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.
X11 Forwarding
Opening an X11 session over an established SSH connection is as easy as running an X program on the local machine. 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 your client machine transparently.
X11 forwarding can be very useful. For example, you can use X11 forwarding to create a secure, interactive session with up2date on the server to update packages. To do this, connect to the server using ssh and type:
up2date & |
You will be asked to supply the root password for the server. Then, Red Hat Update Agent will appear and you can update your packages on the server as though you were sitting in front of the machine.
Port Forwarding
With SSH you 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 allows you to map any port from the server to any port on the client; the port numbers do not need to match for it 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 access. |
So if you want to check your email on a server called mail.domain.com using POP through an encrypted connection, you can use the following command:
ssh -L 1100:mail.domain.com:110 mail.domain.com |
Once the port forwarding channel is in place between your machine and the mail server, you can direct a POP mail client to use port 1100 on localhost to check for new mail. Any requests sent to port 1100 on your system will be directed securely to the mail.domain.com server.
If mail.domain.com is not running an SSH server daemon, but you can log in via SSH to a machine on the same network, you can still use SSH to secure the part of the POP connection. However, a slightly different command is necessary:
ssh -L 1100:mail.domain.com:110 other.domain.com |
In this example, POP requests from port 1100 on your machine are forwarded through the SSH connection on port 22 to the ssh server, other.domain.com. Then, other.domain.com connects to port 110 on mail.domain.com to allow you to check for new mail. Note that by using this technique, only the connection between your system and other.domain.com 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 block 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 the service to which you are forwarding connections. If the client system becomes compromised, the attacker will also have 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. |