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Versions: 00 01 02 03 04 RFC 4335

Secure Shell Working Group                                  J. Galbraith
Internet-Draft                                          VanDyke Software
Expires: January 16, 2006                                     P. Remaker
                                                      Cisco Systems, Inc
                                                           July 15, 2005


           Secure Shell (SSH) Session Channel Break Extension
                       draft-ietf-secsh-break-04

Status of this Memo

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   This Internet-Draft will expire on January 16, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The Session Channel Break Extension provides a means to send a BREAK
   signal over a Secure Shell (SSH) terminal session.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in this Document  . . . . . . . . . . . . . .  4
   3.  The Break Request  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1   Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2   Informative References . . . . . . . . . . . . . . . . . .  9
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 11







































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1.  Introduction

   The Secure Shell (SSH) session channel provides a mechanism for the
   client-user to interactively enter commands and receive output from a
   remote host while taking advantage of the SSH transport's privacy and
   integrity features.  SSH is increasingly being used to replace Telnet
   for terminal access applications.

   A common application of the Telnet protocol is the "Console Server"
   [7] whereby a Telnet Network Virtual Terminal (NVT) can be connected
   to a physical RS-232/V.24 asynchronous port, making the Telnet NVT
   appear as a locally attached terminal to that port, and making that
   physical port appear as a network addressable device.  A number of
   major computer equipment vendors provide high level administrative
   functions through an asynchronous serial port and generally expect
   the attached terminal to be capable of sending a BREAK signal.

   A BREAK signal is defined as the TxD signal being held in a SPACE
   ("0") state for a time greater than a whole character time.  In
   practice, a BREAK signal is typically 250 to 500 ms in length.

   The Telnet protocol furnishes a means to send a "BREAK" signal, which
   RFC0854 [1] defines as a "a signal outside the USASCII set which is
   currently given local meaning within many systems." [1]  Console
   Server vendors interpret the TELNET BREAK signal as a physical BREAK
   signal, which can then allow access to the full range of
   administrative functions available on an asynchronous serial console
   port.

   The lack of a similar facility in the SSH session channel has forced
   users to continue the use of Telnet for the "Console Server"
   function.



















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2.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [2].

   The "byte", "boolean", "uint32", and "string" data types are defined
   in [3].











































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3.  The Break Request

   The following channel specific request can be sent over a session
   channel to request that the remote host perform a BREAK operation.

        byte      SSH_MSG_CHANNEL_REQUEST
        uint32    recipient channel
        string    "break"
        boolean   want_reply
        uint32    break-length in milliseconds

   If the BREAK length cannot be controlled by the application receiving
   this request, the BREAK length parameter SHOULD be ignored and the
   default BREAK signal length of the chipset or underlying chipset
   driver SHOULD be sent.  If no default exists, 500ms can be used as
   the BREAK length.

   If the application receiving this request can control the BREAK-
   length, the following suggestions are made regarding BREAK duration.
   If a BREAK duration request of greater than 3000ms is received, it
   SHOULD be intepreted as a request for a 3000ms BREAK.  This safeguard
   prevents an unreasonably long BREAK request from causing a port to
   become unavailable for as long as 49.7 days while executing the
   BREAK.  Applications that require a longer BREAK may choose to ignore
   this suggestion.  If BREAK duration request of less than 500ms is
   received, it SHOULD be interpreted as a 500ms BREAK since most
   devices will recognize a BREAK of that length.  Applications that
   require a shorter BREAK may choose to ignore this suggestion.  If the
   BREAK-length parameter is 0 or not present, the BREAK SHOULD be
   interpreted as the default BREAK signal length of the chipset or
   underlying chipset driver.  If no default exists, 500ms can be used
   as the BREAK length.

   If the SSH connection does not terminate on a physical serial port,
   the BREAK indication SHOULD be handled in a manner consistent with
   the general use of BREAK as an attention/interrupt signal; for
   instance, a service processor which requires an out-of-band facility
   to get the attention of a system it manages.

   In a case where an SSH connection cascades to another connection, the
   BREAK SHOULD be passed along the cascaded connection.  For example, a
   Telnet session from an SSH shell should carry along an SSH initiated
   BREAK and an SSH client initiated from a Telnet connection SHOULD
   pass a BREAK indication from the Telnet connection.

   If the 'want_reply' boolean is set, the server MUST reply using an
   SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE [5] message.  If a
   BREAK of any kind was preformed, SSH_MSG_CHANNEL_SUCCESS MUST be



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   sent.  If no BREAK was preformed, SSH_MSG_CHANNEL_FAILURE MUST be
   sent.

   This operation SHOULD be supported by any general purpose SSH client.















































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4.  Security Considerations

   Many computer systems treat serial consoles as local and secured, and
   interpret a BREAK signal as an instruction to halt execution of the
   operating system or to enter privileged configuration modes.  Because
   of this, extra care should be taken to ensure that SSH access to
   BREAK-enabled ports are limited to users with appropriate privileges
   to execute such functions.  Alternatively, support for the BREAK
   facility MAY be implemented as configurable on a per-port or per-
   server basis.

   Implementations that literally interpret the BREAK length parameter
   without imposing the suggested BREAK time limit may cause a denial of
   service to or unexpected results from attached devices receiving the
   very long BREAK signal.




































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5.  IANA Considerations

   IANA is requested to assign the Connection Protocol Channel Request
   Name "break" in accordance with [6].















































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6.  References

6.1  Normative References

   [1]  Postel, J. and J. Reynolds, "Telnet Protocol Specification",
        STD 8, RFC 854, May 1983.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
        draft-ietf-secsh-architecture-22 (work in progress), March 2005.

   [4]  Lonvick, C., "SSH Transport Layer Protocol",
        draft-ietf-secsh-transport-24 (work in progress), March 2005.

   [5]  Lonvick, C. and T. Ylonen, "SSH Connection Protocol",
        draft-ietf-secsh-connect-25 (work in progress), March 2005.

   [6]  Lehtinen, S. and C. Lonvick, "SSH Protocol Assigned Numbers",
        draft-ietf-secsh-assignednumbers-12 (work in progress),
        March 2005.

6.2  Informative References

   [7]  Harris, D., "Greater Scroll of Console Knowledge", March 2004,
        <http://www.conserver.com/consoles/>.


Authors' Addresses

   Joseph Galbraith
   VanDyke Software
   4848 Tramway Ridge Blvd
   Suite 101
   Albuquerque, NM  87111
   US

   Phone: +1 505 332 5700
   Email: galb-list@vandyke.com











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   Phillip Remaker
   Cisco Systems, Inc
   170 West Tasman Drive
   San Jose, CA  95120
   US

   Phone: +1 408 526 8614
   Email: remaker@cisco.com











































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   Copyright (C) The Internet Society (2005).  This document is subject
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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