SSH login without password

Your Goal

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.

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

If you want to config rsa for multiple hosts with different keys. You need to configure .ssh/config file like below:

Host bitbucket.org #host base url
  IdentityFile ~/.ssh/bitbucket #private key

or to create an alias as below:

Host alias
  User user #user name
  HostName host_url
  IdentityFile ~/.ssh/rsa #private key

Important Note

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

One more thing

If you want to login directly to tmux, you can add alias below:

alias thost="ssh user@host -t \"tmux new-session -s user || tmux attach-session -t user\""

Original Blog is written by Mathias Kettner and posted on linuxproble.org