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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 founded
SSH Communications Security 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 VNC connections or even forwarding Windows file shares
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 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 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 and not available in TLS
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 is similar to the BSD rcp protocol 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 SCP clients are rare 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 can also refer to Secure file transfer program 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 program actually
use the SFTP protocol to perform file transfers Some such implementations are still able to fallback to
the SCP protocol if the server does not provide SFTP service