Ssh, passwordless authentication

Sometimes, you need to be able to SSH into a remote machine for scripted maintenance purposes and not get challenged with a password. To do this, you need to set up key-based authentication between the user account you'll be using on your local computer, and the user account you'll be logging into on the remote computer. Here's a quick and dirty how-to.

Creating a public/private keyset with ssh-keygen on the computer and under the user account you want to log in FROM:

ph34r# mkdir ~/.ssh
ph34r# chmod 700 ~/.ssh
ph34r# cd ~/.ssh
ph34r# ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key ("your_local_home"/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again: 
Your identification has been saved in id_rsa.
Your public key has been saved in id_rsa.pub.
The key fingerprint is:
17:5a:e7:77:ad:2c:0b:8e:f3:97:f8:20:53:79:69:55 root@ph34r

Getting the public half of the key to the REMOTE computer and user account you want to log in TO:

ph34r# scp ~/.ssh/id_rsa.pub jimbo@l0ath1ng.tehinterweb.net:/home/jimbo/id_rsa.ph34r.pub
ph34r# ssh jimbo@l0ath1ng.tehinterweb.net
Password:
% mkdir .ssh
% chmod 700 .ssh
% cat id_rsa.ph34r.pub >> .ssh/authorized_keys
% chmod 644 .ssh/authorized_keys

Checking to make sure it worked:

% exit
ph34r# ssh jimbo@l0ath1ng.tehinterweb.net
%

Bingo.

From here on out, whenever logged in as root on the computer ph34r, I will be able to SSH into my account jimbo on the machine l0ath1ng without being presented with a password challenge (assuming I did NOT enter a passphrase when I generated the RSA key in the first step). Note that I will not be able to use this key to bypass the password when logging into jimbo@l0ath1ng from any account OTHER than root@ph34r - if I were try it from jimbo@ph34r, I would still need a password.

If I wanted to log in from or to any other user accounts, the steps would be the same, just do them as the appropriate user.

NOTE: it is highly HIGHLY recommended that you only set up passwordless authentication to extremely neutered accounts on the target machine; perhaps an account with absolutely no privileges at all beyond sudo permission (if necessary) to run a single script which the account in question DOES NOT have write permission on. This limits the damage a potential rogue user who compromises the computer on the other end could cause.

There are great stories, on how to securely use passwordless ssh authentication using password protected private key.
Understanding RSA/DSA authentication, Part 1
OpenSSH key management, Part 2
OpenSSH key management, Part 3