SSH login without password

Your aim

You want to use Linux and OpenSSH to automize your tasks. Therefore you need an automatic login from host A / user a to Host B / user b. You don't want to enter any passwords, because you want to call ssh from a within a shell script.

How to do it

First log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:

a@A:~> ssh-keygen -t rsa 

Generating public/private rsa key pair. 

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

Created directory '/home/a/.ssh'. 

Enter passphrase (empty for no passphrase): 

Enter same passphrase again: 

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

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

The key fingerprint is: 3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A


Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):

a@A:~> ssh b@B mkdir -p .ssh 

b@B's password:

Finally append a's new public key to b@B:.ssh/authorized_keys and enter b's password one last time:

a@A:~> cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys' 

b@B's password:

From now on you can log into B as b from A as a without password:

a@A:~> ssh b@B hostname B


A note from one of our readers: Depending on your version of SSH you might also have to do the following changes:
Put the public key in .ssh/authorized_keys2
Change the permissions of .ssh to 700
Change the permissions of .ssh/authorized_keys2 to 640

Reference