How to configure passwordless SSH login

We normally log in to a remote SSH server using password authentication method. This is the most common authentication method for SSH and is a good enough method for most people. Oftentimes though you might want to automatically log in to an SSH server without having to enter your username and password combination.

This could just be a convenience factor for some, but is vitally important for those doing automation work. A script could be configured to automatically run commands on a remote server or for transferring files using scp command.

You can automatically log in to an SSH server without entering username and password by using public key authentication method.

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 enable or disable public key authentication in SSH

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

   $ ssh-copy-id [email protected]
   /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
   [email protected]'s password:
   
   Number of key(s) added: 2
   
   Now try logging into the machine, with:   "ssh '[email protected]'"
   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 [email protected]
   Last login: Fri Jun 28 00:12:15 2019 from 192.168.111.135
   [[email protected] ~]$

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