This article describes ways to generate and use secure shell (SSH) keys on a Windows computer to create and connect to a Linux virtual machine (VM) in Azure. To use SSH keys from a Linux or macOS client, see the quick or detailed guidance.
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 remains on your local system. Protect this private key. Do not share it.
PuTTYgen is a key generator tool for creating pairs of public and private SSH keys. It is one of the components of the open-source networking client PuTTY. Although originally written for Microsoft Windows operating system, it is now officially available for multiple operating systems including macOS, Linux.
When you use an SSH client to connect to your Linux VM (which has the public key), the remote VM tests the client to make sure it possesses the private key. If the client has the private key, it's granted access to the VM.
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.
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.
Windows packages and SSH clients
You connect to and manage Linux VMs in Azure using an SSH client. Computers running Linux or macOS usually have a suite of SSH commands to generate and manage SSH keys and to make SSH connections.
Windows computers do not always have comparable SSH commands installed. Recent versions of Windows 10 provide OpenSSH client commands to create and manage SSH keys and make SSH connections from a command prompt. Recent Windows 10 versions also include the Windows Subsystem for Linux to run and access utilities such as an SSH client natively within a Bash shell.
Other common Windows SSH clients you can install locally are included in the following packages:
You can also use the SSH utilities available in Bash in the Azure Cloud Shell.
- Access Cloud Shell in your web browser at https://shell.azure.com or in the Azure portal.
- Access Cloud Shell as a terminal from within Visual Studio Code by installing the Azure Account extension.
Create an SSH key pair
The following sections describe two options to create an SSH key pair on Windows. You can use a shell command (ssh-keygen) or a GUI tool (PuTTYgen). Also note, when using Powershell to create a key, upload the public key as ssh.com(SECSH) format. When using CLI, convert the key into OpenSSH format prior to uploading.
Generating Ssh Keys On Windows
Create SSH keys with ssh-keygen
If you run a command shell on Windows that supports SSH client tools (or you use Azure Cloud Shell), create an SSH key pair using the ssh-keygen command. Type the following command, and answer the prompts. If an SSH key pair exists in the chosen location, those files are overwritten.
For more background and information, see the quick or detailed steps to create SSH keys using ssh-keygen.
Create SSH keys with PuTTYgen
If you prefer to use a GUI-based tool to create SSH keys, you can use the PuTTYgen key generator, included with the PuTTY download package.
To create an SSH RSA key pair with PuTTYgen:
Start PuTTYgen.
Click Generate. By default PuTTYgen generates a 2048-bit SSH-2 RSA key.
Move the mouse around in the blank area to provide randomness for the key.
After the public key is generated, optionally enter and confirm a passphrase. You will be prompted for the passphrase when you authenticate to the VM with your private SSH key. Without a passphrase, if someone obtains your private key, they can sign in to any VM or service that uses that key. We recommend you create a passphrase. However, if you forget the passphrase, there is no way to recover it.
The public key is displayed at the top of the window. You can copy this entire public key and then paste it into the Azure portal or an Azure Resource Manager template when you create a Linux VM. You can also select Save public key to save a copy to your computer:
Optionally, to save the private key in PuTTy private key format (.ppk file), select Save private key. You will need the .ppk file later to use PuTTY to make an SSH connection to the VM.
If you want to save the private key in the OpenSSH format, the private key format used by many SSH clients, select Conversions > Export OpenSSH key.
Provide an 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 or other methods.
The following example shows how you would copy and paste this public key into the Azure portal when you create a Linux VM. The public key is typically then stored in the ~/.ssh/authorized_key directory on your new VM.
Connect to your VM
One way to make an SSH connection to your Linux VM from Windows is to use an SSH client. This is the preferred method if you have an SSH client installed on your Windows system, or if you use the SSH tools in Bash in Azure Cloud Shell. If you prefer a GUI-based tool, you can connect with PuTTY.
