There are two steps to implement automatic authentication with SSH. They are as follows:

• Creating the SSH key on the local machine
• Transferring the public key to the remote host and appending it to ~/.ssh/authorized_keys (which requires access to the remote machine)

To create an SSH key, run the ssh-keygen command with the encryption algorithm type specified as RSA:

$ ssh-keygen -t rsa 
Generating public/private rsa key pair.  
Enter file in which to save the key (/home/username/.ssh/id_rsa):  
Created directory '/home/username/.ssh'.  
Enter passphrase (empty for no passphrase):  
Enter same passphrase again:  
Your identification has been saved in /home/username/.ssh/id_rsa.  
Your public key has been saved in /home/username/.ssh/id_rsa.pub.  
The key fingerprint is:  
f7:17:c6:4d:c9:ee:17:00:af:0f:b3:27:a6:9c:0a:05 username@slynux-laptop 
The key'srandomart image is:  
+--[ RSA 2048]----+  
|           .     |  
|            o . .| 
|     E       o o.| 
|      ...oo |  
|       .S .+  +o.|  
|      .  . .=....|  
|     .+.o...|  
|      . . + o.  .| 
|       ..+       |  
+-----------------+  

You need to enter a passphrase to generate the public-private key pair. It is possible to generate the key pair without entering a passphrase, but it is insecure.

If you intend to write scripts that use automated login to several machines, you should leave the passphrase empty to prevent the script from asking for a passphrase while running.

The ssh-keygen program creates two files. ~/.ssh/id_rsa.pub and ~/.ssh/id_rsa:id_rsa.pub is the generated public key and id_rsa is the private key. The public key has to be appended to the ~/.ssh/authorized_keys file on remote servers where we need to auto-login from the current host.

This command will append a key file:

$ ssh USER@REMOTE_HOST \
    "cat >> ~/.ssh/authorized_keys" < ~/.ssh/id_rsa.pub
Password:

Provide the login password in the previous command.

The auto-login has been set up from now onwards, so SSH will not prompt for passwords during execution. Test this with the following command:

$ ssh USER@REMOTE_HOST uname 
Linux 

You will not be prompted for a password. Most Linux distros include ssh-copy-id, which will append your private key to the appropriate authorized_keys file on the remote server. This is shorter than the ssh technique described earlier:

ssh-copy-id USER@REMOTE_HOST