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.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

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.

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/>.

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