Setup SSH Key Authentication on Ubuntu

Create SSH keys keypair

Create your SSH keys pair on Ubuntu to start using SSH key authentication. This process makes a private key you keep safe and a public key you can share. The `ssh-keygen` command is used to generate these keys, allowing you to set things like key strength.

The command below generates a new 4096 bits SSH key pair with your email address as a comment.

ssh-keygen -t rsa -b 4096 -C "your_username@example.com"

After running the commands above, you’ll be prompted to specify a filename for the keys. In most cases, the default location and filename should work.

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

Next, you’ll be asked to type a secure passphrase. A passphrase adds an extra layer of security, so you must type the passphrase before you use the key to log in to the remote machine.

Enter passphrase (empty for no passphrase):

Press ENTER without typing a passphrase.

On your screen, the entire interaction should look similar to the one below.

Generating public/private rsa key pair.
Enter file in which to save the key (/home/richard/.ssh/id_rsa): 
Created directory '/home/richard/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/richard/.ssh/id_rsa
Your public key has been saved in /home/richard/.ssh/id_rsa.pub
The key fingerprint is:
SHA256:F217Tplf9iVDvyTRBRfkeXEdQfCugtgC16BrpRqQYpE admin@example.com
The key's randomart image is:
+---[RSA 4096]----+
|             .=OO|
|  .        .  +.*|
| E     .  . o..=.|
|  o   . o  o oo+.|
|.+   o oS.. ..Bo=|
|o .   * o..  ++==|
|   . + o o . ...o|
|    +   .   .    |
|   .             |
+----[SHA256]-----+

That’s it! You have successfully created a key pair.

Copy the public key to target system

Copy your SSH public key to the target system in Ubuntu after you’ve created it. This step is crucial for SSH key authentication because it tells the server that your computer is allowed to log in without a password. The `ssh-copy-id` command makes this easy.

Run the command below to copy your public key to a remote server.

ssh-copy-id username@server_ip_address

Since key-based authentication isn’t yet configured, you’ll be prompted to type in your SSH password.

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

richard@10.0.2.17's password: 

Number of key(s) added: 1

Now try logging into the machine, with:   "ssh 'richard@10.0.2.17'"
and check to make sure that only the key(s) you wanted were added.

Configure SSH for passwordless login

Configure SSH on Ubuntu to allow passwordless login by disabling password authentication. You’ll do this by logging into your server and changing the SSH configuration file, usually found at `/etc/ssh/sshd_config`. The goal is to turn off the option for using passwords to log in.

Log on to the remote server with your password, then open the SSH configuration file by running the commands below.

sudo nano /etc/ssh/sshd_config

In the file, find the lines below and change the value to match these.

PasswordAuthentication no
ChallengeResponseAuthentication no
UsePAM no

Save the file and exit.

Restart the SSH server on the remote host.

sudo systemctl restart ssh

After that, password login should be disabled.

Using the command `ssh-copy-id user@remote_host` sends your SSH public key to a remote server. This action configures the remote server to recognize your key, allowing passwordless logins for future connections.

ssh username@server_ip_address

That should do it!

Conclusion:

Setting up key-based SSH authentication on Ubuntu Linux uses a process that this article explained. Users can report errors or share additions using the comment form.