SSH login without password

By ValkaTR, 17 April, 2023

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/
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|                 |
|               . |
|         .      o|
|        S o   .Eo|
|         o * = o.|
|          = B + o|
|         = + oo..|
|          o o. o=|

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/ | 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

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.