Secure Shell

In computing, Secure Shell or SSH is a set of standards and an associated network protocol that allows establishing a secure channel between a local and a remote computer. It uses public-key cryptography to authenticate the remote computer and (optionally) to allow the remote computer to authenticate the user. SSH provides confidentiality and integrity of data exchanged between the two computers using encryption and message authentication codes. SSH is typically used to login to a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; it can transfer files using the associated SFTP or SCP protocols. An SSH server, by default, listens on the standard TCP port 22.

History

In 1995, Tatu Ylönen, a researcher at Helsinki University of Technology, Finland, designed the first version of the protocol (now called SSH-1) prompted by a password-sniffing attack at his university network. The goal of SSH was to replace the earlier rlogin, TELNET and rsh protocols, which did not provide strong authentication or guarantee confidentiality. Ylönen released his implementation as freeware in July 1995, and the tool quickly gained in popularity. Towards the end of 1995, the SSH user base had grown to 20,000 users in fifty countries.

In December 1995, Ylönen founded SSH Communications Security to market and develop SSH. 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), who later sold it to WRQ (now Attachmate), who markets it under the name Reflection for Secure IT [1]. SSH Secure Communications has a USA subsidiary in Palo Alto, California.

In 1996, a revised version of the protocol, SSH-2, was designed, incompatible with SSH-1. In 2006, this protocol became a proposed Internet standard with the publication by the IETF "secsh" working group of RFCs (see references). SSH-2 features both security and feature improvements over SSH-1. Better security, for example, comes through Diffie-Hellman key exchange and strong integrity checking via MACs. New features of SSH-2 include the ability to run any number of shell sessions over a single SSH connection [2].

In 1999, developers wanting a free software version to be available went back to the older 1.2.12 release of the original ssh program, which was the last released under an open source license. Björn Grönvall's OSSH was subsequently developed from this codebase. Shortly thereafter, OpenBSD developers branched Björn's code and did extensive work on it, creating OpenSSH which shipped with the 2.6 release of OpenBSD. From this version, a "portability" branch was formed to port to other operating systems. As of 2005, a large number of operating systems use the OpenSSH codebase.

An ssh program commonly appears for use on Unix-like systems for client connections as well as a daemon such as sshd for accepting remote connections. Implementations of SSH exist for most modern platforms including Sun OS, Mac OS, Linux-based distributions, Microsoft Windows, and BSD operating systems. Commercial, freeware and open source versions of various levels of complexity and completeness exist.

It is estimated that, at the end of 2000, there were 2,000,000 users of SSH.

Uses of SSH

Example of tunnelling an X11 application over SSH.

SSH is most commonly used:

• with an SSH client that supports terminal protocols, for remote administration of the SSH server computer via terminal (character-mode) console;
• in combination with SFTP, as a secure alternative to FTP which can be set up more easily on a small scale without a public key infrastructure and X.509 certificates;
• in combination with rsync to backup, copy and mirror files efficiently and securely
• in combination with SCP, as a secure alternative for rcp file transfers - more often used in environments involving Unix;
• for port forwarding or tunneling, frequently as an alternative to a full-fledged VPN. In this type of use, a (non-secure) TCP/IP connection of an external application is redirected to the SSH program (client or server), which forwards it to the other SSH party (server or client), which in turn forwards the connection to the desired destination host. The forwarded connection is encrypted and protected on the path between the SSH client and server only. Uses of SSH port forwarding include accessing database servers, email servers, securing X11, Windows Remote Desktop and VNC connections or even forwarding Windows file shares.
• with an SSH client that supports dynamic port forwarding (presenting to other programs a SOCKS or HTTP 'CONNECT' proxy interface), SSH can even be used for generally browsing the web through an encrypted proxy connection, using the SSH server as a proxy;
• with an SSH client that supports SSH exec requests (frequently embedded in other software, e.g. a network monitoring program), for automated remote monitoring and management of servers.
• Using just a normal ssh login on a server, the SSH Filesystem can securely mount a directory on the server as a filesystem on the local computer.

