How to configure passwordless SSH login

The usual way to log in to remote servers using SSH is via the challenge–response protocol by the use of username and password combination.

Yo can also log in using public and private key pair method and when configured, can be used to passwordlessly log in to remote SSH servers; which could be a nice convenience and useful for automation work.

You can configure passwordless SSH login from the terminal.

Setup passwordless SSH login:

1. Launch terminal application.
2. Generate an SSH key pair on the local host if you don't already have one.

   $ ssh-keygen
   Generating public/private rsa key pair.
   Enter file in which to save the key (/home/user/.ssh/id_rsa):
   Enter passphrase (empty for no passphrase):
   Enter same passphrase again:
   Your identification has been saved in /home/user/.ssh/id_rsa.
   Your public key has been saved in /home/user/.ssh/id_rsa.pub.
   ##### snipped

   Make sure to not set any passphrase for the key pair

   How to generate SSH key pair using ssh-keygen

3. Enable public key authentication in the remote SSH server.

   $ sudo grep PubkeyAuthentication /etc/ssh/sshd_config
   [sudo] password for user:
   PubkeyAuthentication yes

   Public key authentication is normally enabled by default.

   How to disable public key authentication in SSH

4. Copy your SSH public key from local host to the SSH server.

   $ ssh-copy-id user@remote-host
   /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
   /usr/bin/ssh-copy-id: INFO: 2 key(s) remain to be installed -- if you are prompted now it is to install the new keys
   user@remote-host's password:
   
   Number of key(s) added: 2
   
   Now try logging into the machine, with:   "ssh 'user@remote-host'"
   and check to make sure that only the key(s) you wanted were added.

   How to copy SSH public key to remote server

5. Log in to the remote server to test.

   $ ssh user@remote-host
   Last login: Fri Jun 28 00:12:15 2019 from 192.168.111.135
   [user@remote-host ~]$

   You will no longer be prompted for password when logging in to the server.