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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<!-- %EricssonCopyright% -->
<chapter>
<header>
<copyright>
<year>2013</year><year>2016</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/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>
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