How to configure a SSH connection with no password

This is needed to:

• Move from a system's user to another system's user without typing the password.
• Allow automatic scripts to execute remote commands or copy / move data from one system to another.

Be aware of:

• If the user account you use to connect to remote systems is hacked, all your systems might be compromised.
• Do not allow root user to remotely connect without password unless necessary, for the risk is higher.
• The configuration is not bidirectional. You must do all the steps to configure the connection back from the remote to the local system.

In my case, I will show you how to configure a SSH connection from the user sergio of the local system odin (Ubuntu 12.04 with kernel 3.2.0-29) to the user coord of the remote system mudel (Debian 6.0 with kernel 2.6.32-5).

The steps are:

1. Create a pair of keys (public and private) in the user's home of the system you will use to connect to the remote system. The command to use is ssh-keygen.
   You can change the filenames where your keys will be stored.
   When asked about the passphrase, keep it empty, otherwise it will be required when establishing a connection to the remote system.
   sergio@odin:~$ ssh-keygen
   Generating public/private rsa key pair.
   Enter file in which to save the key (/home/sergio/.ssh/id_rsa):
   Enter passphrase (empty for no passphrase):
   Enter same passphrase again:
   Your identification has been saved in /home/sergio/.ssh/id_rsa.
   Your public key has been saved in /home/sergio/.ssh/id_rsa.pub.
   The key fingerprint is:
   ce:bb:e5:75:e4:8f:23:67:09:12:c8:9c:39:ad:3e:bd sergio@odin
   The key's randomart image is:
   +--[ RSA 2048]----+
   |                 |
   |                 |
   |       o =       |
   |        B o      |
   |        So .  .  |
   |       o. . .o   |
   |       .o.....o. |
   |        o+....=o |
   |        ooE. +...|
   +-----------------+
2. A directory .ssh will be created, containing the two files. Be aware of keeping the permissions of the private key file (id_rsa) readable only by the owner.
   sergio@odin:~$ ls -l .ssh
   total 8
   -rw------- 1 sergio sergio 1679 ago 15 11:46 id_rsa
   -rw-r--r-- 1 sergio sergio  393 ago 15 11:46 id_rsa.pub
3. Append the content of the public key file (id_rsa.pub) to the file ~/.ssh/authorized_keys of the remote system. To do so, you might send it by e-mail, copy and paste, remote copy to a temporary file or use the ssh-copy-id tool, which will do all the work.
   sergio@odin:~$ ssh-copy-id coord@mudel
   The authenticity of host 'mudel (192.168.56.1)' can't be established.
   RSA key fingerprint is 0c:48:50:6c:67:df:1f:8c:ac:22:c2:ee:0b:a8:98:55.
   Are you sure you want to continue connecting (yes/no)? yes
   Warning: Permanently added 'mudel,192.168.56.1' (RSA) to the list of known hosts.
   coord@mudel's password:
   Now try logging into the machine, with "ssh 'coord@mudel'", and check in:
   
     ~/.ssh/authorized_keys
   
   to make sure we haven't added extra keys that you weren't expecting.
4. The first time you establish a SSH connection, the fingerprint of the origin system will be shown and you will be asked if it should be append to the ~/.ssh/known_hosts file of the remote system. Answer yes.
5. Now try to connect.
   sergio@odin:~$ ssh coord@mudel
   Linux mudel 2.6.32-5-686 #1 SMP Tue Mar 8 21:36:00 UTC 2011 i686
   
   The programs included with the Debian GNU/Linux system are free software;
   the exact distribution terms for each program are described in the
   individual files in /usr/share/doc/*/copyright.
   
   Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
   permitted by applicable law.
   Last login: Wed Aug 15 12:07:26 2012
   coord@mudel:~$