passphrase. For this exercise, just take the defaults. It will create two files: id_rsa and id_rsa.pub in the ~/.ssh directory. The files will be overwritten if they exist. There are various options you can use to provide the file name or a passphrase to avoid the prompt.

Private key passphrase

You can optionally provide a passphrase while generating your private key. This is a password you must enter when you use the key. This passphrase is used 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.

It is strongly recommended that you add a passphrase. 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 passphrase protects a private key, it cannot be used by that attacker. This provides an additional layer of security for your infrastructure on Azure.

Use the SSH key pair with an Azure Linux VM

Once you have the key pair generated, you can use it with a Linux VM in Azure. You can supply the public key during the VM creation or add it after the VM has been created.

You can view the contents of the file in Azure Cloud Shell with the following command:

cat ~/.ssh/id_rsa.pub

It will be a single line and look something like:

ssh-rsa XXXXXXXXXXc2EAAAADAXABAAABAXC5Am7+fGZ+5zXBGgXS6GUvmsXCLGc7tX7/rViXk3+eShZzaXnt75gUmT1I2f75zFn2hlAIDGKWf4g12KWcZxy81TniUOTjUsVlwPymXUXxESL/UfJKfbdstBhTOdy5EG9rYWA0K43SJmwPhH28BpoLfXXXXXGX/ilsXXXXXKgRLiJ2W19MzXHp8z3Lxw7r9wx3HaVlP4XiFv9U4hGcp8RMI1MP1nNesFlOBpG4pV2bJRBTXNXeY4l6F8WZ3C4kuf8XxOo08mXaTpvZ3T1841altmNTZCcPkXuMrBjYSJbA8npoXAXNwiivyoe3X2KMXXXXXdXXXXXXXXXXCXXXXX/ [email protected]

Use the SSH key when creating a Linux VM

To apply the SSH key while creating a new Linux VM, you will need to copy the contents of the public key and supply it to the Azure portal, or supply the public key file to the Azure CLI or Azure PowerShell command. We’ll use this approach when we create our Linux VM.

Add the SSH key to an existing Linux VM

If you have already created a VM, you can install the public key onto your Linux VM with the ssh-copy-id command. Once the key has been authorized for SSH, it grants access to the server without a password, though you will still be prompted for the passphrase on the key if you set one.

For example, if we had a Linux VM named myserver with a user azureuser, we could use the following command to install the public key file and authorize the user with the key:

ssh-copy-id -i ~/.ssh/id_rsa.pub [email protected]

Now that we have our public key, let’s switch to the Azure portal and create a Linux VM.