SSH Secure Shell From: http://en.wikipedia.org/wiki/Ssh

SSH  Secure Shell (SSH)  Network protocol  Allows data to be exchanged over a secure channel between two computers  Encryption provides confidentiality and integrity of data  SSH uses public-key cryptography to authenticate the remote computer  Allows the remote computer to authenticate the user, if necessary

SSH  Typically used to log into a remote machine and execute commands  Supports tunneling  Forwards arbitrary TCP ports and X11 connections  Can transfer files using the associated SFTP or SCP protocols

SSH  An SSH server  Listens on the TCP port 22 (default) 

ssh

client program typically used for establishing connections to an

sshd

daemon accepting remote connections.  Both typically present on operating systems  Mac OS X, Linux, Solaris, OpenVMS, …  Proprietary, freeware and open source versions of various levels of complexity and completeness exist

History

History

 SSH-1  1995: Tatu Ylönen designed the first version of the protocol (

SSH-1

)  Prompted by a password sniffing attack at his university’s network   Goal of SSH was to replace the earlier rlogin , TELNET  and rsh protocols Did not provide strong authentication or guarantee confidentiality Ylönen released his implementation as freeware in July 1995  Tool quickly gained in popularity   Towards the end of 1995, the SSH user base had grown to 20,000 users in fifty countries.

December 1995:

Ylönen

Security founded SSH Communications to market and develop SSH.   Original version of the SSH software used various pieces of free software  such as GNU libgmp Later versions released by SSH Secure Communications evolved into increasingly proprietary software

History

 SSH-2  1996:

SSH-2

designed  incompatible with SSH-1.  SSH-2 featured both security and feature improvements over SSH-1  Better security through Diffie-Hellman key exchange  Strong integrity checking via message authentication codes  New features of SSH-2 included the ability to run any number of shell sessions over a single SSH connection

History

  1999: developers wanted a free software version    Went back to the 1.2.12 release of the original ssh program  Last released under an open source license Björn Grönvall's OSSH developed from this codebase OpenBSD developers forked Björn's code  Extensive work done   Created OpenSSH  Shipped with the 2.6 release of OpenBSD “Portability" branch was formed to port OpenSSH to other operating systems It is estimated that, at the end of 2000 , there were 2,000,000 users of SSH

History

 As of 2005  OpenSSH is the single most popular ssh implementation  The default in a large number of operating systems.  OSSH meanwhile has become obsolete  In 2006, SSH-2 protocol became a proposed Internet standard  Publication by the IETF "secsh" working group of RFCs

Uses of SSH

SSH is most commonly used:

Uses of SSH:

    With an SSH client that supports terminal protocols  Remote administration    Of the SSH server computer Via terminal (character-mode) console Can be used as an alternative to a terminal on a headless server In combination with SFTP , as a secure alternative to FTP  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  A secure alternative for rcp file transfers  More often used in environments involving Unix

Uses of SSH

 For port forwarding or tunneling  Frequently as an alternative to a full-fledged VPN  A (non-secure) TCP/IP connection of an external application is redirected to the SSH program (client or server)   Forwards it to the other SSH party (server or client) In turn forwards the connection to the desired destination host    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 file shares VNC connections or even forwarding Windows Primarily useful for tunneling connections through firewalls   Ordinarily block that type of connection Encrypting protocols which are not normally encrypted (e.g. VNC).

Uses of SSH

 ssh and rdesktop.  Three computers    The computer that will run rdesktop and ssh A computer used to obtain access to a remote network The computer you want rdesktop to display.     " ssh -L3389:mytarget.mycompany.net:3389 sshtarget.mycompany.net

" log into the middle computer and do nothing on it Open another shell from the first computer running ssh and type rdesktop localhost.   This example uses the middle computer to port forward 3389 from the end computer to the first computer. If on Windows, run ssh using another local port, e.g.  "ssh -L3390:mydesktop.mycompany.net:3389 sshserver.mycompany.net". Start the native Windows Remote Desktop client and type localhost:3390 to remote into "mydesktop.mycompany.net"

Uses of SSH

    Log into one machine from your local host  Login from there to another machine  Run an X application (eg. xterm, matlab) on the last machine to display on your local display This is especially useful for running X applications on a department host from off campus  Had to connect through another department host which is available for ssh login through the campus firewall Channel the X-window through a series of logins back to the host at which you are sitting  Best way to do this is to make use of the X11-forwarding feature of ssh For unix/linux to unix/linux, force an X11-forwarding request with the '-X' option (capitalized x). ssh -X host.com

