Network Working Group                                         P. Hoffman
Request for Comments: 3491                                    IMC & VPNC
Category: Standards Track                                    M. Blanchet
                                                               Viagenie
                                                             March 2003


                  Nameprep: A Stringprep Profile for
                 Internationalized Domain Names (IDN)

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

  This document describes how to prepare internationalized domain name
  (IDN) labels in order to increase the likelihood that name input and
  name comparison work in ways that make sense for typical users
  throughout the world.  This profile of the stringprep protocol is
  used as part of a suite of on-the-wire protocols for
  internationalizing the Domain Name System (DNS).

1. Introduction

  This document specifies processing rules that will allow users to
  enter internationalized domain names (IDNs) into applications and
  have the highest chance of getting the content of the strings
  correct.  It is a profile of stringprep [STRINGPREP].  These
  processing rules are only intended for internationalized domain
  names, not for arbitrary text.

  This profile defines the following, as required by [STRINGPREP].

  -  The intended applicability of the profile: internationalized
     domain names processed by IDNA.

  -  The character repertoire that is the input and output to
     stringprep:  Unicode 3.2, specified in section 2.




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  -  The mappings used: specified in section 3.

  -  The Unicode normalization used: specified in section 4.

  -  The characters that are prohibited as output: specified in section
     5.

  -  Bidirectional character handling: specified in section 6.

1.1 Interaction of protocol parts

  Nameprep is used by the IDNA [IDNA] protocol for preparing domain
  names; it is not designed for any other purpose.  It is explicitly
  not designed for processing arbitrary free text and SHOULD NOT be
  used for that purpose.  Nameprep is a profile of Stringprep
  [STRINGPREP].  Implementations of Nameprep MUST fully implement
  Stringprep.

  Nameprep is used to process domain name labels, not domain names.
  IDNA calls nameprep for each label in a domain name, not for the
  whole domain name.

1.2 Terminology

  The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
  in this document are to be interpreted as described in BCP 14, RFC
  2119 [RFC2119].

2. Character Repertoire

  This profile uses Unicode 3.2, as defined in [STRINGPREP] Appendix A.

3. Mapping

  This profile specifies mapping using the following tables from
  [STRINGPREP]:

  Table B.1
  Table B.2

4. Normalization

  This profile specifies using Unicode normalization form KC, as
  described in [STRINGPREP].







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5. Prohibited Output

  This profile specifies prohibiting using the following tables from
  [STRINGPREP]:

  Table C.1.2
  Table C.2.2
  Table C.3
  Table C.4
  Table C.5
  Table C.6
  Table C.7
  Table C.8
  Table C.9

  IMPORTANT NOTE: This profile MUST be used with the IDNA protocol.
  The IDNA protocol has additional prohibitions that are checked
  outside of this profile.

6. Bidirectional characters

  This profile specifies checking bidirectional strings as described in
  [STRINGPREP] section 6.

7. Unassigned Code Points in Internationalized Domain Names

  If the processing in [IDNA] specifies that a list of unassigned code
  points be used, the system uses table A.1 from [STRINGPREP] as its
  list of unassigned code points.

8. References

8.1 Normative References

  [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

  [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
               Internationalized Strings ("stringprep")", RFC 3454,
               December 2002.

  [IDNA]       Faltstrom, P., Hoffman, P. and A. Costello,
               "Internationalizing Domain Names in Applications
               (IDNA)", RFC 3490, March 2003.







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8.2 Informative references

  [STD13]      Mockapetris, P., "Domain names - concepts and
               facilities", STD 13, RFC 1034, and "Domain names -
               implementation and specification", STD 13, RFC 1035,
               November 1987.

9. Security Considerations

  The Unicode and ISO/IEC 10646 repertoires have many characters that
  look similar.  In many cases, users of security protocols might do
  visual matching, such as when comparing the names of trusted third
  parties.  Because it is impossible to map similar-looking characters
  without a great deal of context such as knowing the fonts used,
  stringprep does nothing to map similar-looking characters together
  nor to prohibit some characters because they look like others.

  Security on the Internet partly relies on the DNS.  Thus, any change
  to the characteristics of the DNS can change the security of much of
  the Internet.

  Domain names are used by users to connect to Internet servers.  The
  security of the Internet would be compromised if a user entering a
  single internationalized name could be connected to different servers
  based on different interpretations of the internationalized domain
  name.

  Current applications might assume that the characters allowed in
  domain names will always be the same as they are in [STD13].  This
  document vastly increases the number of characters available in
  domain names.  Every program that uses "special" characters in
  conjunction with domain names may be vulnerable to attack based on
  the new characters allowed by this specification.


















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10. IANA Considerations

  This is a profile of stringprep.  It has been registered by the IANA
  in the stringprep profile registry
  (www.iana.org/assignments/stringprep-profiles).

     Name of this profile:
        Nameprep

     RFC in which the profile is defined:
        This document.

     Indicator whether or not this is the newest version of the
     profile:
        This is the first version of Nameprep.

11. Acknowledgements

  Many people from the IETF IDN Working Group and the Unicode Technical
  Committee contributed ideas that went into this document.

  The IDN Nameprep design team made many useful changes to the
  document.  That team and its advisors include:

     Asmus Freytag
     Cathy Wissink
     Francois Yergeau
     James Seng
     Marc Blanchet
     Mark Davis
     Martin Duerst
     Patrik Faltstrom
     Paul Hoffman

  Additional significant improvements were proposed by:

     Jonathan Rosenne
     Kent Karlsson
     Scott Hollenbeck
     Dave Crocker
     Erik Nordmark
     Matitiahu Allouche









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12. Authors' Addresses

  Paul Hoffman
  Internet Mail Consortium and VPN Consortium
  127 Segre Place
  Santa Cruz, CA  95060 USA

  EMail: [email protected] and [email protected]


  Marc Blanchet
  Viagenie inc.
  2875 boul. Laurier, bur. 300
  Ste-Foy, Quebec, Canada, G1V 2M2

  EMail: [email protected]



































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13.  Full Copyright Statement

  Copyright (C) The Internet Society (2003).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS 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.

Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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