Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)
RFC 3491

Document Type RFC - Proposed Standard (March 2003; No errata)
Obsoleted by RFC 5891
Last updated 2015-10-14
Stream IETF
Formats plain text pdf html bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state RFC 3491 (Proposed Standard)
Consensus Boilerplate Unknown
Telechat date
Responsible AD Erik Nordmark
IESG note Updated drafts are available. Need to be reviewed to see
if the IESG comments have been addressed.
Send notices to <>
Network Working Group                                         P. Hoffman
Request for Comments: 3491                                    IMC & VPNC
Category: Standards Track                                    M. Blanchet
                                                              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.


   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

   -  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

   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

   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

   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]
Show full document text