Subject: SSH Break Draft, resurrected (draft-ietf-secsh-break-04)

From: "Phillip Remaker" <[email-masked]>
Date: Thu, 19 May 2005 08:46:58 -0700

THANKS to Ben Harris at Vandyke for stepping up and editing this.  It has
been languishing for months on my TO DO list.  I plan to resubmit this draft
for SSH over break to this noble body.

All just nits and tweaks, the content is unchanged, largely.  But comments
are most welcome!!

 --------------------------------------------------------------------

Secure Shell Working Group                                  J. Galbraith
Internet-Draft                                          VanDyke Software
Expires: November 15, 2005                                    P. Remaker
                                                      Cisco Systems, Inc
                                                            May 14, 2005

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

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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 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 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
   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 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
   driver.

   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 could use some other 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
   sent.  If no BREAK was preformed, SSH_MSG_CHANNEL_FAILURE MUST be

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   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 configurable or 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: [email-masked]

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

   Phone: +1 408 526 8614
   Email: [email-masked]

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---

From: Ben Harris <[email-masked]>
Date: Thu, 19 May 2005 16:53:12 +0100

In article <[email-masked]> you write:
>THANKS to Ben Harris at Vandyke for stepping up and editing this.

Point of Information, I'm not of Vandyke.  I'm either of the University of
Cambridge or of the PuTTY project, but not acting on behalf of either.

-- 
Ben Harris