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