Setup SSH Passwordless Login

Secure Shell (SSH) is a cryptographic network protocol used for 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.

In this tutorial, we will show you how to setup an SSH key-based authentication as well how to connect to your Linux server without entering a password.

To set up a passwordless SSH login in Linux all you need to do is to generate a public authentication key and append it to the remote hosts \~/.ssh/authorized\_keys file.

The following steps will describe the process for configuring passwordless SSH login:

Before generating a new SSH key pair first check if you already have an SSH key on your client machine because you don’t want to overwrite your existing keys.

Run the following ls command to see if existing SSH keys are present:

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

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

If you see No such file or directory or no matches found it means that you do not have an SSH key and you can proceed with the next step and generate a new one.

The following command will generate a new 4096 bits SSH key pair with your email address as a comment:

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

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

Next, the ssh-keygen tool will ask you 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. In most cases, developers and system administrators use SSH without a passphrase because they are useful for fully automated processes. If you don’t want to use a passphrase just press Enter.

To be sure that the SSH keys are generated you can list your new private and public keys with:

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

Now that you have generated an SSH key pair, in order to be able to login to your server without a password you need to copy the public key to the server you want to manage.

The easiest way to copy your public key to your server is to use a command 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:

Once the user is authenticated, the public key will be appended to the remote user authorized\_keys file and connection will be closed.

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"

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

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

  ssh remote_username@server_ip_address

If everything went well, you will be logged in immediately.

To add an extra layer of security to your server you can disable the password authentication for SSH.

Before disabling the 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.

The following tutorials describe how to configure sudo access:

Log into your remote server with SSH keys, either as a user with sudo privileges or root:

  ssh sudo_user@server_ip_address

Open the SSH configuration file /etc/ssh/sshd\_config, search for the following directives and modify as it follows:

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

Once you are done save the file and restart the SSH service.

On Ubuntu or Debian servers, run the following command:

  sudo systemctl restart ssh

On CentOS or Fedora servers, run the following command:

  sudo systemctl restart sshd

PASO 1 - Copiar llaves SSH para que no se nos pida siempre ingresar la contraseña a los servidores que queramos automatizar su control:

Y ya tenemos un par de claves SSH dentro del directorio .ssh:

Ahora tengo mi máquina anfitrión de KDE o Kali Linux y mi máquina Ubuntu que voy a administrar con ansible:

Ahora vamos a importar esta clave SSH al Ubuntu con este comando:

Ahora ya puedo entrar en el servidor sin necesitad de proporcionar ninguna contraseña, por ejemplo si ejecuto el comando de ssh directamente podremos entrar sin problemas:

Ahora por último tenemos que configurar que también podamos conectarnos como usuario root a través de SSH; y para ello debemos editar el fichero de configuración /etc/ssh/sshd_config y cambiar a “yes” el apartado de PermitRootLogin yes:

Y ahora dentro de los equipos que queramos administrar tenemos que establecer una nueva contraseña para root:

Y ya podremos conectarnos como root en ssh:

Y ahora por último vamos a hacer que la conexión sea automática al igual que ocurre si usamos el usuario mario:

Y ya podremos conectarnos de forma automática tanto con el usuario mario como con el usuario root dentro de este equipo: