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+<?xml version="1.0" encoding="iso-8859-1" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+<!-- %EricssonCopyright% -->
+<chapter>
+ <header>
+ <copyright>
+ <year>2013</year>
+ <holder>Ericsson AB. All Rights Reserved.</holder>
+ </copyright>
+ <legalnotice>
+ The program may be used and/or copied only with the written permission from
+ Ericsson AB, or in accordance with the terms and conditions stipulated in
+ the agreement/contract under which the program has been supplied.
+ </legalnotice>
+ <title>Secure Shell (SSH)</title>
+ <prepared>OTP</prepared>
+ <date></date>
+ <rev>%VSN%</rev>
+ <file>ssh_protocol.xml</file>
+ </header>
+
+ <section>
+ <title>SSH Protocol Overview</title>
+
+ <p> Conceptually the SSH protocol can be partitioned into four
+ layers:</p>
+
+ <image file="SSH_protocols.png">
+ <icaption>SSH Protocol Architecture</icaption>
+ </image>
+
+ <section>
+ <title>Transport Protocol</title>
+
+ <p> The SSH Transport Protocol is a secure, low level transport.
+ It provides strong encryption, cryptographic host
+ authentication and integrity protection. Currently, only a
+ minimum of MAC- (message authentication code, a short piece of
+ information used to authenticate a message) and encryption
+ algorithms are supported see <seealso marker="ssh">ssh(3)</seealso>
+ </p>
+ </section>
+
+ <section>
+ <title>Authentication Protocol</title>
+
+ <p>The SSH authentication protocol is a general-purpose user
+ authentication protocol run over the SSH transport
+ protocol. Erlang SSH supports user authentication using public
+ key technology (RSA and DSA, X509-certificates are currently not
+ supported). It is also possible to use a so called keyboard
+ interactive authentication. This method is suitable for
+ interactive authentication methods that do not need any special
+ software support on the client side. Instead, all authentication
+ data should be entered via the keyboard. It is also possible
+ to use a pure password based authentication scheme, note that in
+ this case the the plain text password will be encrypted before sent
+ over the network. There are several configuration options for
+ authentication handling available in
+ <seealso marker="ssh#connect-3">ssh:connect/[3,4]</seealso>
+ and <seealso marker="ssh#daemon-2">ssh:daemon/[2,3]</seealso>
+ It is also possible to customize the public key handling
+ by implementing the behaviours <seealso
+ marker="ssh_client_key_api">ssh_client_key_api</seealso> and
+ <seealso
+ marker="ssh_server_key_api">ssh_server_key_api</seealso>
+ </p>
+ </section>
+
+ <section>
+ <title>Connection Protocol</title>
+
+ <p>The SSH Connection Protocol provides application-support
+ services over the transport pipe, such as channel multiplexing,
+ flow control, remote program execution, signal propagation,
+ connection forwarding, etc. Functions for handling the SSH
+ Connection Protocol can be found in the module <seealso
+ marker="ssh_connection">ssh_connection</seealso>.
+ </p>
+ </section>
+
+ <section>
+ <title>Channels</title>
+
+ <p>All terminal sessions, forwarded connections etc., are
+ channels. Multiple channels are multiplexed into a single
+ connection, and all channels are flow-controlled. Typically an
+ SSH client will open a channel, send data/commands, receive
+ data/"control information" and when it is done close the
+ channel. The <seealso
+ marker="ssh_channel">ssh_channel</seealso> behaviour makes it easy to
+ write your own SSH client/server processes that use flow
+ control. It handles generic parts of SSH channel management and
+ lets you focus on the application logic.
+ </p>
+
+ <p>Channels comes in three flavors</p>
+
+ <list type="bulleted">
+ <item><em>Subsystem</em> - named services that can be run as
+ part of an SSH server such as SFTP <seealso
+ marker="ssh_sftpd">ssh_sftpd</seealso>, that is built in to the
+ SSH daemon (server) by default but may be disabled. The Erlang SSH
+ daemon may be configured to run any Erlang
+ implemented SSH subsystem.
+ </item>
+ <item><em>Shell</em> - interactive shell. By default the
+ Erlang daemon will run the Erlang shell. It is
+ possible to customize the shell by providing your own
+ read-eval-print loop. It is also possible, but much more work,
+ to provide your own CLI (Command Line Interface) implementation.
+ </item>
+ <item><em>Exec</em> - one-time remote execution of commands. See <seealso
+ marker="ssh_connection#exec-4">ssh_connection:exec/4</seealso></item>
+ </list>
+ </section>
+
+ <p>Channels are flow controlled. No data may be sent to a channel
+ peer until a message is received to indicate that window space is
+ available. The 'initial window size' specifies how many bytes of
+ channel data that can be sent to the channel peer without adjusting the
+ window.
+ </p>
+
+ <p>
+ For more detailed information about the SSH protocol, see the
+ following RFCs:
+ </p>
+
+ <list type="bulleted">
+ <item><url href="http://www.ietf.org/rfc/rfc4250.txt">RFC 4250</url> -
+ Protocol Assigned Numbers.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4251.txt">RFC 4251</url> -
+ Protocol Architecture.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4252.txt">RFC 4252</url> -
+ Authentication Protocol.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4253.txt">RFC 4253</url> -
+ Transport Layer Protocol.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4254.txt">RFC 4254</url> -
+ Connection Protocol.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4255.txt">RFC 4255</url> -
+ Key Fingerprints.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4344.txt">RFC 4344</url> -
+ Transport Layer Encryption Modes.</item>
+ <item><url href="http://www.ietf.org/rfc/rfc4716.txt">RFC 4716</url> -
+ Public Key File Format.</item>
+ </list>
+ </section>
+</chapter>