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-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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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: galb-list@vandyke.com
Phillip Remaker
Cisco Systems, Inc
170 West Tasman Drive
San Jose, CA 95120
US
EMail: remaker@cisco.com
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