SSH login without a password

If you are using Linux and OpenSSH to automate your tasks, you will almost always find it easier to have an automatic login that does not require a password. This is especially true if you are using something like rcp for easy file transfers.

Let's configure SSH (version 2) with a secure key to bypass password login from host A / user a to Host B / user b.

Step by step

First login on system 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, although in that case you may have to manually reset the mode with chmod 700 ~/.ssh ):

a@A:~> ssh b@B mkdir -p .ssh -m 700
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_keys2'
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

If that does not work, from system A do:

a@A:~gt; ssh b@B 'chmod 640 ~/.ssh/authorized_keys2'

and you should be good to go.