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NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.

17.7. More Than a Secure Shell

A secure command line interface is just the beginning of the many ways SSH can be used. Given the proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by using TCP/IP forwarding, previously insecure port connections between systems can be mapped to specific SSH channels.

17.7.1. X11 Forwarding

Opening an X11 session over an SSH connection is as easy as connecting to the SSH server using the -Y option and running an X program on a local machine.

ssh -Y <user>@example.com

When an X program is run from the secure shell prompt, the SSH client and server create a new secure channel, and the X program data is sent over that channel to the client machine transparently.

X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive session of the Printer Configuration Tool. To do this, connect to the server using ssh and type:

system-config-printer &

After supplying the root password for the server, the Printer Configuration Tool appears and allows the remote user to safely configure printing on the remote system.

17.7.2. Port Forwarding

SSH can secure otherwise insecure TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.

Port forwarding works by mapping a local port on the client to a remote port on the server. SSH can map any port from the server to any port on the client; port numbers do not need to match for this technique to work.

To create a TCP/IP port forwarding channel which listens for connections on the localhost, use the following command:

ssh -L local-port:remote-hostname:remote-port username@hostname

Note

Setting up port forwarding to listen on ports below 1024 requires root level access.

To check email on a server called mail.example.com using POP3 through an encrypted connection, use the following command:

ssh -L 1100:mail.example.com:110 mail.example.com

Once the port forwarding channel is in place between the client machine and the mail server, direct a POP3 mail client to use port 1100 on the localhost to check for new mail. Any requests sent to port 1100 on the client system are directed securely to the mail.example.com server.

If mail.example.com is not running an SSH server, but another machine on the same network is, SSH can still be used to secure part of the connection. However, a slightly different command is necessary:

ssh -L 1100:mail.example.com:110 other.example.com

In this example, POP3 requests from port 1100 on the client machine are forwarded through the SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com connects to port 110 on mail.example.com to check for new mail. Note, when using this technique only the connection between the client system and other.example.com SSH server is secure.

Port forwarding can also be used to get information securely through network firewalls. If the firewall is configured to allow SSH traffic via its standard port (22) but blocks access to other ports, a connection between two hosts using the blocked ports is still possible by redirecting their communication over an established SSH connection.

Note

Using port forwarding to forward connections in this manner allows any user on the client system to connect to that service. If the client system becomes compromised, the attacker also has access to forwarded services.

System administrators concerned about port forwarding can disable this functionality on the server by specifying a No parameter for the AllowTcpForwarding line in /etc/ssh/sshd_config and restarting the sshd service.

17.7.3. Generating Key Pairs

If you do not want to enter your password every time you use ssh, scp, or sftp to connect to a remote machine, you can generate an authorization key pair.

Keys must be generated for each user. To generate keys for a user, use the following steps as the user who wants to connect to remote machines. If you complete the steps as root, only root will be able to use the keys.

Starting with OpenSSH version 3.0, ~/.ssh/authorized_keys2, ~/.ssh/known_hosts2, and /etc/ssh_known_hosts2 are obsolete. SSH Protocol 1 and 2 share the ~/.ssh/authorized_keys, ~/.ssh/known_hosts, and /etc/ssh/ssh_known_hosts files.

Red Hat Enterprise Linux 5 uses SSH Protocol 2 and RSA keys by default.

Tip

If you reinstall and want to save your generated key pair, backup the .ssh directory in your home directory. After reinstalling, copy this directory back to your home directory. This process can be done for all users on your system, including root.

17.7.3.1. Generating an RSA Key Pair for Version 2

Use the following steps to generate an RSA key pair for version 2 of the SSH protocol. This is the default starting with OpenSSH 2.9.

  1. To generate an RSA key pair to work with version 2 of the protocol, type the following command at a shell prompt:

    ssh-keygen -t rsa
    

    Accept the default file location of ~/.ssh/id_rsa. Enter a passphrase different from your account password and confirm it by entering it again.

    The public key is written to ~/.ssh/id_rsa.pub. The private key is written to ~/.ssh/id_rsa. Never distribute your private key to anyone.

  2. Change the permissions of the .ssh directory using the following command:

    chmod 755 ~/.ssh
    
  3. Copy the contents of ~/.ssh/id_rsa.pub into the file ~/.ssh/authorized_keys on the machine to which you want to connect. If the file ~/.ssh/authorized_keys exist, append the contents of the file ~/.ssh/id_rsa.pub to the file ~/.ssh/authorized_keys on the other machine.

  4. Change the permissions of the authorized_keys file using the following command:

    chmod 644 ~/.ssh/authorized_keys
    
  5. If you are running GNOME or are running in a graphical desktop with GTK2+ libraries installed, skip to Section 17.7.3.4, “Configuring ssh-agent with a GUI”. If you are not running the X Window System, skip to Section 17.7.3.5, “Configuring ssh-agent”.

17.7.3.2. Generating a DSA Key Pair for Version 2

Use the following steps to generate a DSA key pair for version 2 of the SSH Protocol.

  1. To generate a DSA key pair to work with version 2 of the protocol, type the following command at a shell prompt:

    ssh-keygen -t dsa
    

    Accept the default file location of ~/.ssh/id_dsa. Enter a passphrase different from your account password and confirm it by entering it again.

