Secure Shell Working Group J. Galbraith Internet-Draft VanDyke Software Expires:
SeptemberFebruary 17, 20032004 P. Remaker Cisco Systems, Inc MarchAugust 19, 2003 Session Channel Break Extension draft-ietf-secsh-break-00.txtdraft-ietf-secsh-break-01.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 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 SeptemberFebruary 17, 2003.2004. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract The Session Channel Break Extension provides a waymeans to send a breakBREAK signal during a over an SSH terminal session.session . Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The Break Request . . . . . . . . . . . . . . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 Normative References . . . . . . . . . . . . . . . . . . . . . . 7 Informative References . . . . . . . . . . . . . . . . . . . . 5. 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 58 Intellectual Property and Copyright Statements . . . . . . . . . 69 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"  whereby a telnet NVT can be connected to a physical RS-232/V.24 asynchronous port, allowingmaking the telnet NVT toappear as a locally attached terminal to that port, and allowingmaking that physical port toappear 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, whichsignal. 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,time. In practice, a BREAK signal is typically interpreted as250 to 500 ms.ms in length. The telnet protocolprovidesprotocol furnishes a means to send a "BREAK" signal, which is definedRFC0854 defines as a "a signal outside the USASCII set which is currently given local meaning within many systems."  Console Server vendors interpret the TELNET breakBREAK signal as a physical breakBREAK signal, which can then allow access to the full range of administartiveadminisrative 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. The Break Request The following following channel specific request can be sent to request that the remote host perform a breakBREAK operation. byte SSH_MSG_CHANNEL_REQUEST uint32 recipient channel string "break" boolean want_reply uint32 break-length in milliseconds If the breakBREAK length cannot be controlled by the application receiving this request, the breakBREAK length parameter SHOULD be ignored and the default breakBREAK signal length of the chipset or underlying chipset driver SHOULD be sent. If the application receiving this request can control the break-length,BREAK-length, the following suggestions are made reagarding breakregarding BREAK duration. If a breakBREAK duration request of greater than 3000ms is received, it SHOULD be processed as a 3000ms break,BREAK, in order to prevent an unreasonably long breakBREAK request causing the port to become unavailable for as long as 4749.7 days while executing the break.BREAK. Applications that require a longer breakBREAK may choose to ignore this requirement. If breakBREAK duration request of less than 500ms, is requested a breakBREAK of 500ms SHOULD be sent since most devices will recognize a breakBREAK of that length. In the event that an application needs a shorter break,BREAK, this suggestion can be ignored. If the break-lengthBREAK-length parameter is 0, the breakBREAK SHOULD be sent as 500ms or the default breakBREAK signal length of the chipset or underlying chipset driver .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  messages. If a breakBREAK of any kind was preformed, SSH_MSG_CHANNEL_SUCCESS MUST be sent. If no breakBREAK was preformed, SSH_MSG_CHANNEL_FAILURE MUST be sent. This operation SHOULD be supportsupported by mostany general purpose SSH clients.client. 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. Normative References  Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD 8, RFC 854, May 1983. Informative References  Harris, D., "Greater Scroll of Console Knowledge", April 2003.  Rinne, T., Ylonen, T., Kivinen, T. and S. Lehtinen, "SSH Protocol Architecture", draft-ietf-secsh-architecture-13draft-ietf-secsh-architecture-14 (work in progress), September 2002. July 2003.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Transport Layer Protocol", draft-ietf-secsh-transport-15draft-ietf-secsh-transport-16 (work in progress), September 2002. July 2003.  Rinne, T., Ylonen, T., Kivinen, T. and S. Lehtinen, "SSH Connection Protocol", draft-ietf-secsh-connect-16draft-ietf-secsh-connect-17 (work in progress), September 2002.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@example.com Phillip Remaker Cisco Systems, Inc 170 West Tasman Drive San Jose, CA 95120 US EMail: firstname.lastname@example.org Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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