SSH architecture

The SSH-2 protocol has a clean internal architecture (defined in RFC 4251) with well-separated layers. These are:

• The transport layer (RFC 4253). This layer handles initial key exchange and server authentication and sets up encryption, compression and integrity verification. It exposes to the upper layer an interface for sending and receiving plaintext packets of up to 32,768 bytes each (more can be allowed by the implementation). The transport layer also arranges for key re-exchange, usually after 1 GB of data have been transferred or after 1 hour has passed, whichever is sooner.
• The user authentication layer (RFC 4252). This layer handles client authentication and provides a number of authentication methods. Authentication is client-driven, a fact commonly misunderstood by users; when one is prompted for a password, it is the SSH client prompting, not the server. The server merely responds to client's authentication requests. Widely used user authentication methods include the following:
  • "password": a method for straightforward password authentication, including a facility allowing a password to be changed. This method is not implemented by all programs.
  • "publickey": a method for public key-based authentication, usually supporting at least DSA or RSA keypairs, with other implementations also supporting X.509 certificates)
  • "keyboard-interactive" (RFC 4256): a versatile method where the server sends one or more prompts to enter information and the client displays them and sends back responses keyed-in by the user. Used to provide one-time password authentication such as S/Key or SecurID. Used by some OpenSSH configurations when PAM is the underlying host authentication provider to effectively provide password authentication, sometimes leading to inability to log in with a client that supports just the plain "password" authentication method.
  • GSSAPI authentication methods which provide an extensible scheme to perform SSH authentication using external mechanisms such as Kerberos 5 or NTLM, providing single sign on capability to SSH sessions. These methods are usually implemented by commercial SSH implementations for use in organizations.
• The connection layer (RFC 4254). This layer defines the concept of channels, channel requests and global requests using which SSH services are provided. A single SSH connection can host multiple channels simultaneously, each transferring data in both directions. Channel requests are used to relay out-of-band channel specific data, such as the changed size of a terminal window or the exit code of a server-side process. The SSH client requests a server-side port to be forwarded using a global request. Standard channel types include:
  • "shell" for terminal shells, SFTP and exec requests (including SCP transfers)
  • "direct-tcpip" for client-to-server forwarded connections
  • "forwarded-tcpip" for server-to-client forwarded connections

This open architecture provides considerable flexibility, allowing SSH to be used for a variety of purposes beyond secure shell. The functionality of the transport layer alone is comparable to TLS; the user authentication layer is highly extensible with custom authentication methods; and the connection layer provides the ability to multiplex many secondary sessions into a single SSH connection, a feature comparable to BEEP and not available in TLS.

Security cautions

Since SSH-1 has inherent design flaws which make it vulnerable to e.g. man in the middle attacks, it is now generally considered obsolete and should no longer be used. In practice most modern servers and clients support SSH-2, which should be used exclusively (by explicitly disabling fallback to SSH-1). However, software not supporting SSH-2 is still used by many organizations, which can make it hard to avoid the use of SSH-1.

In all versions of SSH, it is important to verify unknown public keys before accepting them as valid. Accepting an attacker owned public key as a valid public key has the effect of disclosing transmitted password, and allowing man in the middle attacks.

See also

• Comparison of SSH clients
• VNC can be tunneled through SSH to securely access a remote machine that is behind a firewall.
• Cygwin allows many Linux/BSD programs to run on Windows, including the OpenSSH client and server.
• Corkscrew - a tool that enables a user to run SSH over HTTPS proxy servers
• Proxytunnel, another program, offers more generic capabilities.
• connect.c consists of a small C program which enables users to run SSH over HTTPS proxy servers.
• autossh, Automatically restarts SSH sessions and tunnels
• Transport Layer Security
• Ident

References

• Daniel J. Barrett, Richard E. Silverman, and Robert G. Byrnes — SSH: The Secure Shell (The Definitive Guide), O'Reilly 2005 (2^nd edition). ISBN 0-596-00895-3 [3].
• Michael Stahnke — Pro OpenSSH, Apress 2005 ISBN 1-590-59476-2 [4].
• Kurt Seifried, What's in a Name?, February 14, 2001, [5].
• "ANNOUNCEMENT: Ssh (Secure Shell) Remote Login Program". comp.security.unix. Jul. 12, 1995. {{cite news}}: Check date values in: |date= (help) Original announcement of Ssh by Tatu Ylönen

External links

• Java based SSH from Duke University
• Info about SSH
• Using SSH
• A Guide to SSH Port Forwarding
• X11 display fowarding with SSH in Cygwin/X
• IETF 'secsh' working group (for SSH-2)
  • RFC 4251 (SSH-2 architecture); other documents documenting other aspects of the protocol are referenced from here.
• SSH Communications Security
• OpenSSH key management
• History of OpenSSH
• SSH Tutorial for Linux (Also good as a general SSH tutorial)
• ssh man page via OpenBSD
• Info about SSH in German
• SSH tricks - useful for newbies

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.