ssh login without password

First log in on hostA as user a and generate a pair of authentication keys. Do not enter a passphrase:

# 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 ....

Now use ssh to create a directory ~/.ssh as on hostB (the directory may already exist, which is fine):

# ssh user@hostB mkdir -p .ssh
user@hostB's password: 

Finally append a’s new public key to user@B:.ssh/authorized_keys and enter b’s password one last time:

# cat .ssh/id_rsa.pub | ssh user@hostB 'cat >> .ssh/authorized_keys'
user@hostB's password: 

From now on you can log into hostB as “user” from hostA as a without password:

# ssh user@hostB hostname

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

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