    Network Working Group                                       J. Galbraith
    Request for Comments: 4335                              VanDyke Software
    Category: Standards Track                                     P. Remaker
                                                          Cisco Systems, Inc
                                                                January 2006

             The Secure Shell (SSH) Session Channel Break Extension

    Status of This Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2006).

    Abstract

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

    Table of Contents

       1. Introduction ....................................................2
       2. Conventions Used in This Document ...............................2
       3. The Break Request ...............................................3
       4. Security Considerations .........................................4
       5. IANA Considerations .............................................4
       6. References ......................................................4
          6.1. Normative References .......................................4
          6.2. Informative References .....................................5

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    1.  Introduction

       The Secure Shell (SSH) [5] 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
       RFC 854 [1] defines as "a signal outside the USASCII set which is
       currently given local meaning within many systems".  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.

    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 (as described in [4]) 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 3000 ms is received, it
       SHOULD be interpreted as a request for a 3000 ms 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 500
       ms is received, it SHOULD be interpreted as a 500 ms 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, the BREAK SHOULD be interpreted as the
       default BREAK signal length of the chipset or underlying chipset
       driver.  If no default exists, 500 ms 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 that 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.

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       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
       sent.

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

    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.

    5.  IANA Considerations

       IANA has assigned the Connection Protocol Channel Request Name
       "break" in accordance with [6].

    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, Ed., "The Secure Shell (SSH) Protocol
            Architecture", RFC 4251, January 2006.

       [4]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
            Transport Layer Protocol", RFC 4253, January 2006.

       [5]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
            Connection Protocol", RFC 4254, January 2006.

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    RFC 4335                  SSH Break Extension               January 2006

       [6]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
            Protocol Assigned Numbers", RFC 4250, January 2006.

    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

       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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    RFC 4335                  SSH Break Extension               January 2006

    Full Copyright Statement

       Copyright (C) The Internet Society (2006).

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