Secure Shell Working Group J. Galbraith Internet-Draft VanDyke Software Expires:
November 24, 2005January 16, 2006 P. Remaker Cisco Systems, Inc May 23,July 15, 2005 Secure Shell (SSH) Session Channel Break Extension draft-ietf-secsh-break-03draft-ietf-secsh-break-04 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 24, 2005.January 16, 2006. 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. 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 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"  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 sendsending 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."  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 . The "byte", "boolean", "uint32", and "string" data types are defined in . 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 no default exists, 500ms can be used as the BREAK length. 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 processedintepreted as a request for a 3000ms BREAK, in order to preventBREAK. This safeguard prevents an unreasonably long BREAK request from causing thea 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.suggestion. If BREAK duration request of less than 500ms, is requested a BREAK of500ms is received, it SHOULD be sentinterpreted as a 500ms BREAK since most devices will recognize a BREAK of that length. In the eventApplications that an application needsrequire a shorter BREAK,BREAK may choose to ignore this suggestion can be ignored.suggestion. If the BREAK- lengthBREAK-length parameter is 0,0 or not present, the BREAK SHOULD be sentinterpreted as 500ms orthe default BREAK signal length of the chipset or underlying chipset driver. If no default exists, 500ms 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 could use some otherwhich 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. If the 'want_reply' boolean is set, the server MUST reply using an SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE  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 oron a per portper-port or perper- 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 is requested to assign the Connection Protocol Channel Request Name "break" in accordance with . 6. References 6.1 Normative References  Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD 8, RFC 854, May 1983.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Ylonen, T. and C. Lonvick, "SSH Protocol Architecture", draft-ietf-secsh-architecture-22 (work in progress), March 2005.  Lonvick, C., "SSH Transport Layer Protocol", draft-ietf-secsh-transport-24 (work in progress), March 2005.  Lonvick, C. and T. Ylonen, "SSH Connection Protocol", draft-ietf-secsh-connect-25 (work in progress), March 2005.  Lehtinen, S. and C. Lonvick, "SSH Protocol Assigned Numbers", draft-ietf-secsh-assignednumbers-12 (work in progress), March 2005. 6.2 Informative References  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: firstname.lastname@example.org Phillip Remaker Cisco Systems, Inc 170 West Tasman Drive San Jose, CA 95120 US Phone: +1 408 526 8614 Email: email@example.com Intellectual Property Statement 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 in BCP 78 and BCP 79. 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