How to Set Up SSH Keys on CentOS 8

Posted 

4 min read

Secure Shell (SSH) is a cryptographic network protocol designed for a secure connection between a client and a server.

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

This article describes how to generate SSH keys on CentOS 8 systems. We’ll also show you how to set up an SSH key-based authentication and connect to remote Linux servers without entering a password.

Creating SSH keys on CentOS

The chances are that you already have an SSH key pair on your CentOS client machine. If you are generating a new key pair, the old one will be overwritten.

Run the following ls command to check whether the key files exist:

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

If the output of the command returns something like No such file or directory, or no matches found it means that the user does not have SSH keys, and you can proceed with the next step and generate SSH key pair.

Otherwise, if you have an SSH key pair, you can either use those or backup up the old keys and generate new ones.

To generate a new 4096 bits SSH key pair with your email address as a comment, run:

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

You will be prompted to specify the file name:

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 asked to type a secure passphrase. Whether you want to use passphrase, it’s up to you. A passphrase will add an extra layer of security. If you don’t want to use passphrase just press Enter.

Enter passphrase (empty for no passphrase):

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

That’s it. You’ve successfully generated an SSH key pair on your CentOS client machine.

Copy the Public Key to the Server

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

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

ssh-copy-id remote_username@server_ip_address

The command will ask you to enter the remote_username password:

remote_username@server_ip_address's password:

Once the user is authenticated, the content of the public key file (~/.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 ssh-copy-id is not available on your local computer, 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 verify 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 asked to enter the passphrase.

Disabling SSH Password Authentication

To add an additional layer of security to your remote server, you can disable SSH password authentication.

Before continuing, make sure you can log in to your server without a password as a user with sudo privileges .

Follow the steps below to disable SSH password authentication:

  1. Log into your remote server:

    ssh sudo_user@server_ip_address
  2. Open the SSH configuration file /etc/ssh/sshd_config with your text editor :

    sudo nano /etc/ssh/sshd_config
  3. Search for the following directives and modify as it follows:

    /etc/ssh/sshd_config
    PasswordAuthentication no
    ChallengeResponseAuthentication no
    UsePAM no
  4. 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

We’ve shown you how to generate a new SSH key pair and set up an SSH key-based authentication. You can use the same key to manage multiple remote servers. You have also learned 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. To simplify your workflow, use the SSH config file to define all your SSH connections.

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