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SSH login without password

 ·  ☕ 2 min read
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  • #ssh

You want to use Linux and OpenSSH to automate 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. First log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:

[email protected]:~> 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:
ae:ff:03:74:ba:7f:66:73:3x:35:45:f5:c7:ac:57:a6 [email protected]

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

[email protected]:~> ssh [email protected] mkdir -p .ssh
[email protected]'s password: 

Finally append a's new public key to [email protected]:.ssh/authorized_keys and enter b's password one last time:

[email protected]:~> cat .ssh/id_rsa.pub | ssh [email protected] 'cat >> .ssh/authorized_keys'
[email protected]'s password:

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

[email protected]:~> ssh [email protected]

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

Summary

  1. Run the command below on A and keep type enter while following the prompt: ssh-keygen -t rsa

  2. Create the ~/.ssh on B from A via ssh: ssh [email protected] mkdir -p .ssh

  3. Append public key from A to B’s ~/.ssh/authorized_keys by running the command below on A: cat ~/.ssh/id_rsa.pub | ssh [email protected] 'cat >> .ssh/authorized_keys'

  4. Verify: ssh [email protected]Permission:– ~/.ssh needs to be 700 – .ssh/authorized_key needs to be 640

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Carlos Dagorret
WRITTEN BY
Carlos Dagorret
SysAdmin