Secure Shell

Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices. Used primarily on Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for Telnet and other insecure remote shells, which send information, notably passwords, in plaintext, rendering them susceptible to packet analysis. The encryption used by SSH provides confidentiality and integrity of data over an insecure network, such as the Internet.

Definition

SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary.

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

The standard TCP port 22 has been assigned for contacting SSH servers.

An SSH client program is typically used for establishing connections to an SSH daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, Linux, FreeBSD, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.

History and development

Version 1.0

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.

Version 2.0

"Secsh" was the official Internet Engineering Task Force's (IETF) name for the IETF working group responsible for version 2 of the SSH protocol. In 1996, a revised version of the protocol, SSH-2, was adopted as a standard. This version is incompatible with SSH-1. 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 message authentication codes. New features of SSH-2 include the ability to run any number of shell sessions over a single SSH connection.

SSH compensation attack detector

In 1998 a vulnerability was described in SSH 1.5 which allowed unauthorized insertion of content into encrypted SSH stream due to insufficient data integrity protection from CRC-32 used in this version of protocol. A fix known as SSH Compensation Attack Detector was introduced into most implementations.

OpenSSH

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 forked Grönvall'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 OpenSSH to other operating systems.

It is estimated that, as of 2000, there were 2 million users of SSH.

As of 2005, OpenSSH is the single most popular SSH implementation, coming by default in a large number of operating systems. OSSH meanwhile has become obsolete.

SSH-2 Internet standard

In 2006, the aforementioned SSH-2 protocol became a proposed Internet standard with the publication by the IETF "secsh" working group of RFCs. It was first published in January 2006.

• RFC 4250, The Secure Shell (SSH) Protocol Assigned Numbers
• RFC 4251, The Secure Shell (SSH) Protocol Architecture
• RFC 4252, The Secure Shell (SSH) Authentication Protocol
• RFC 4253, The Secure Shell (SSH) Transport Layer Protocol
• RFC 4254, The Secure Shell (SSH) Connection Protocol
• RFC 4255, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
• RFC 4256, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
• RFC 4335, The Secure Shell (SSH) Session Channel Break Extension
• RFC 4344, The Secure Shell (SSH) Transport Layer Encryption Modes
• RFC 4345, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol

It was later modified and expanded by the following publications.

• RFC 4419, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol (March 2006)
• RFC 4432, RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol (March 2006)
• RFC 4716, The Secure Shell (SSH) Public Key File Format (November 2006)

Plaintext recovery attack

In 2008 a cryptographic vulnerability was found in SSH-2 which allowed retrieval of up to 4 bytes of plaintext from single SSH stream under special conditions. It was fixed by changing default encryption modes in OpenSSH 5.2.

File transfer protocols using SSH

There are multiple mechanisms for transferring files using the Secure Shell protocols.

• SSH File Transfer Protocol (SFTP), a secure alternative to FTP (not to be confused with FTP over SSH)
• Secure copy (SCP), which evolved from RCP protocol over SSH
• Files transferred over shell protocol (A.K.A. FISH), released in 1998, which evolved from Unix shell commands over SSH

IETF Standards have not been developed for these protocols. A series of IETF drafts have been created for SFTP but as of 2006 they have become stalled due to controversy that SFTP is actually a file system.

Uses

Example of tunneling an X11 application over SSH: the user 'josh' has SSHed from the local machine 'foofighter' to the remote machine 'tengwar' to run xeyes.

Logging into OpenWrt via SSH using PuTTY running on Windows.

SSH is a protocol that can be used for many applications across many platforms including UNIX, Microsoft Windows, Apple Mac and Linux. Some of the applications below may require features that are only available or compatible with specific SSH clients or servers. For example, using the SSH protocol to implement a VPN is possible, but presently only with the OpenSSH server and client implementation.

• For login to a shell on a remote host (replacing Telnet and rlogin)
• For executing a single command on a remote host (replacing rsh)
• For copying files from a local server to a remote host. See SCP, as an alternative for rcp
• In combination with SFTP, as a secure alternative to FTP file transfer
• In combination with rsync to backup, copy and mirror files efficiently and securely
• For forwarding or tunneling a port (not to be confused with a VPN which routes packets between different networks or bridges two broadcast domains into one.).
• For using as a full-fledged encrypted VPN. Note that only OpenSSH server and client supports this feature.
• For forwarding X from a remote host (possible through multiple intermediate hosts)
• For browsing the web through an encrypted proxy connection with SSH clients that support the SOCKS protocol.
• For securely mounting a directory on a remote server as a filesystem on a local computer using SSHFS.
• For automated remote monitoring and management of servers through one or more of the mechanisms as discussed above.
• For secure collaboration of multiple SSH shell channel users where session transfer, swap, sharing, and recovery of disconnected sessions is possible.

Architecture

Diagram of the SSH-2 binary packet.

The SSH-2 protocol has an 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 has 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: when one is prompted for a password, it may be 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, though OpenSSH does have a working GSSAPI implementation.
• 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
• The SSHFP DNS record (RFC 4255) provides the public host key fingerprints in order to aid in verifying the authenticity of the host.

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 Transport Layer Security (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 issues

Since SSH-1 has inherent design flaws which make it vulnerable (e.g., man-in-the-middle attacks), it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. While most modern servers and clients support SSH-2, some organizations still use software with no support for SSH-2, and thus SSH-1 cannot always be avoided.

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

See also

• Ident
• Shell account
• SSH file transfer protocol (SFTP) - a file transfer protocol using SSH
• Secure copy (SCP) - a file transfer protocol using SSH
• Web-based SSH – Access to SSH servers through standard web browsers

Software

• Comparison of SSH clients
• Comparison of SSH servers
• Corkscrew – a tool that enables a user to run SSH over HTTPS proxy servers
• Autossh - a tool to maintain a constant SSH connection, restarting it as necessary
• NX technology uses SSH to tunnel secure access to remote machines
• VNC can be tunneled through SSH to securely access a remote machine that is behind a firewall

Further reading

• 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
• Michael Stahnke – Pro OpenSSH, Apress 2005 ISBN 1-59059-476-2
• "Announcement: Ssh (Secure Shell) Remote Login Program". comp.security.unix. 12 July 1995. Original announcement of Ssh by Tatu Ylönen
• Himanshu Dwivedi; Implementing SSH, Wiley 2003. ISBN 978-0-471-45880-7
• 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.

External links

• Old homepage for IETF 'secsh' working group, which has concluded (for SSH-2)
• SSH Protocols