Internet Draft                                          Paul Hoffman
draft-ietf-idn-nameprep-09.txt                            IMC & VPNC
April 30, 2002                                         Marc Blanchet
Expires in six months                                       ViaGenie

   Nameprep: A Stringprep Profile for Internationalized Domain Names

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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This document describes how to prepare internationalized domain name
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 DNS.

1. Introduction

This document specifies processing rules that will allow users to enter
internationalized domain name labels in 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 name labels

- The character repertoire that is the input and output to stringprep:
Unicode 3.1, specified in Section 2

- 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

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.

1.2 Terminology

The key words "MUST", "SHOULD", and "MAY" in this document are to be
interpreted as described in RFC 2119 [RFC2119].

2. Character Repertoire

This profile uses Unicode 3.1, as defined in [STRINGPREP] Appendix

3. Mapping

This profile specifies mapping using the following tables from
Table B.1
Table B.2

4. Normalization

This profile specifies using Unicode normalization form KC, as described

5. Prohibited Output

This profile specifies prohibiting using the following tables from
Table C.1
Table C.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.

In addition, this profile adds the prohibitions. Thus, the full set of
prohibited characters are those from the tables above plus those listed
individually below.

5.1 Inappropriate characters from common input mechanisms

U+3002 is used as if it were U+002E in many domain name input mechanisms,
particularly in Asia. This prohibition allows input mechanisms to safely
map U+3002 to U+002E before doing stringprep without worrying about
preventing users from accessing legitimate domain name labels.


6. Unassigned Code Points in Internationalized Domain Names

This profile allows unassigned code points in DNS requests but not
in stored domain names. It uses [STRINGPREP] table A.1 as its list
of unassigned code points.

7. 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.
This profile 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

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.

8. References

[IDNA] Patrik Faltstrom, Paul Hoffman, and Adam M. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
draft-ietf-idn-idna, work-in-progress.

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

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

[STRINGPREP] Paul Hoffman and Marc Blanchet, "Preparation of
Internationalized Strings ("stringprep")", draft-hoffman-stringprep,
work in progress.

A. Acknowledgements

Many people from the IETF IDN Working Group and the Unicode Technical
Committee contributed ideas that went into the first draft of this

The IDN namprep design team made many useful changes to the first
draft. 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

B. IANA Considerations

This is a profile of stringprep. When it becomes an RFC, it should be
registered in the stringprep profile registry.

C. Author Contact Information

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

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