Secure Shell Working Group                                  J. Galbraith
Internet-Draft                                          VanDyke Software
Expires: February 17, 2004                                    P. Remaker
                                                      Cisco Systems, Inc
                                                         August 19, 2003

                    Session Channel Break Extension

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on February 17, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   The Session Channel Break Extension provides a means to send a BREAK
   signal [2] over an SSH terminal session [5].

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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2. The Break Request  . . . . . . . . . . . . . . . . . . . . . . . 4
   3. Security Considerations  . . . . . . . . . . . . . . . . . . . . 6
      Normative References . . . . . . . . . . . . . . . . . . . . . . 7
      Informative References . . . . . . . . . . . . . . . . . . . . . 8
      Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
      Intellectual Property and Copyright Statements . . . . . . . . . 9

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1. Introduction

   The 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"
   [2] whereby a telnet 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 send 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 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
   adminisrative functions available on an asynchronous serial console

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

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2. The Break Request

   The following following channel specific request can be sent 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 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 processed as a 3000ms BREAK, in order to
   prevent an unreasonably long BREAK request causing the 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
   requirement.  If  BREAK duration request of less than 500ms, is
   requested a BREAK of 500ms SHOULD be sent since most devices will
   recognize a BREAK of that length.  In the event that an application
   needs a shorter BREAK, this suggestion can be ignored.  If the
   BREAK-length parameter is 0, the BREAK SHOULD be sent as 500ms or the
   default BREAK signal length of the chipset or underlying chipset

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

   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 initited 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
   a BREAK of any kind was preformed, SSH_MSG_CHANNEL_SUCCESS MUST be
   sent.  If no BREAK was preformed, SSH_MSG_CHANNEL_FAILURE MUST be

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

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3. 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 priviliged 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 priviliges
   to execute such functions. Alternatively, support for the BREAK
   facility MAY be imlemented configurable or a per port or per server

   Implementations that literally intepret 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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Normative References

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

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Informative References

   [2]  Harris, D., "Greater Scroll of Console Knowledge", April 2003.

   [3]  Rinne, T., Ylonen, T., Kivinen, T. and S. Lehtinen, "SSH
        Protocol Architecture", draft-ietf-secsh-architecture-14 (work
        in progress), July 2003.

   [4]  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S.
        Lehtinen, "SSH Transport Layer Protocol",
        draft-ietf-secsh-transport-16 (work in progress), July 2003.

   [5]  Rinne, T., Ylonen, T., Kivinen, T. and S. Lehtinen, "SSH
        Connection Protocol", draft-ietf-secsh-connect-17 (work in
        progress), July 2003.

Authors' Addresses

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

   Phone: +1 505 332 5700

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


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Intellectual Property Statement

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

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