What Is SSH Protocol?

SSH is a popular protocol used mainly by Linux and Unix users to connect from one computer to another. The main use of SSH is to establish an encrypted connection so that nobody in between can read the traffic, especially for sensitive items such as passwords. SSH can also establish encrypted data tunnels underneath, which allow users to forward traffic such as a graphical program's output to a distant server.

Definition

SSH stands for Secure Shell. SSH is secured because the SSH protocol requires encryption of all data, including the login portion. Linux and Unix users' command line is called a shell, which is used to manage a file system as well as run programs once logged in. SSH can also create SSH tunnels, which are encrypted data channels for other uses such as for graphical programs.

History

Before SSH, there was another protocol called telnet that also allowed a user on one computer to connect to another computer via command line text without encryption. Unfortunately, telnet connections are vulnerable to having passwords and other data captured in between. SSH was invented 1995 as a replacement for telnet. By the late 1990s, SSH was in wide use and replaced telnet.

Function

SSH begins with a user launching the SSH client to connect to a remote system's SSH server with encryption. After the SSH client connects to the remote system, the SSH client transfers an optional key to the remote system's SSH server. If the key matches a copy on the SSH server, access is automatically granted. If the key is not correct or missing, the SSH server will ask for a user name and password. If they are correct, the user is granted access, otherwise the user is disconnected. After the user has been authenticated, the SSH client works with the SSH server to set up the command line session as well as other SSH tunnels.

Benefits

SSH's main benefit is to allow users on one computer to securely connect to another computer via text command line without worrying about their traffic being intercepted. On SSH connections between two Unix or Linux systems, all that's needed to run graphical programs is for the SSH client to click the option for X11 forwarding. Once the X11 forwarding tunnel has been established, a user simply needs to run a graphical program on the remote system, which will show up in the user's GUI automatically.

Expert Insight

SSH tunnels are often used in countries where internet access is censored such as China and the UAE (United Arab Emirates). Users in those countries buy SSH access outside of the country in order to set up SSH tunnels that carry web browsing traffic. The SSH tunnel forwards the traffic to a proxy server, which itself is used to forward traffic to any website on the internet. Because SSH is encrypted, the web browsing traffic over SSH tunnels cannot be blocked based on content.