Secure Shell (SSH) is a cryptographic network protocol used for a  secure connection between a client and a server and supports various  authentication mechanisms.

The two most popular mechanisms are  passwords based authentication and public key-based authentication.  Using SSH keys is more secure and convenient than traditional password  authentication.

In  this tutorial, we will walk through how to generate SSH keys on Ubuntu  18.04 machines. We will also show you how to set up an SSH key-based  authentication and connect to your remote Linux servers without entering  a password.

Creating SSH keys on Ubuntu

Before  generating a new SSH key pair first, check for existing SSH keys on  your Ubuntu client machine. You can do that by running the following ls command:

ls -l ~/.ssh/id_*.pub

If the command above prints something like No such file or directory or no matches found it means that you don’t have SSH keys on your client machine and you can proceed with the next step, and generate SSH key pair.

If there are existing keys, you can either use those and skip the next step or backup up the old keys and generate new ones.

Generate a new 4096 bits SSH key pair with your email address as a comment by typing:

ssh-keygen -t rsa -b 4096 -C "your_email@domain.com"

The output will look something like this:

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

Press Enter to accept the default file location and file name.

Next,  you’ll be prompted to type a secure passphrase. Whether you want to use  passphrase, it’s up to you. If you choose to use passphrase you will  get an extra layer of security.

Enter passphrase (empty for no passphrase):

If you don’t want to use passphrase just press Enter.

The whole interaction looks like this:

To verify your new SSH key pair is generated, type:

ls ~/.ssh/id_*
/home/yourusername/.ssh/id_rsa /home/yourusername/.ssh/id_rsa.pub

Copy the Public Key to Ubuntu Server

Now that you generated your SSH key pair, the next step is to copy the public key to the server you want to manage.

The easiest and the recommended way to copy your public key to the server is to use a utility called ssh-copy-id. On your local machine terminal type:

ssh-copy-id remote_username@server_ip_address

You will be prompted to enter the remote_username password:

remote_username@server_ip_address's password:

Once the user is authenticated, the public key ~/.ssh/id_rsa.pub will be appended to the remote user ~/.ssh/authorized_keys file and connection will be closed.

Number of key(s) added: 1

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

If by some reason the ssh-copy-id utility is not available on your local computer, you can use the following command to copy the public key:

cat ~/.ssh/id_rsa.pub | ssh remote_username@server_ip_address "mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"

Login to your server using SSH keys

After completing the steps above you should be able to log in to the remote server without being prompted for a password.

To test it, try to login to your server via SSH:

ssh remote_username@server_ip_address

If  you haven’t set a passphrase for the private key, you will be logged in  immediately. Otherwise, you will be prompted to enter the passphrase.

Disabling SSH Password Authentication

Disabling the password authentication adds an extra layer of security to your server.

Before  disabling SSH password authentication, make sure you can log in to your  server without a password, and the user you are logging in with has sudo privileges.

Log into your remote server:

ssh sudo_user@server_ip_address

Open the SSH configuration file /etc/ssh/sshd_config with your text editor:

sudo nano /etc/ssh/sshd_config

Search for the following directives and modify as it follows:

/etc/ssh/sshd_config

PasswordAuthentication no
ChallengeResponseAuthentication no
UsePAM no

Copy

Once you are done, save the file and restart the SSH service by typing:

sudo systemctl restart ssh

At this point, the password-based authentication is disabled.

Conclusion

In  this tutorial, you have learned how to generate a new SSH key pair and  set up an SSH key-based authentication. You can add the same key to  multiple remote servers. We have also shown you how to disable SSH  password authentication and add an extra layer of security to your  server.

By default, SSH listens on port 22. Changing the default SSH port reduces the risk of automated attacks.

If you are regularly connecting to multiple systems, you can simplify your workflow by defining all of your connections in the SSH config file.

If you have any questions or feedback, feel free to leave a comment.