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 password based and public-key based authentication. Using SSH keys is more secure and convenient than traditional password authentication.
In this tutorial, we will describe how to generate SSH keys on Debian 9 systems. 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 Debian
Before generating a new SSH key pair first, check for existing SSH keys on your Debian client machine. You can do that by running the following ls command:
ls -l ~/.ssh/id_*.pub
If the output of the command above contains something like
No such file or directory or
no matches found it means that you don’t have SSH keys, and you can continue with the next step and generate a new 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.
Start by generating a new 4096 bits SSH key pair with your email address as a comment using the following command:
ssh-keygen -t rsa -b 4096 -C "email@example.com"
The output will look similar to the following:
Enter file in which to save the key (/home/yourusername/.ssh/id_rsa):
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. With passphrase, an extra layer of security is added to your key.
Enter passphrase (empty for no passphrase):
If you don’t want to use passphrase just press
The whole interaction looks like this:
To verify that the SSH key pair was generated, type:
The output should look something like this:
Copy the Public Key to the Server
Now that you have 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 the public key to the remote server is to use the
On your local machine terminal tun the following command:
You will be prompted to enter the
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.
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 the Server using SSH Keys
At this point, you should be able to log in to the remote server without being prompted for a password.
To test it, try to connect to the server via SSH:
If you haven’t set a passphrase, you will be logged in immediately. Otherwise, you will be prompted to enter the passphrase.
Disabling SSH Password Authentication
To add an extra layer of security to your server, you can disable the password authentication for SSH.
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:
Open the SSH configuration file
sudo nano /etc/ssh/sshd_config
Search for the following directives and modify as it follows:
PasswordAuthentication no ChallengeResponseAuthentication no UsePAM no
Once you are done, save the file and restart the SSH service using the following command:
sudo systemctl restart ssh
At this point, the password-based authentication is disabled.
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.