Ssh - Definition and Overview

In computing, Secure shell, or SSH, is both a computer program and an associated network protocol designed for logging into and executing commands on a remote computer. It is intended to replace the earlier rlogin, telnet and rsh protocols, and provides secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP ports can also be forwarded over the secure channel, and files can be transferred using the associated scp or sftp programs. The standard TCP port that an ssh server listens to is port 22.

The first version of the protocol (now called SSH-1) was designed, and the first software written, by Tatu Ylönen from Espoo, Finland in 1995. He soon formed a company called SSH Communications Security to exploit this innovation. The original version of the SSH software used various pieces of free software, such as GNU libgmp, but later versions released by SSH Secure Communications evolved into increasingly proprietary software. SSH Communications Security subsequently relicensed SSH to F-Secure (formerly known as Data Fellows). SSH Secure Communications has a USA subsidiary in Palo Alto, California.

A later version of the protocol was released under the name SSH-2. It is being standardised by the IETF "secsh" working group, and features both security and feature improvements over SSH-1. Examples of the former are Diffie-Hellman key exchange and strong integrity checking via MACs; of the latter, the ability to run any number of shell sessions over a single SSH connection. [1] (http://www.snailbook.com/faq/ssh-1-vs-2.auto.html)

The program is a common Unix shell program for client connections, accompanied by a daemon for accepting remote connections. Implementations exist for most modern platforms, including Microsoft Windows (where one of the most popular is PuTTY) and Mac OS. There are commercial versions, freeware versions, and open source versions.

List of implementations

• Lsh, the GNU Projects' implementation of SSH. (client and server)
• OpenSSH is an open source implementation of SSH. OpenSSH derives from the original, free implementation of SSH. (client and server)
• PuTTY (client)
• SSH Tectia Client [2] (http://www.ssh.com/products/tectia/client/)
• PenguiNet [3] (http://www.siliconcircus.com/penguinet/)
• SSHDOS [4] (http://sshdos.sourceforge.net/)
• WinSCP [5] (http://winscp.sourceforge.net/eng/) (file transfer client)
• JavaSSH [6] (http://javassh.org/)
• Dropbear (http://matt.ucc.asn.au/dropbear/dropbear.html), small client and server for POSIX systems

References

See also

• Corkscrew is a tool enabling the user to run SSH over HTTPS proxy servers.

• Connect.c (http://www.taiyo.co.jp/~gotoh/ssh/connect.html&e=1102) is a small C program to enable the user to run SSH over HTTPS proxy servers.

• Proxytunnel (http://proxytunnel.sourceforge.net) is another program, but of a more generic nature.

External links

• IETF working group (http://www.ietf.org/html.charters/secsh-charter.html) (for SSH-2)