    Tip

    A passphrase is a string of words and characters used to authenticate a user. Passphrases differ from passwords in that you can use spaces or tabs in the passphrase. Passphrases are generally longer than passwords because they are usually phrases instead of a single word.

    The public key is written to ~/.ssh/id_dsa.pub. The private key is written to ~/.ssh/id_dsa. It is important never to give anyone the private key.

  2. Change the permissions of the .ssh directory with the following command:

    chmod 755 ~/.ssh
    
  3. Copy the contents of ~/.ssh/id_dsa.pub into the file ~/.ssh/authorized_keys on the machine to which you want to connect. If the file ~/.ssh/authorized_keys exist, append the contents of the file ~/.ssh/id_dsa.pub to the file ~/.ssh/authorized_keys on the other machine.

  4. Change the permissions of the authorized_keys file using the following command:

    chmod 644 ~/.ssh/authorized_keys
    
  5. If you are running GNOME or a graphical desktop environment with the GTK2+ libraries installed, skip to Section 17.7.3.4, “Configuring ssh-agent with a GUI”. If you are not running the X Window System, skip to Section 17.7.3.5, “Configuring ssh-agent”.

17.7.3.3. Generating an RSA Key Pair for Version 1.3 and 1.5

Use the following steps to generate an RSA key pair, which is used by version 1 of the SSH Protocol. If you are only connecting between systems that use DSA, you do not need an RSA version 1.3 or RSA version 1.5 key pair.

  1. To generate an RSA (for version 1.3 and 1.5 protocol) key pair, type the following command at a shell prompt:

    ssh-keygen -t rsa1
    

    Accept the default file location (~/.ssh/identity). Enter a passphrase different from your account password. Confirm the passphrase by entering it again.

    The public key is written to ~/.ssh/identity.pub. The private key is written to ~/.ssh/identity. Do not give anyone the private key.

  2. Change the permissions of your .ssh directory and your key with the commands chmod 755 ~/.ssh and chmod 644 ~/.ssh/identity.pub.

  3. Copy the contents of ~/.ssh/identity.pub into the file ~/.ssh/authorized_keys on the machine to which you wish to connect. If the file ~/.ssh/authorized_keys does not exist, you can copy the file ~/.ssh/identity.pub to the file ~/.ssh/authorized_keys on the remote machine.

  4. If you are running GNOME, skip to Section 17.7.3.4, “Configuring ssh-agent with a GUI”. If you are not running GNOME, skip to Section 17.7.3.5, “Configuring ssh-agent”.

17.7.3.4. Configuring ssh-agent with a GUI

The ssh-agent utility can be used to save your passphrase so that you do not have to enter it each time you initiate an ssh or scp connection. If you are using GNOME, the gnome-ssh-askpass package contains the application used to prompt you for your passphrase when you log in to GNOME and save it until you log out of GNOME. You will not have to enter your password or passphrase for any ssh or scp connection made during that GNOME session. If you are not using GNOME, refer to Section 17.7.3.5, “Configuring ssh-agent”.

To save your passphrase during your GNOME session, follow the following steps:

  1. You will need to have the package gnome-ssh-askpass installed; you can use the command rpm -q openssh-askpass to determine if it is installed or not. If it is not installed, install it from your Red Hat Enterprise Linux CD-ROM set, from a Red Hat FTP mirror site, or using Red Hat Network.

  2. Select Main Menu Button (on the Panel) => Preferences => More Preferences => Sessions, and click on the Startup Programs tab. Click Add and enter /usr/bin/ssh-add in the Startup Command text area. Set it a priority to a number higher than any existing commands to ensure that it is executed last. A good priority number for ssh-add is 70 or higher. The higher the priority number, the lower the priority. If you have other programs listed, this one should have the lowest priority. Click Close to exit the program.

  3. Log out and then log back into GNOME; in other words, restart X. After GNOME is started, a dialog box will appear prompting you for your passphrase(s). Enter the passphrase requested. If you have both DSA and RSA key pairs configured, you will be prompted for both. From this point on, you should not be prompted for a password by ssh, scp, or sftp.

17.7.3.5. Configuring ssh-agent

The ssh-agent can be used to store your passphrase so that you do not have to enter it each time you make a ssh or scp connection. If you are not running the X Window System, follow these steps from a shell prompt. If you are running GNOME but you do not want to configure it to prompt you for your passphrase when you log in (refer to Section 17.7.3.4, “Configuring ssh-agent with a GUI”), this procedure will work in a terminal window, such as an XTerm. If you are running X but not GNOME, this procedure will work in a terminal window. However, your passphrase will only be remembered for that terminal window; it is not a global setting.

  1. At a shell prompt, type the following command:

    exec /usr/bin/ssh-agent $SHELL
    
  2. Then type the command:

    ssh-add
    

    and enter your passphrase(s). If you have more than one key pair configured, you will be prompted for each one.

  3. When you log out, your passphrase(s) will be forgotten. You must execute these two commands each time you log in to a virtual console or open a terminal window.


 
 
  Published under the terms of the GNU General Public License Design by Interspire