draft-ietf-secsh-break-04.txt   rfc4335.txt 
Secure Shell Working Group J. Galbraith Network Working Group J. Galbraith
Internet-Draft VanDyke Software Request for Comments: 4335 VanDyke Software
Expires: January 16, 2006 P. Remaker Category: Standards Track P. Remaker
Cisco Systems, Inc Cisco Systems, Inc
July 15, 2005 January 2006
Secure Shell (SSH) Session Channel Break Extension
draft-ietf-secsh-break-04
Status of this Memo
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This Internet-Draft will expire on January 16, 2006. This document specifies an Internet standards track protocol for the
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and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
The Session Channel Break Extension provides a means to send a BREAK The Session Channel Break Extension provides a means to send a BREAK
signal over a Secure Shell (SSH) terminal session. signal over a Secure Shell (SSH) terminal session.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction ....................................................2
2. Conventions Used in this Document . . . . . . . . . . . . . . 4 2. Conventions Used in This Document ...............................2
3. The Break Request . . . . . . . . . . . . . . . . . . . . . . 5 3. The Break Request ...............................................3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations .........................................4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations .............................................4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. References ......................................................4
6.1 Normative References . . . . . . . . . . . . . . . . . . . 9 6.1. Normative References .......................................4
6.2 Informative References . . . . . . . . . . . . . . . . . . 9 6.2. Informative References .....................................5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 11
1. Introduction 1. Introduction
The Secure Shell (SSH) session channel provides a mechanism for the The Secure Shell (SSH) [5] session channel provides a mechanism for
client-user to interactively enter commands and receive output from a the client-user to interactively enter commands and receive output
remote host while taking advantage of the SSH transport's privacy and from a remote host while taking advantage of the SSH transport's
integrity features. SSH is increasingly being used to replace Telnet privacy and integrity features. SSH is increasingly being used to
for terminal access applications. replace Telnet for terminal access applications.
A common application of the Telnet protocol is the "Console Server" A common application of the Telnet protocol is the "Console Server"
[7] whereby a Telnet Network Virtual Terminal (NVT) can be connected [7] whereby a Telnet Network Virtual Terminal (NVT) can be connected
to a physical RS-232/V.24 asynchronous port, making the Telnet NVT 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 appear as a locally attached terminal to that port, and making that
physical port appear as a network addressable device. A number of physical port appear as a network-addressable device. A number of
major computer equipment vendors provide high level administrative major computer equipment vendors provide high-level administrative
functions through an asynchronous serial port and generally expect functions through an asynchronous serial port and generally expect
the attached terminal to be capable of sending a BREAK signal. 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 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 ("0") state for a time greater than a whole character time. In
practice, a BREAK signal is typically 250 to 500 ms in length. practice, a BREAK signal is typically 250 to 500 ms in length.
The Telnet protocol furnishes a means to send a "BREAK" signal, which The Telnet protocol furnishes a means to send a "BREAK" signal, which
RFC0854 [1] defines as a "a signal outside the USASCII set which is RFC 854 [1] defines as "a signal outside the USASCII set which is
currently given local meaning within many systems." [1] Console currently given local meaning within many systems". Console Server
Server vendors interpret the TELNET BREAK signal as a physical BREAK vendors interpret the TELNET BREAK signal as a physical BREAK signal,
signal, which can then allow access to the full range of which can then allow access to the full range of administrative
administrative functions available on an asynchronous serial console functions available on an asynchronous serial console port.
port.
The lack of a similar facility in the SSH session channel has forced The lack of a similar facility in the SSH session channel has forced
users to continue the use of Telnet for the "Console Server" users to continue the use of Telnet for the "Console Server"
function. function.
2. Conventions Used in this Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [2]. document are to be interpreted as described in [2].
The "byte", "boolean", "uint32", and "string" data types are defined The "byte", "boolean", "uint32", and "string" data types are defined
in [3]. in [3].
3. The Break Request 3. The Break Request
The following channel specific request can be sent over a session The following channel-specific request can be sent over a session
channel to request that the remote host perform a BREAK operation. channel (as described in [4]) to request that the remote host perform
a BREAK operation.
byte SSH_MSG_CHANNEL_REQUEST byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel uint32 recipient channel
string "break" string "break"
boolean want_reply boolean want_reply
uint32 break-length in milliseconds uint32 break-length in milliseconds
If the BREAK length cannot be controlled by the application receiving If the BREAK length cannot be controlled by the application receiving
this request, the BREAK length parameter SHOULD be ignored and the this request, the BREAK length parameter SHOULD be ignored and the
default BREAK signal length of the chipset or underlying chipset default BREAK signal length of the chipset or underlying chipset
driver SHOULD be sent. If no default exists, 500ms can be used as driver SHOULD be sent.
the BREAK length.
If the application receiving this request can control the BREAK- If the application receiving this request can control the BREAK
length, the following suggestions are made regarding BREAK duration. length, the following suggestions are made regarding BREAK duration.
If a BREAK duration request of greater than 3000ms is received, it If a BREAK duration request of greater than 3000ms is received, it
SHOULD be intepreted as a request for a 3000ms BREAK. This safeguard SHOULD be interpreted as a request for a 3000 ms BREAK. This
prevents an unreasonably long BREAK request from causing a port to safeguard prevents an unreasonably long BREAK request from causing a
become unavailable for as long as 49.7 days while executing the port to become unavailable for as long as 49.7 days while executing
BREAK. Applications that require a longer BREAK may choose to ignore the BREAK. Applications that require a longer BREAK may choose to
this suggestion. If BREAK duration request of less than 500ms is ignore this suggestion. If BREAK duration request of less than 500
received, it SHOULD be interpreted as a 500ms BREAK since most ms is received, it SHOULD be interpreted as a 500 ms BREAK since most
devices will recognize a BREAK of that length. Applications that devices will recognize a BREAK of that length. Applications that
require a shorter BREAK may choose to ignore this suggestion. If the require a shorter BREAK may choose to ignore this suggestion. If the
BREAK-length parameter is 0 or not present, the BREAK SHOULD be BREAK length parameter is 0, the BREAK SHOULD be interpreted as the
interpreted as the default BREAK signal length of the chipset or default BREAK signal length of the chipset or underlying chipset
underlying chipset driver. If no default exists, 500ms can be used driver. If no default exists, 500 ms can be used as the BREAK
as the BREAK length. length.
