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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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 [RFC 2119].
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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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
[RFC 2119] 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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RFC 3491 IDN Nameprep March 2003
8.2 Informative references
[STD 13] 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 [STD 13]. 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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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
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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
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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
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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
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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.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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