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Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 3491.
Authors Paul E. Hoffman , Marc Blanchet
Last updated 2015-10-14 (Latest revision 2002-05-17)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 3491 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Erik Nordmark
IESG note
Send notices to <>
Internet Draft                                          Paul Hoffman
draft-ietf-idn-nameprep-10.txt                            IMC & VPNC
May 17, 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.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

To view the list Internet-Draft Shadow Directories, see


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 names 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 names processed by IDNA

- 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.

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

1.2 Terminology

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and
"MAY" in this document are to be interpreted as described in RFC 2119

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
used by applications, particularly in Asia. Thus, U+3002 is prohibited
in domain names by this specification.


6. 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.

7. References

7.1 Normative references

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

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

7.2 Informative references

[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.

8. 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

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.

9. IANA Considerations

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

10. 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
Erik Nordmark

11. 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