How to secure Debian Server 9.X – Setup SSH keys

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 Debian 9 installation. SSH keys provide an easy, secure way of logging into your server and are recommended for all users.

Step 1 – How is an SSH key used

I will first try to visualize the use of SSH keys a bit. Once a client is trying to get access to your server, the SSH service will check with what kind of authentication the client is coming in.

If password authentication is used, and the SSH service is configured to disallow this, the client will be rejected immediately. If the client is using a private key, the server will check if this matches with the public key on the server.

Please check the diagram below.

The first step is to create the SSH key(s). There are 2 possible ways you can do this:

1. You create the private & public key on your server, then download the private key to your client PC.
2. You create the public & private key on your client, and then insert it inside the server configuration.

I personally like to use the second version. The way to accomplish this is different per client (Linux/Windows/Mac) so let’s take a look at that now.

Step 2 – Creating the SSH key

Linux

If you are on a Linux client (Ubuntu, Debian etc.) you will have the ability to use a terminal and create your SSH key easily:

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). However, you would need a supercomputer to crack the 2048-bit RSA 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:

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.

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.

Windows

Most of the users will have a Windows client to connect to your server. Since Windows doesn’t have the same terminal as Linux does, you will not be able to follow the steps above.

Therefor we will use a program called PuTTYgen to create our SSH keys. If you have downloaded Putty as an SSH client before, with the Windows installer, you will also have PuTTYgen.

First run PuTTYgen and click on Generate. The program will ask you to move your mouse randomly on the blank area, this way a random key will be generated.

Here you optionally may enter a secure passphrase and a comment, which is highly recommended. A passphrase adds an additional layer of security to prevent unauthorized users from logging in.
The comment can be used to distinguish your different keys if you have multiple.

Once done, you will have a public key output which we can use to secure our server.

Important: First of all save your private key somewhere safely! This will be used to authenticate to your server once we configured it.

Step 2 — Copy the Public Key to the 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 the alternate method provided in this section.

Copying Public Key Using ssh-copy-id (Linux Client)

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 (Your server) 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 “y” 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 of your server(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:

Now try logging into the machine, with:

and check to make sure that only the key(s) you wanted were added.

At this point, your id_rsa.pub key has been uploaded to the remote account.

Copying Public Key Manually (Windows)

If you do not have a Linux client and not able to use the ssh-copy-id utility, you will have to copy the public key to your server manually.

We will manually append the content of your id_rsa.pub file to the ~/.ssh/authorized_keys file on your remote machine.

Since we already create our SSH keys with PuTTYgen, we can just use the output from this program, copy it, and place it into our remote server.

You will see the key’s content, which should look something like this:

You will have to copy the fully output (check the scroll wheel) of this public key. If you are not logged in yet, login to your remote server with the password authentication.

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 the file if necessary:

In the above command, substitute the public_key_string with the public key output from PuTTYgen on your local PC so it will looke something like this:

Finally, we’ll ensure that the ~/.ssh directory and authorized_keys file have the appropriate permissions set:

This recursively removes all “group” and “other” permissions for the ~/.ssh/ directory.

If you’re using the root account to set up keys for another user account, it’s also important that the ~/.sshdirectory belongs to the other 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 (linux):

On windows & using PuTTY, you will have to change some settings. First enter the hostname (IP) from the remote host again, put this time also use a private key. You can find this under:

Connection –> SSH –> Auth

And browse to your private key we created earlier.

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.

The next step will disable 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 plain 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 the current session:

Once you have verified your SSH service, you can safely close all current server sessions.

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 would like to know more about hardening your Linux server, check the next parts.