How to configure password-less authentication with Open SSH

 In my experience as a network admin is not a good ideea to let users login with their machine credentials so I will show you how to ssh login using public key and private key authentication.
Note . I presume that you have already installed the openssh-server package.
1.Open a shell and type
ssh-keygen (this is the ssh utility for PKI authentication.It will generate a public key and a private key.)

Generating public/private rsa key pair.

Enter file in which to save the key (/home/cata/.ssh/id_rsa):

Enter passphrase (empty for no passphrase):

Enter same passphrase again:

Your identification has been saved in id_rsa.

Your public key has been saved in id_rsa.pub.

The key fingerprint is:

7a:df:3b:ac:6f:72:24:44:31:9d:c1:61:bb:59:9f:61 cata@krishnamurti2

The key's randomart image is:

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

|          o+++   |

|          .o+.   |

|         .  . .E |

|          .  +..o|

|        S.  o  ..|

|       .  . .    |

|      . .  +     |

|       . ...=    |

|          o*+o   |

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

2.Lets presume you want to login without a password to the pc named xubuntu(192.168.2.55).

ssh-copy-id -i ~/.ssh/id_rsa.pub  cata@192.168.2.55 - this will copy your public key(id_rsa.pub) to the remote sistem and generate the file /home/cata.ssh/authorized_keys where your public key will be inserted.

3.Restart ssh server on the remote host :

suso service ssh restart

4.Now from your shell type:

ssh 192.168.2.55 (and you will be loged in without a password)

5.If you want to test that everything is ok type:

ssh-add -l (this will show you all the public keys thata are stored in your public agent)
If there are no keys here and ssh-copy-id has made an error to add a public key manualy do this:

ssh-add

If you want to delete a key from the agent do this
ssh-add -d or ssh-add -D (deletes all keys)
Recommandation
Change your public key password at least once a year .To change the password without generating a new key type:

ssh-keygen -p