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Network Working Group                                       J. GalbraithRequest for Comments: 4335                              VanDyke SoftwareCategory: Standards Track                                     P. Remaker                                                      Cisco Systems, Inc                                                            January 2006The Secure Shell (SSH) Session Channel Break ExtensionStatus 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 Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. The Break Request ...............................................34. Security Considerations .........................................45. IANA Considerations .............................................46. References ......................................................46.1. Normative References .......................................46.2. Informative References .....................................5Galbraith & Remaker         Standards Track                     [Page 1]

RFC 4335                  SSH Break Extension               January 20061.  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, whichRFC 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].Galbraith & Remaker         Standards Track                     [Page 2]

RFC 4335                  SSH Break Extension               January 20063.  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.Galbraith & Remaker         Standards Track                     [Page 3]

RFC 4335                  SSH Break Extension               January 2006   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.  References6.1.  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.Galbraith & Remaker         Standards Track                     [Page 4]

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.comGalbraith & Remaker         Standards Track                     [Page 5]

RFC 4335                  SSH Break Extension               January 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Galbraith & Remaker         Standards Track                     [Page 6]