14.2.4. Using Key-Based Authentication

To improve the system security even further, you can enforce key-based authentication by disabling the standard password authentication. To do so, open the /etc/ssh/sshd_config configuration file in a text editor such as vi or nano, and change the PasswordAuthentication option as follows:
PasswordAuthentication no
To be able to use ssh, scp, or sftp to connect to the server from a client machine, generate an authorization key pair by following the steps below. Note that keys must be generated for each user separately.
Red Hat Enterprise Linux 6 uses SSH Protocol 2 and RSA keys by default (see Section 14.1.3, “Protocol Versions” for more information).

Important

Do not generate key pairs as root, as only root would be able to use those keys.

Note

Before reinstalling your system, back up the ~/.ssh/ directory to keep the generated key pair. Copy the backed-up data to the home directory in the new system for any user you require, including root.

14.2.4.1. Generating Key Pairs

To generate an RSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa
    Generating public/private rsa key pair.
    Enter file in which to save the key (/home/john/.ssh/id_rsa):
  2. Press Enter to confirm the default location (that is, ~/.ssh/id_rsa) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/id_rsa.
    Your public key has been saved in /home/john/.ssh/id_rsa.pub.
    The key fingerprint is:
    e7:97:c7:e2:0e:f9:0e:fc:c4:d7:cb:e5:31:11:92:14 john@penguin.example.com
    The key's randomart image is:
    +--[ RSA 2048]----+
    |             E.  |
    |            . .  |
    |             o . |
    |              . .|
    |        S .    . |
    |         + o o ..|
    |          * * +oo|
    |           O +..=|
    |           o*  o.|
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 700 ~/.ssh
  5. Copy the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 600 ~/.ssh/authorized_keys
To generate a DSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate a DSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t dsa
    Generating public/private dsa key pair.
    Enter file in which to save the key (/home/john/.ssh/id_dsa):
  2. Press Enter to confirm the default location (that is, ~/.ssh/id_dsa) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/id_dsa.
    Your public key has been saved in /home/john/.ssh/id_dsa.pub.
    The key fingerprint is:
    81:a1:91:a8:9f:e8:c5:66:0d:54:f5:90:cc:bc:cc:27 john@penguin.example.com
    The key's randomart image is:
    +--[ DSA 1024]----+
    |   .oo*o.        |
    |  ...o Bo        |
    | .. . + o.       |
    |.  .   E o       |
    | o..o   S        |
    |. o= .           |
    |. +              |
    | .               |
    |                 |
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 700 ~/.ssh
  5. Copy the content of ~/.ssh/id_dsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 600 ~/.ssh/authorized_keys
To generate an RSA key pair for version 1 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa1
    Generating public/private rsa1 key pair.
    Enter file in which to save the key (/home/john/.ssh/identity):
  2. Press Enter to confirm the default location (that is, ~/.ssh/identity) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log into your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/identity.
    Your public key has been saved in /home/john/.ssh/identity.pub.
    The key fingerprint is:
    cb:f6:d5:cb:6e:5f:2b:28:ac:17:0c:e4:62:e4:6f:59 john@penguin.example.com
    The key's randomart image is:
    +--[RSA1 2048]----+
    |                 |
    |     . .         |
    |    o o          |
    |     + o E       |
    |    . o S        |
    |       = +   .   |
    |      . = . o . .|
    |       . = o o..o|
    |       .o o  o=o.|
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 700 ~/.ssh
  5. Copy the content of ~/.ssh/identity.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 600 ~/.ssh/authorized_keys
See Section 14.2.4.2, “Configuring ssh-agent” for information on how to set up your system to remember the passphrase.

Important

Never share your private key with anybody; it is for your personal use only.