Uses of SSH

  X11-forwarding for through multiple hosts ssh -X hostA.com → ssh -X hostB.com → ssh -X hostC.com ensure the tunnel is working every step of the way by running something like xterm on host B then C. If this does not work the -Y may be needed. ssh -X -Y hostA.com → ssh -X -Y hostB.com → ssh -X -Y hostC.com 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

Uses of SSH

 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  SSH Filesystem can securely mount a directory on the server as a filesystem on the local computer

SSH architecture

SSH architecture

 The SSH-2 protocol has a clean internal architecture with well-separated layers:  Transport Layer  User Authentication Layer  Connection Layer  Defined in RFC 4251

SSH architecture – Transport Layer

 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  After 1 hour has passed  Whichever is sooner

SSH architecture – User Authentication Layer

 The

user authentication

layer ( RFC 4252 )  This layer handles client authentication and provides a number of authentication methods.  Authentication is

client-driven

 Commonly misunderstood by users  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

SSH architecture – User Authentication Layer

 Widely used user authentication methods include the following: 

"password"



"publickey"



"keyboard-interactive" ( RFC 4256 )



GSSAPI authentication

SSH architecture – User Authentication Layer



"password“



A method for straightforward password authentication

 Includes a facility allowing a password to be changed  This method is not implemented by all programs

SSH architecture – User Authentication Layer



"publickey“



a method for public key-based authentication

 Usually supporting at least DSA or RSA keypairs  Other implementations also supporting X.509

certificates

SSH architecture – User Authentication Layer

 "keyboard-interactive" ( RFC 4256 )  The server sends one or more prompts to enter information  The client displays them and sends back responses keyed-in by the user  Used to provide one-time password such as S/Key or SecurID . authentication  Used by some OpenSSH configurations when PAM is the underlying host authentication provider to effectively provide password authentication  Sometimes leads to inability to log in with a client that supports just the plain "password" authentication method

SSH architecture – User Authentication Layer



GSSAPI authentication methods



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.

SSH architecture – Connection Layer

 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

SSH architecture – Connection Layer

 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

SSH architecture

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

Security cautions

Security cautions

 SSH-1 has inherent design flaws which make it vulnerable to, e.g., man-in-the-middle attacks  Now generally considered obsolete  Should be avoided by explicitly disabling fallback to SSH-1  Most modern servers and clients support SSH-2  Some organizations still use software with no support for SSH-2  SSH-1 cannot always be avoided

Security cautions

 In all versions of SSH  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

Security cautions

 As with any encrypted protocol:  SSH can be considered a security risk by companies or governments who do not trust their users  Wish to eavesdrop on their communications  Furthermore SSH has built in tunneling features which make it easier for users to achieve passage of large volumes of information or to establish an entry point for unauthorized inward access over a SSH link than with other protocols

How SSH uses public-key cryptography

How SSH uses public-key cryptography (with analogy)

 First, a pair of cryptographic keys is generated.  One is the private key, the other is the public key.  As an analogy, they can be thought of as a matching private key and a public padlock.       The public padlock is what is installed on the remote machine  Used by ssh to authenticate users using the matching private key As a user of the system, don’t care who can see or copy the padlock (ie the public key)  Only the secret private key fits it The private key is the part you keep secret inside a secure box  Can only be opened with the correct passphrase When the user wants to access a remote system  opens the secure box with his passphrase  uses the private-key to authenticate him with the padlock on the remote computer Neither the passphrase nor the private key leave the user's machine. However, the user still needs to trust the local machine not to scrape his passphrase or copy his private-key while it's out of the secure box.

SCP

http://en.wikipedia.org/wiki/Secure_copy

SCP



Secure Copy

 A means of securely transferring computer files using the Secure Shell (SSH) protocol  between a local computer and a remote host  between two remote hosts  SCP can refer to two related things: 

SCP protocol



SCP program

SCP protocol

 The

SCP protocol

protocol is similar to the BSD rcp  Unlike rcp, data is encrypted during transfer  Avoid potential packet sniffers extracting usable information from the data packets  The protocol itself does not provide authentication and security  Relies on the underlying protocol, SSH, to provide these features

SCP protocol

 SCP can interactively request any passwords or passphrases required to make a connection to a remote host  Unlike rcp which fails in this situation  The SCP protocol implements file transfers only  It does so by connecting to the host using SSH and there executes an SCP server (scp)  The SCP server program is typically the same program as the SCP client

