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Network Working Group                                         P. HoffmanRequest for Comments: 3491                                    IMC & VPNCCategory: Standards Track                                    M. Blanchet                                                                Viagenie                                                              March 2003Nameprep: A Stringprep Profile forInternationalized 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 insection 2.Hoffman & Blanchet          Standards Track                     [Page 1]

RFC 3491                      IDN Nameprep                    March 2003   -  The mappings used: specified insection 3.   -  The Unicode normalization used: specified insection 4.   -  The characters that are prohibited as output: specified insection5.   -  Bidirectional character handling: specified insection 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 inBCP 14,RFC2119 [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.24. Normalization   This profile specifies using Unicode normalization form KC, as   described in [STRINGPREP].Hoffman & Blanchet          Standards Track                     [Page 2]

RFC 3491                      IDN Nameprep                    March 20035. 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. References8.1 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 20038.2 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 200310. 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 AlloucheHoffman & Blanchet          Standards Track                     [Page 5]

RFC 3491                      IDN Nameprep                    March 200312. 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.caHoffman & Blanchet          Standards Track                     [Page 6]

RFC 3491                      IDN Nameprep                    March 200313.  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]