How to login via ssh without password

A user might need the ability of login to a remote machine without typing password each time. First you need to generate a public authentication key:

Generating authentication keys

Use this command on a machine you want to login from:

ssh-keygen -t rsa

the output looks like this (empty passphrase was used):

user@host1 $ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user/.ssh/id_rsa.
Your public key has been saved in /home/user/.ssh/id_rsa.pub.

there will also be a hexadecimal key fingerprint and the key's randomart image. Anyway your public key is /home/user/.ssh/id_rsa.pub.

Copy the key to remote host

Now you have to copy the public key to the remote host you want to be able to login without a password. You can do it manually: create the file ~/.ssh/authorized_keys (if it is not already there) and append the contents of the public key to it. The other way is to use the dedicated tool:

ssh-copy-id -i ~/.ssh/id_rsa.pub user@remote.host

Next login into the remote host, check if the file ~/.ssh/authorized_keys contains your key, then make sure the permissions are as follows:

chmod 700 ~/.ssh/
chmod 640 ~/.ssh/authorized_keys

Then you should be able to login via ssh without pasword, like this:

ssh user@remote.host

Using different keys for different servers

You can use different keys for different servers to increase security and also specify your e-mail address:

user@host1 $ ssh-keygen -t rsa -C "my@email.com"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa): /home/user/.ssh/id_rsa.example.com
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user/.ssh/id_rsa.
Your public key has been saved in /home/user/.ssh/id_rsa.pub.

To automatically chose the right key create the file ~/.ssh/config and add something like this:

Host example.com
        IdentityFile ~/.ssh/id_rsa.example.com

Host github.com
        IdentityFile ~/.ssh/id_rsa.github.com