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Internet Engineering Task Force (IETF)                         S. TathamRequest for Comments: 8160                                         PuTTYCategory: Standards Track                                      D. TuckerISSN: 2070-1721                                                  OpenSSH                                                              April 2017IUTF8 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8160.Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Tatham & Tucker              Standards Track                    [Page 1]

RFC 8160               IUTF8 Terminal Mode in SSH             April 2017Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .23.  New IUTF8 Opcode for Terminal Mode Encoding . . . . . . . . .24.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . .36.  References  . . . . . . . . . . . . . . . . . . . . . . . . .36.1.  Normative References  . . . . . . . . . . . . . . . . . .36.2.  Informative References  . . . . . . . . . . . . . . . . .3   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .4   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .41.  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 inSection 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 20174.  IANA Considerations   This document augments the list of "Pseudo-Terminal Encoded Terminal   Modes" defined inSection 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.  References6.1.  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 2017Acknowledgements   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.comTatham & Tucker              Standards Track                    [Page 4]