How to configure Passwordless SSH login in Linux ?

SSH is often used to login from one machine to another machine, There are number of methods to achieve this but mostly in every method it requires authentication..... It also does requires authentication but for one time only i.e. for the first time you need to do a setup and for rest of the times when you will try to login via ssh it will not ask for any password.

For achieving this you just need to generate your own personal set of private/public key pair. ssh-keygen is used to generate that key pair for you.

A HOWTO for generating your own private/public key pair is given below:

Firstly generate your private/public key pair by following command:

[root@linuxforfreshers.com Desktop]$ ssh-keygen -t rsa

Generating public/private rsa key pair.

Enter file in which to save the key (/home/root/.ssh/id_rsa): [Press enter if you want to save your keys to default location]

Enter passphrase (empty for no passphrase): [Enter your passphrase]

Enter same passphrase again:  [Enter your passphrase again to verify]

Your identification has been saved in /home/root/.ssh/id_rsa.

Your public key has been saved in /home/root/.ssh/id_rsa.pub.

The key fingerprint is:

52:17:88:67:f9:ae:03:e2:59:39:97:f9:7d:95:b6:ed root@linuxforfreshers.com

The key's randomart image is:

+--[ RSA 2048]----+

|       . o.      |

|      . =  .     |

|       o...      |

|       . ..      |

|      ..S+      .|

|    . =.+ .    o.|

|   . + + o .  ..o|

|    o   o . . ...|

|         .   . .E|

+-----------------+

Now just copy your public key to remote machine, in this case I have generated key pair for server.example.com and copied server's public key to client's machine via following command.

[root@linuxforfreshers.com .ssh]$ ssh-copy-id -i ~/.ssh/id_rsa.pub  client@192.168.86.129

client@192.168.86.129's password:  [Enter the password for the first time]

Now try logging into the machine, with "ssh 'client@192.168.86.129'", and check in:

cat  /home/client/ .ssh/authorized_keys

Now when you will try to login to client's machine it will not prompt for the password.

[root@linuxforfreshers.com ~]$ ssh client@192.168.86.129

Last login: Thu Dec  9 10:45:35 2015 from server.redhat.com