SCP protocol

 For upload, the client feeds the server with files to be uploaded  Optionally including their basic attributes  Permissions  Timestamps  This is an advantage over the common FTP protocol  Does not have provision for uploads to include the original date/timestamp attribute

SCP protocol

 For downloads, the client sends a request for files or directories to be downloaded  Server feeds the client with its subdirectories and files  Download is server-driven  Imposes a security risk when connected to a malicious server

SCP protocol

 For most applications, the SCP protocol is superseded by the more comprehensive SFTP protocol  Also based on SSH

SCP program

 A client implementing the SCP protocol  A program to perform secure copying  The most widely used SCP client is the command line scp program  Provided in most SSH implementations  scp program is the secure analog of the rcp command  scp program must be part of all SSH servers that want to provide SCP service  scp functions as SCP server too

SCP program

 Some SSH implementations provide the scp2 program  Uses the SFTP protocol instead of SCP  Provides the very same command line interface as scp  scp is then typically a symbolic link to scp2  Typically, a syntax of scp program is like the syntax of cp :  scp

SourceFile user

@

host

:

directory

/

TargetFile

scp

user

@

host

:

folder

/

SourceFile TargetFile

SCP program

  As the SCP protocol implements file transfers only, GUI are rare SCP clients     Implementing it requires additional functionality  Directory listing at least For example, WinSCP defaults to the SFTP protocol. Even when operating in SCP mode, clients like WinSCP are typically not pure SCP clients  They must use other means to implement the additional functionality  This in turn brings platform-dependency problems Thus it may not be possible to work with a particular SCP server using a GUI SCP client  Even if you are able to work with the same server using a traditional command line client More comprehensive tools for managing files over SSH are SFTP clients

SFTP http://en.wikipedia.org/wiki/SSH_file_transfer_protocol

SFTP



SSH File Transfer Protocol

 A network protocol that provides file transfer and manipulation functionality over any reliable data stream  It is typically used with the SSH -2 protocol (TCP port 22) to provide secure file transfer  Intended to be usable with other protocols as well

Capabilities

 The SFTP protocol allows for a range of operations on remote files  It is more like a remote file system protocol  An SFTP client's extra capabilities compared to an SCP client include:  Resuming interrupted transfers  Directory listings  Remote file removal.  For the same reason it is reasonable to implement a GUI SFTP client, but not a GUI SCP client

Capabilities

  SFTP attempts to be more platform-independent than SCP    With SCP, the expansion of wildcards specified by the client was up to the server SFTP's design avoids this problem While SCP was most frequently implemented on Unix platforms, there exist SFTP servers for most platforms A common misconception is that SFTP is simply FTP run over SSH   In fact it is a new protocol designed from the ground up by the IETF SECSH working group. It is sometimes confused with Simple File Transfer Protocol .

Capabilities

 The protocol itself does not provide authentication and security  It expects the underlying protocol to secure this     SFTP is most often used as subsystem of SSH protocol version 2 implementations  Designed by the same working group However, it is possible to run it over SSH-1 or other data streams Running SFTP server over SSH-1 is not platform independent as SSH-1 does not support the concept of subsystems An SFTP client willing to connect to an SSH-1 server needs to know the path to the SFTP server binary on the server side

Capabilities

  The Secure Internet Live Conferencing (SILC) protocol defines the SFTP as its default file transfer protocol   In SILC the SFTP data is not protected with SSH but SILC's secure packet protocol   Used to encapsulate the SFTP data into SILC packet Deliver it peer-to-peer SFTP is designed to be protocol independent.

For uploads, the transferred files may be associated with their basic attributes, such as timestamps  An advantage over the common FTP  protocol Does not have provision for uploads to include the original date/timestamp attribute

Standardization

  The protocol is not yet an Internet standard .

   The latest specification is an expired Internet Draft  Defines version 6 of the protocol Currently the most widely used version is 3, implemented by the popular OpenSSH SFTP server. Many Microsoft Windows -based SFTP implementations use version 4 of the protocol, which lessened its ties with the Unix platform The Internet Engineering Task Force (IETF) "Secsh Status Pages" search tool contains links to all versions of the Internet draft-ietf-secsh-filexfer which describes this protocol

SFTP client

   The term

SFTP program

can also refer to

Secure file transfer

  A command-line program Implements the client part of this protocol  Such as that supplied with OpenSSH The sftp program provides an interactive interface  Similar to that of traditional FTP clients Some implementations of the scp use the SFTP

protocol program

actually to perform file transfers  Some such implementations are still able to fallback to the SCP protocol if the server does not provide SFTP service