Use 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):
If you configured a passphrase when you created your key pair, enter the passphrase when prompted during the sign-in process.
If the VM is using the just-in-time access policy, you need to request access before you can connect to the VM. For more information about the just-in-time policy, see Manage virtual machine access using the just in time policy.
Connect with PuTTY
If you installed the PuTTY download package and previously generated a PuTTY private key (.ppk) file, you can connect to a Linux VM with PuTTY.
Start PuTTy.
Fill in the host name or IP address of your VM from the Azure portal:
Select the Connection > SSH > Auth category. Browse to and select your PuTTY private key (.ppk file):
Click Open to connect to your VM.
Next steps
For detailed steps, options, and advanced examples of working with SSH keys, see Detailed steps to create SSH key pairs.
You can also use PowerShell in Azure Cloud Shell to generate SSH keys and make SSH connections to Linux VMs. See the PowerShell quickstart.
If you have difficulty using SSH to connect to your Linux VMs, see Troubleshoot SSH connections to an Azure Linux VM.
Related
Introduction
SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with a Debian server, chances are you will spend most of your time in a terminal session connected to your server through SSH.
In this guide, we’ll focus on setting up SSH keys for a vanilla Debian 10 installation. SSH keys provide an easy, secure way of logging into your server and are recommended for all users.
Step 1 — Create the RSA Key Pair
The first step is to create a key pair on the client machine (usually your computer):
By default ssh-keygen will create a 2048-bit RSA key pair, which is secure enough for most use cases (you may optionally pass in the -b 4096 flag to create a larger 4096-bit key).
After entering the command, you should see the following output:
Press enter to save the key pair into the .ssh/ subdirectory in your home directory, or specify an alternate path.
If you had previously generated an SSH key pair, you may see the following prompt:
Warning: If you choose to overwrite the key on disk, you will not be able to authenticate using the previous key anymore. Be very careful when selecting yes, as this is a destructive process that cannot be reversed.
You should then see the following prompt:
Here you optionally may enter a secure passphrase, which is highly recommended. A passphrase adds an additional layer of security to prevent unauthorized users from logging in. To learn more about security, consult our tutorial on How To Configure SSH Key-Based Authentication on a Linux Server.
You should then see the following output:
You now have a public and private key that you can use to authenticate. The next step is to place the public key on your server so that you can use SSH-key-based authentication to log in.
Step 2 — Copy the Public Key to Debian Server
The quickest way to copy your public key to the Debian host is to use a utility called ssh-copy-id. Due to its simplicity, this method is highly recommended if available. If you do not have ssh-copy-id available to you on your client machine, you may use one of the two alternate methods provided in this section (copying via password-based SSH, or manually copying the key).
Copying Public Key Using ssh-copy-id
The ssh-copy-id tool is included by default in many operating systems, so you may have it available on your local system. For this method to work, you must already have password-based SSH access to your server.
To use the utility, you simply need to specify the remote host that you would like to connect to and the user account that you have password SSH access to. This is the account to which your public SSH key will be copied.
The syntax is:
You may see the following message:
This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press ENTER to continue.
Next, the utility will scan your local account for the id_rsa.pub key that we created earlier. When it finds the key, it will prompt you for the password of the remote user’s account:
Type in the password (your typing will not be displayed for security purposes) and press ENTER. The utility will connect to the account on the remote host using the password you provided. It will then copy the contents of your ~/.ssh/id_rsa.pub key into a file in the remote account’s home ~/.ssh directory called authorized_keys.
You should see the following output:
At this point, your id_rsa.pub key has been uploaded to the remote account. You can continue on to Step 3.
Copying Public Key Using SSH
If you do not have ssh-copy-id available, but you have password-based SSH access to an account on your server, you can upload your keys using a conventional SSH method.
We can do this by using the cat command to read the contents of the public SSH key on our local computer and piping that through an SSH connection to the remote server.
On the other side, we can make sure that the ~/.ssh directory exists and has the correct permissions under the account we’re using.
