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Internet Engineering Task Force (IETF) S. Tatham
Request for Comments: 8160 PuTTY
Category: Standards Track D. Tucker
ISSN: 2070-1721 OpenSSH
April 2017
IUTF8 Terminal Mode in Secure Shell (SSH)
Abstract
This document specifies a new opcode in the Secure Shell terminal
modes encoding. The new opcode describes the widely used IUTF8
terminal mode bit, which indicates that terminal I/O uses UTF-8
character encoding.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8160.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Tatham & Tucker Standards Track [Page 1]
RFC 8160 IUTF8 Terminal Mode in SSH April 2017 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 3. New IUTF8 Opcode for Terminal Mode Encoding . . . . . . . . . 2 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . 3 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 3 6.1. Normative References . . . . . . . . . . . . . . . . . . 3 6.2. Informative References . . . . . . . . . . . . . . . . . 3 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 4 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction The Secure Shell (SSH) connection protocol [RFC4254] provides an encoding for terminal modes, used in the "pty-req" channel request type. A commonly used terminal mode is IUTF8, which indicates that the terminal driver should assume that terminal I/O uses the UTF-8 character encoding [RFC3629]. This is typically used by the kernel's terminal driver on the server to decide how many bytes of input to treat as a single logical character during line editing. SSH does not currently provide an encoding for IUTF8. This document specifies one. 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 [RFC2119]. 3. New IUTF8 Opcode for Terminal Mode Encoding The opcode value 42 is defined for the IUTF8 terminal mode. As specified in Section 8 of [RFC4254], all opcodes in the range 1 to 159 have a single uint32 argument; therefore, the IUTF8 opcode is followed by a single uint32 argument. The value 0 indicates that the IUTF8 mode is disabled, and the value 1 indicates that it is enabled. As with all other encoded terminal modes, the client SHOULD transmit a value for this mode if it knows about one, and the server MAY ignore it. Tatham & Tucker Standards Track [Page 2]
RFC 8160 IUTF8 Terminal Mode in SSH April 2017 4. IANA Considerations This document augments the list of "Pseudo-Terminal Encoded Terminal Modes" defined in Section 8 of [RFC4254]. IANA has added the following opcode to the "Pseudo-Terminal Encoded Terminal Modes" registry: opcode mnemonic description ------ -------- ----------- 42 IUTF8 Terminal input and output is assumed to be encoded in UTF-8. 5. Security Considerations The security considerations of [RFC4254] apply. This additional terminal mode encoding is believed to have no security implications differing from the existing set of encoded terminal modes. Since the IUTF8 terminal mode is intended for use in conjunction with the UTF-8 character encoding, the security considerations of [RFC3629] also apply to systems in which this mode is enabled. In particular, terminal drivers that interpret this bit as enabling UTF-8-aware line-editing behavior should carefully consider how that behavior will treat illegal sequences, overlong encodings, and redundant representations of composed characters (see [UNICODE]). 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <http://www.rfc-editor.org/info/rfc3629>. [RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006, <http://www.rfc-editor.org/info/rfc4254>. 6.2. Informative References [UNICODE] The Unicode Consortium, "The Unicode Standard", <http://www.unicode.org/versions/latest/>. Tatham & Tucker Standards Track [Page 3]
RFC 8160 IUTF8 Terminal Mode in SSH April 2017 Acknowledgements The authors are indebted to Colin Watson for originally suggesting this terminal mode in 2005, and David Madore and Jakub Jelen for prior implementation efforts. Authors' Addresses Simon Tatham PuTTY Email: anakin@pobox.com Darren Tucker OpenSSH Email: dtucker@openssh.com Tatham & Tucker Standards Track [Page 4]