Configure SSH Key-Based Authentication

How To Create SSH Keys

The first step to configure SSH key authentication to your server is to generate an SSH key pair on your local computer.

To do this, we can use a special utility called ssh-keygen, which is included with the standard OpenSSH suite of tools. By default, this will create a 2048 bit RSA key pair, which is fine for most uses.

On your local computer, generate a SSH key pair by typing:

<code>$bash>ssh-keygen</code>

or 

Generating public/private rsa key pair.
Enter file in which to save the key (/home/<span class="highlight">username</span>/.ssh/id_rsa):

The utility will prompt you to select a location for the keys that will be generated. By default, the keys will be stored in the ~/.ssh directory within your user's home directory. The private key will be called id_rsa and the associated public key will be called id_rsa.pub.

Usually, it is best to stick with the default location at this stage. Doing so will allow your SSH client to automatically find your SSH keys when attempting to authenticate. If you would like to choose a non-standard path, type that in now, otherwise, press ENTER to accept the default.

If you had previously generated an SSH key pair, you may see a prompt that looks like this:

/home/<span class="highlight">username</span>/.ssh/id_rsa already exists.
Overwrite (y/n)?

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.

Created directory '/home/<span class="highlight">username</span>/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again: 

Next, you will be prompted to enter a passphrase for the key. This is an optional passphrase that can be used to encrypt the private key file on disk.

You may be wondering what advantages an SSH key provides if you still need to enter a passphrase. Some of the advantages are:

• The private SSH key (the part that can be passphrase protected), is never exposed on the network. The passphrase is only used to decrypt the key on the local machine. This means that network-based brute forcing will not be possible against the passphrase.
• The private key is kept within a restricted directory. The SSH client will not recognize private keys that are not kept in restricted directories. The key itself must also have restricted permissions (read and write only available for the owner). This means that other users on the system cannot snoop.
• Any attacker hoping to crack the private SSH key passphrase must already have access to the system. This means that they will already have access to your user account or the root account. If you are in this position, the passphrase can prevent the attacker from immediately logging into your other servers. This will hopefully give you time to create and implement a new SSH key pair and remove access from the compromised key.

Since the private key is never exposed to the network and is protected through file permissions, this file should never be accessible to anyone other than you (and the root user). The passphrase serves as an additional layer of protection in case these conditions are compromised.

A passphrase is an optional addition. If you enter one, you will have to provide it every time you use this key (unless you are running SSH agent software that stores the decrypted key). We recommend using a passphrase, but if you do not want to set a passphrase, you can simply press ENTER to bypass this prompt.

<code>Your identification has been saved in /home/<span class="highlight">username</span>/.ssh/id_rsa.
Your public key has been saved in /home/<span class="highlight">username</span>/.ssh/id_rsa.pub.
The key fingerprint is:
a9:49:2e:2a:5e:33:3e:a9:de:4e:77:11:58:b6:90:26 <span class="highlight">username</span>@remote_host
The key's randomart image is:
+--[ RSA 2048]----+
|     ..o         |
|   E o= .        |
|    o. o         |
|        ..       |
|      ..S        |
|     o o.        |
|   =o.+.         |
|. =++..          |
|o=++.            |
+-----------------+
</code>

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 authentication to log in.

How To Copy a Public Key to your Server

If you already have a server available and did not embed keys upon creation, you can still upload your public key and use it to authenticate to your server.

The method you use depends largely on the tools you have available and the details of your current configuration. The following methods all yield the same end result. The easiest, most automated method is first and the ones that follow each require additional manual steps if you are unable to use the preceding methods.

Copying your Public Key Using SSH-Copy-ID

The easiest way to copy your public key to an existing server is to use a utility called ssh-copy-id. Because of its simplicity, this method is recommended if available.

The ssh-copy-id tool is included in the OpenSSH packages in many distributions, 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 where your public SSH key will be copied.

The syntax is:

<code>ssh-copy-id <span class="highlight">username</span>@<span class="highlight">remote_host</span>
</code>

You may see a message like this:

<code>The authenticity of host '111.111.11.111 (111.111.11.111)' can't be established.
ECDSA key fingerprint is fd:fd:d4:f9:77:fe:73:84:e1:55:00:ad:d6:6d:22:fe.
Are you sure you want to continue connecting (yes/no)? yes
</code>

This just 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:

<code>/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
username@111.111.11.111's password:
</code>

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 will see output that looks like this:

<code>Number of key(s) added: 1

Now try logging into the machine, with:   "ssh 'username@111.111.11.111'"
and check to make sure that only the key(s) you wanted were added.
</code>

At this point, your id_rsa.pub key has been uploaded to the remote account. You can continue onto the next section.

Copying your 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 outputting the content of our public SSH key on our local computer and piping it through an SSH connection to the remote server. On the other side, we can make sure that the ~/.sshdirectory exists under the account we are using and then output the content we piped over into a file called authorized_keys within this directory.

We will 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 will look like this:

<code>cat ~/.ssh/id_rsa.pub | ssh <span class="highlight">username</span>@<span class="highlight">remote_host</span> "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"
</code>

You may see a message like this:

<code>The authenticity of host '111.111.11.111 (111.111.11.111)' can't be established.
ECDSA key fingerprint is fd:fd:d4:f9:77:fe:73:84:e1:55:00:ad:d6:6d:22:fe.
Are you sure you want to continue connecting (yes/no)? yes
</code>

This just 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 will be prompted with the password of the account you are attempting to connect to:

<code>username@111.111.11.111's password:
</code>

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 to the next section if this was successful.

Authenticate to your 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:

<code>ssh <span class="highlight">username</span>@<span class="highlight">remote_host</span>
</code>

If this is your first time connecting to this host (if you used the last method above), you may see something like this:

<code>The authenticity of host '111.111.11.111 (111.111.11.111)' can't be established.
ECDSA key fingerprint is fd:fd:d4:f9:77:fe:73:84:e1:55:00:ad:d6:6d:22:fe.
Are you sure you want to continue connecting (yes/no)? yes
</code>

This just 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 required to enter it now. Afterwards, a new shell session should be spawned for you with the account on the remote system.

If successful, continue on to find out how to lock down the server.

Disabling Password Authentication on your Server

If you were able to login to 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 an account on this server with sudo access. This step will lock down password-based logins, so ensuring that you have will still be able to get administrative access is essential.

Once the above conditions are true, log into your remote server with SSH keys, either as root or with an account with sudo privileges. Open the SSH daemon's configuration file:

<code>sudo nano /etc/ssh/sshd_config
</code>

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 through SSH using account passwords:

<code>PasswordAuthentication no
</code>

Save and close the file when you are finished. To actually implement the changes we just made, you must restart the service.

On Ubuntu or Debian machines, you can issue this command:

<code>sudo service ssh restart
</code>

On CentOS/Fedora machines, the daemon is called sshd:

<code>sudo service sshd restart
</code>

After completing this step, you've successfully transitioned your SSH daemon to only respond to SSH keys.

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