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We can then output the content we piped over into a file called authorized_keys within this directory. We’ll use the >> redirect symbol to append the content instead of overwriting it. This will let us add keys without destroying previously added keys.
The full command looks like this:
You may see the following message:
This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press ENTER to continue.
Afterwards, you should be prompted to enter the remote user account password:
After entering your password, the content of your id_rsa.pub key will be copied to the end of the authorized_keys file of the remote user’s account. Continue on to Step 3 if this was successful.
Copying Public Key Manually
If you do not have password-based SSH access to your server available, you will have to complete the above process manually.
We will manually append the content of your id_rsa.pub file to the ~/.ssh/authorized_keys file on your remote machine.
To display the content of your id_rsa.pub key, type this into your local computer:
You will see the key’s content, which should look something like this:
Access your remote host using whichever method you have available.
Once you have access to your account on the remote server, you should make sure the ~/.ssh directory exists. This command will create the directory if necessary, or do nothing if it already exists:
Now, you can create or modify the authorized_keys file within this directory. You can add the contents of your id_rsa.pub file to the end of the authorized_keys file, creating it if necessary, using this command:
In the above command, substitute the public_key_string with the output from the cat ~/.ssh/id_rsa.pub command that you executed on your local system. It should start with ssh-rsa AAAA...
Finally, we’ll ensure that the ~/.ssh directory and authorized_keys file have the appropriate permissions set:
Generate Ssh Key Github
This recursively removes all “group” and “other” permissions for the ~/.ssh/ directory.
If you’re using the root account to set up keys for a user account, it’s also important that the ~/.ssh directory belongs to the user and not to root:
In this tutorial our user is named sammy but you should substitute the appropriate username into the above command.
We can now attempt passwordless authentication with our Debian server.
Step 3 — Authenticate to Debian Server Using SSH Keys
If you have successfully completed one of the procedures above, you should be able to log into the remote host without the remote account’s password.
The basic process is the same:
If this is your first time connecting to this host (if you used the last method above), you may see something like this:
This means that your local computer does not recognize the remote host. Type “yes” and then press ENTER to continue.
If you did not supply a passphrase for your private key, you will be logged in immediately. If you supplied a passphrase for the private key when you created the key, you will be prompted to enter it now (note that your keystrokes will not display in the terminal session for security). After authenticating, a new shell session should open for you with the configured account on the Debian server.
If key-based authentication was successful, continue on to learn how to further secure your system by disabling password authentication.
Step 4 — Disable Password Authentication on your Server
If you were able to log into your account using SSH without a password, you have successfully configured SSH-key-based authentication to your account. However, your password-based authentication mechanism is still active, meaning that your server is still exposed to brute-force attacks.
Before completing the steps in this section, make sure that you either have SSH-key-based authentication configured for the root account on this server, or preferably, that you have SSH-key-based authentication configured for a non-root account on this server with sudo privileges. This step will lock down password-based logins, so ensuring that you will still be able to get administrative access is crucial.
Once you’ve confirmed that your remote account has administrative privileges, log into your remote server with SSH keys, either as root or with an account with sudo privileges. Then, open up the SSH daemon’s configuration file:
Inside the file, search for a directive called PasswordAuthentication. This may be commented out. Uncomment the line and set the value to “no”. This will disable your ability to log in via SSH using account passwords:
Save and close the file when you are finished by pressing CTRL + X, then Y to confirm saving the file, and finally ENTER to exit nano. To actually implement these changes, we need to restart the sshd service:
As a precaution, open up a new terminal window and test that the SSH service is functioning correctly before closing this session:
Once you have verified your SSH service, you can safely close all current server sessions.
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The SSH daemon on your Debian server now only responds to SSH keys. Password-based authentication has successfully been disabled.
Conclusion
You should now have SSH-key-based authentication configured on your server, allowing you to sign in without providing an account password.
If you’d like to learn more about working with SSH, take a look at our SSH Essentials Guide.