Subject: BREAK OVER SSH: draft-ietf-secsh-break-02

From: "Phillip Remaker" <[email-masked]>
Date: Tue, 19 Aug 2003 15:52:06 -0700

I submit for your comments the second (third?  Did we start at zero?) draft
of "break over SSH."

- Split normative/non-normative references
- Added verbage on cascaded connections and dealing with BREAK on
psuedo-ttys
- Added a brief security considerations section.
- Updated references to latest SSH drafts.

Also, does anyone have a good, normative reference on the definition of a
physical BREAK signal?  It doesn't seem to show in
any ITU docs, but I did point to a good website that treats the issue
comprehensively as an Informative reference.

(Does mailing to the list constitute an official submission?  Or are there
more steps I need to take?)

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

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
                     draft-ietf-secsh-break-02.txt

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.

Abstract

   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
   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. 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
   driver.

   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
   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 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
   SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE [5] messages.  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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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
   basis.

   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
   US

   Phone: +1 505 332 5700
   EMail: [email-masked]

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

   EMail: [email-masked]

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---

From: RJ Atkinson <[email-masked]>
Date: Wed, 20 Aug 2003 11:04:37 -0400

On Tuesday, Aug 19, 2003, at 18:52 America/Montreal, Phillip Remaker 
> more steps I need to take?)

Official submissions are sent to <[email-masked]>.
The IETF Secretariat folks will, when time permits, then publish
the submitted I-D to the online I-D archives.

I-D submissions that have the approval of the WG Chair should
have a filename of draft-ietf-secsh-*.txt.  Individual proposals
should have a filename of draft-<lastname>-*.txt.  Normally,
stuff starts off as an individual proposal, then becomes a
WG official document later on after some WG discussion.

(I'm copying the whole list since others probably wonder about
the same questions.)

Ran

---

From: Ivan Passos <[email-masked]>
Date: Wed, 20 Aug 2003 09:56:08 -0700

Hello, Phil!

> comprehensively as an Informative reference.

Where is the pointer to that web site?? I couldn't find it in the 
draft you attached to your original msg. Did you send it in some 
other msg??

Anyhow, I would guess that the EIA-232E standard document has the 
formal definition of a BREAK signal. However, I tried to find 
that document through the 'Net to no avail (the company who sells 
EIA standards -- Global Engineering Documents, 
at the Information 
Handling Services (IHS)
, http://global.ihs.com/ -- doesn't seem to 
have it in their EIA section ... :-/ ...).

I know this is not worth much, but I can assure you that the 
definition I provided earlier is correct. :)) That is: a serial 
BREAK is the assertion of the TxD signal to '0' (Space) for a 
period longer than a character time. 

Hope this helps!!

Later,
-- 
Ivan Passos			Phone: 510-771-6202
Product Marketing Manager	Cell:  510-468-3515
Cyclades Corporation		Fax:   510-771-6200
[email-masked]
http://www.cyclades.com		"Everywhere with Linux"

---

From: Jeffrey Altman <[email-masked]>
Date: Wed, 20 Aug 2003 19:08:44 -0400

The following is from the DEC VT100 manual:

"BREAK – Typing the BREAK key causes the transmission line to be forced 
to its zero state for 0.2333 seconds ± 10 percent. If either SHIFT key 
is down, the time is increased to 3.5 seconds ± 10 percent. Data 
Terminal Ready is also deasserted during this interval. At the 
conclusion of the 3.5 second interval Data Terminal Ready will again be 
asserted.

"The SHIFT and BREAK keys typed together provide the 
long-break-disconnect function. Used with properly configured modems 
with RS-232-C levels, it will cause both the local and remote data sets 
to disconnect. For modems that are connected via the 20 mA current loop, 
issuing the long space may disconnect the remote data set only.

"The CTRL and BREAK keys typed together cause the transmission of the 
answerback message.

"The BREAK key does not function when the VT100 is in LOCAL mode."