SSH login without password

Description

It is very useful to automate tasks for several hosts at the same time. Start and stop services, list directories, operations with files. We can do it remotely using ssh but we cannot automate because password is required in interactive mode. To avoid this problem we can use rsa public keys. In this article we are going to view how to configure remote access to other machines via ssh without password prompt.

Generate rsa public key in local machine

Generate the public rsa key with ssh-keygen command

[zoomer@oel6 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/zoomer/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/zoomer/.ssh/id_rsa.
Your public key has been saved in /home/zoomer/.ssh/id_rsa.pub.
The key fingerprint is:
20:d0:44:d5:b8:ae:6e:a8:3a:30:24:93:a1:4c:37:db zoomer@oel6.localdomain
The key's randomart image is:
+--[ RSA 2048]----+
|  .+o..o         |
|.. +. . .        |
|+o. = ..         |
|=o . P..         |
|o.   .  T        |
|o     .          |
|.. . .           |
|. . o            |
|+o o.            |
+-----------------+

Now in $HOME/.ssh we have the public key for our host:

[zoomer@oel6 .ssh]$ cd $HOME/.ssh
[zoomer@oel6 .ssh]$ ls -la
total 16
drwx------.  2 zoomer zoomer 4096 nov 23 11:07 .
drwx------. 28 zoomer zoomer 4096 nov 23 10:43 ..
-rw-------.  1 zoomer zoomer 1675 nov 23 11:07 id_rsa
-rw-r--r--.  1 zoomer zoomer  405 nov 23 11:07 id_rsa.pub
[zoomer@oel6 .ssh]$ cat id_rsa.pub
ssh-rsa AAAAB3NzaC1yc2EBCDABIwAAAQEAw/naiJmXw7GS1TEFr8PixmbkGslyPqWGCgAmEupUFUwn1I++gsjz2+sGuxcknQahPJCReJlOsu3Ik8NiFuQ+kR7XsXpHcp7HmBiHU2Hk+u8fRSE+kxN717a2joufy0mA6V3ST3xmS8frKZIA7OEHOKHyvnE6SluZWYTgSDMKxfm2VJPa59MP+k4d1yRAuiLLJE8dsfMCekZER8k0gHxtyeH14QZVTekrREDQismlFDChhaJarQ9Ii3E2mX2ZD355y7LDdJFhRmsE4kKTb+/UvzRl4zIQdnucULWv1mddM3ypmYzi6E00qxKU1v0RbJpbpVlC8SxtJFe0iDgyKrieuw== zoomer@oel6host.localdomain

Copy public key to target host authorized_keys file

Copy your public key file id_rsa.pub to the target host on $HOME/.ssh directory and include the text in the authorized_keys file

[zoomer@oel6 .ssh]$ cat id_rsa.pub >> $HOME/.ssh/authorized_keys

Access using ssh without password

Now we can access in a passwordless mode with ssh to the remote machine

[zoomer@oel6 ~]$ ssh localhost
Last login: Fri Nov 23 11:12:45 2012 from localhost
[zoomer@oel6 ~]$

Common problems

Be sure that permissions are set as bellow in both machines:

[zoomer@oel6 ] chmod 755 $HOME $HOME/.ssh
[zoomer@oel6 ] chmod 600 $HOME/.ssh/authorized_keys

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