Introduction to SSH

Secure Shell (SSH) is a network protocol for secure data communication, remote shell services or command execution and other secure network services between two networked computers that it connects via a secure channel over an insecure network: a server and a client (running SSH server and SSH client programs, respectively). SSH replaces ancient insecure applications like TELNET for Windows and rlogin, rsh, rcp, and rexec for both Linux and Windows. The protocol specification distinguishes two major versions that are referred to as SSH-1 and SSH-2.

If computer A wants to connect to computer B via a secure SSH connection, computer A must be running an SSH client and computer B must be running an SSH daemon or server. When data goes from computer A, it is encrypted and received at computer B, where it is decrypted.

Definition

SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary. Anyone can produce a matching pair of different keys (public and private). The public key is placed on all computers that must allow access to the owner of the matching private key (the owner keeps the private key secret). While authentication is based on the private key, the key itself is never transferred through the network during authentication.

Usage

SSH is typically used to log into a remote machine and execute commands, but it also supports tunnelling, forwarding TCP ports and X11 connections; it can transfer files using the associated SSH file transfer (SFTP) or secure copy (SCP) protocols. SSH uses the client-server model.

The standard TCP port 22 has been assigned for contacting SSH servers, though administrators frequently change it to a non-standard port as an additional security measure.

What is SSH used for?

• To securely log into a shell on a remote host and to securely execute commands on that remote host.
• To securely transfer files.
• For browsing the web securely via an encrypted proxy connection.

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