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rfc:rfc3491

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].
  1. The intended applicability of the profile: internationalized

domain names processed by IDNA.

  1. The character repertoire that is the input and output to

stringprep: Unicode 3.2, specified in section 2.

Hoffman & Blanchet Standards Track [Page 1] RFC 3491 IDN Nameprep March 2003

  1. The mappings used: specified in section 3.
  1. The Unicode normalization used: specified in section 4.
  1. The characters that are prohibited as output: specified in section

5.

  1. 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].

Hoffman & Blanchet Standards Track [Page 2] RFC 3491 IDN Nameprep March 2003

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.

Hoffman & Blanchet Standards Track [Page 3] RFC 3491 IDN Nameprep March 2003

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.

Hoffman & Blanchet Standards Track [Page 4] RFC 3491 IDN Nameprep March 2003

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

Hoffman & Blanchet Standards Track [Page 5] RFC 3491 IDN Nameprep March 2003

12. Authors' Addresses

 Paul Hoffman
 Internet Mail Consortium and VPN Consortium
 127 Segre Place
 Santa Cruz, CA  95060 USA
 EMail: paul.hoffman@imc.org and paul.hoffman@vpnc.org
 Marc Blanchet
 Viagenie inc.
 2875 boul. Laurier, bur. 300
 Ste-Foy, Quebec, Canada, G1V 2M2
 EMail: Marc.Blanchet@viagenie.qc.ca

Hoffman & Blanchet Standards Track [Page 6] RFC 3491 IDN Nameprep March 2003

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.

Hoffman & Blanchet Standards Track [Page 7]

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