If the SSH connection does not terminate on a physical serial port, If the SSH connection does not terminate on a physical serial port,
the BREAK indication SHOULD be handled in a manner consistent with the BREAK indication SHOULD be handled in a manner consistent with
the general use of BREAK as an attention/interrupt signal; for the general use of BREAK as an attention/interrupt signal; for
instance, a service processor which requires an out-of-band facility instance, a service processor that requires an out-of-band facility
to get the attention of a system it manages. to get the attention of a system it manages.
In a case where an SSH connection cascades to another connection, the In a case where an SSH connection cascades to another connection, the
BREAK SHOULD be passed along the cascaded connection. For example, a BREAK SHOULD be passed along the cascaded connection. For example, a
Telnet session from an SSH shell should carry along an SSH initiated Telnet session from an SSH shell should carry along an SSH-initiated
BREAK and an SSH client initiated from a Telnet connection SHOULD BREAK, and an SSH client initiated from a Telnet connection SHOULD
pass a BREAK indication from the Telnet connection. pass a BREAK indication from the Telnet connection.
If the 'want_reply' boolean is set, the server MUST reply using an 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 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 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. If no BREAK was preformed, SSH_MSG_CHANNEL_FAILURE MUST be
sent. sent.
This operation SHOULD be supported by any general purpose SSH client. This operation SHOULD be supported by any general purpose SSH client.
4. Security Considerations 4. Security Considerations
Many computer systems treat serial consoles as local and secured, and Many computer systems treat serial consoles as local and secured, and
interpret a BREAK signal as an instruction to halt execution of the interpret a BREAK signal as an instruction to halt execution of the
operating system or to enter privileged configuration modes. Because operating system or to enter privileged configuration modes. Because
of this, extra care should be taken to ensure that SSH access to of this, extra care should be taken to ensure that SSH access to
BREAK-enabled ports are limited to users with appropriate privileges BREAK-enabled ports are limited to users with appropriate privileges
to execute such functions. Alternatively, support for the BREAK to execute such functions. Alternatively, support for the BREAK
facility MAY be implemented as configurable on a per-port or per- facility MAY be implemented as configurable on a per-port or
server basis. per-server basis.
Implementations that literally interpret the BREAK length parameter Implementations that literally interpret the BREAK length parameter
without imposing the suggested BREAK time limit may cause a denial of without imposing the suggested BREAK time limit may cause a denial of
service to or unexpected results from attached devices receiving the service to or unexpected results from attached devices receiving the
very long BREAK signal. very long BREAK signal.
5. IANA Considerations 5. IANA Considerations
IANA is requested to assign the Connection Protocol Channel Request IANA has assigned the Connection Protocol Channel Request Name
Name "break" in accordance with [6]. "break" in accordance with [6].
6. References 6. References
6.1 Normative References 6.1. Normative References
[1] Postel, J. and J. Reynolds, "Telnet Protocol Specification", [1] Postel, J. and J. Reynolds, "Telnet Protocol Specification",
STD 8, RFC 854, May 1983. STD 8, RFC 854, May 1983.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[3] Ylonen, T. and C. Lonvick, "SSH Protocol Architecture", [3] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol
draft-ietf-secsh-architecture-22 (work in progress), March 2005. Architecture", RFC 4251, January 2006.
[4] Lonvick, C., "SSH Transport Layer Protocol", [4] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
draft-ietf-secsh-transport-24 (work in progress), March 2005. Transport Layer Protocol", RFC 4253, January 2006.
[5] Lonvick, C. and T. Ylonen, "SSH Connection Protocol", [5] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
draft-ietf-secsh-connect-25 (work in progress), March 2005. Connection Protocol", RFC 4254, January 2006.
[6] Lehtinen, S. and C. Lonvick, "SSH Protocol Assigned Numbers", [6] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
draft-ietf-secsh-assignednumbers-12 (work in progress), Protocol Assigned Numbers", RFC 4250, January 2006.
March 2005.
6.2 Informative References 6.2. Informative References
[7] Harris, D., "Greater Scroll of Console Knowledge", March 2004, [7] Harris, D., "Greater Scroll of Console Knowledge", March 2004,
<http://www.conserver.com/consoles/>. <http://www.conserver.com/consoles/>.
Authors' Addresses Authors' Addresses
Joseph Galbraith Joseph Galbraith
VanDyke Software VanDyke Software
4848 Tramway Ridge Blvd 4848 Tramway Ridge Blvd
Suite 101 Suite 101
Albuquerque, NM 87111 Albuquerque, NM 87111
US US
Phone: +1 505 332 5700 Phone: +1 505 332 5700
Email: galb-list@vandyke.com EMail: galb-list@vandyke.com
Phillip Remaker Phillip Remaker
Cisco Systems, Inc Cisco Systems, Inc
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95120 San Jose, CA 95120
US US
Phone: +1 408 526 8614 Phone: +1 408 526 8614
Email: remaker@cisco.com EMail: remaker@cisco.com
Intellectual Property Statement Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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skipping to change at page 11, line 29 skipping to change at page 6, line 45
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"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.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is provided by the IETF
Internet Society. Administrative Support Activity (IASA).
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