Passwordless SSH login

Your aim

You need an automatic login from host A / user ‘a’ to Host B / user ‘b’. You don’t want to enter any passwords, maybe 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
ssh-copy-id -i ~/.ssh/id_rsa.pub username@mystery

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