Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)
The rulebook for cleaning up internationalized domain name labels so look-alike inputs compare as equal.
Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, DOI 10.17487/RFC3491, March 2003, <https://www.rfc-editor.org/info/rfc3491>.
@misc{rfc3491,
series = {Request for Comments},
number = 3491,
howpublished = {RFC 3491},
publisher = {RFC Editor},
doi = {10.17487/RFC3491},
url = {https://www.rfc-editor.org/info/rfc3491},
author = {P. Hoffman and M. Blanchet},
title = {{Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)}},
pagetotal = 7,
year = 2003,
month = mar,
} RFC text
Contents
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.
- 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].
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.
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.
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
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
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
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followed, or as required to translate it into languages other than
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
ISO Plain Language
On this page
§1What Nameprep Is For
The RFC describes how to prepare internationalized domain name labels so that name input and name comparison work in ways that make sense for typical users throughout the world. It is a profile of the stringprep protocol, used as part of a suite of on-the-wire protocols for internationalizing the Domain Name System.
The processing rules are intended only for internationalized domain names, not for arbitrary text. The profile fixes the things stringprep requires: the intended applicability of the profile, which is internationalized domain names processed by IDNA; the character repertoire that is the input and output to stringprep, which is Unicode 3.2; the mappings used; the Unicode normalization used; the characters prohibited as output; and the handling of bidirectional characters.
The rules apply to one job only: making a domain name label written in a non-English script behave predictably. They are not a general-purpose text cleaner.
§1.1How Nameprep Fits with IDNA and Stringprep
Nameprep is used by the IDNA protocol for preparing domain names. It is not designed for any other purpose, and it is explicitly not designed for processing arbitrary free text and should not be used for that purpose. 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.
The RFC marks one point as important: this profile must be used with the IDNA protocol, because the IDNA protocol has additional prohibitions that are checked outside of this profile. So nameprep does the label-level work, and IDNA applies its own extra rules on top.
Nameprep is one step inside a larger scheme. It cleans up each label of a domain name separately, and the scheme that uses it applies further checks of its own.
§2Character Repertoire and Mapping
The profile uses Unicode 3.2, as defined in the stringprep specification's Appendix A. Unicode 3.2 is the set of characters that can go in and come out of the processing; anything outside it is out of scope for these rules.
The profile specifies mapping using table B.1 and table B.2 from stringprep. These are the tables that say which characters are mapped to other characters, and which are left alone. Mapping is the first of the steps, and everything after it operates on the mapped result.
The set of mappings is the same for every domain name in the profile, so two different domains are compared after the same substitutions have been applied to both.
Only characters from one version of Unicode are allowed, and a fixed table says which of them get rewritten to another character before anything else happens.
§4Normalization, Prohibited Output and Bidirectional Text
The profile specifies using Unicode normalization form KC, as described in stringprep. Normalization puts a string into one canonical form so that two spellings that differ only in how the same characters are combined end up byte-for-byte identical.
The profile specifies prohibiting the following stringprep tables as output: C.1.2, C.2.2, C.3, C.4, C.5, C.6, C.7, C.8 and C.9. These are the characters that must not survive into the prepared string, because they have no place in a domain name label or would let two different labels look the same.
The profile also specifies checking bidirectional strings as described in section 6 of stringprep. This is the check that keeps text written right to left, such as Arabic or Hebrew, in a well-formed shape after mapping and normalization have run.
Finally, the RFC addresses unassigned code points. 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.
After the substitutions, the string is put into one standard form, checked against a list of characters that must not appear, and checked for right-to-left text. Code points that Unicode has not assigned are handled from a fixed table.
character repertoire : Unicode 3.2
mapping : stringprep tables B.1, B.2
normalization : Unicode form KC
prohibited output : stringprep tables C.1.2, C.2.2,
C.3, C.4, C.5, C.6, C.7, C.8, C.9
bidirectional check : stringprep section 6
unassigned code pts : stringprep table A.1§10Registration of the Profile
The RFC records that this is a profile of stringprep and that it has been registered by IANA in the stringprep profile registry. The registration names the profile Nameprep, identifies the RFC in which the profile is defined as the document itself, and records that this is the first version of Nameprep.
The profile has a name in the IANA registry, and the registry records that this document is the first version of it.
§9Security Considerations
The RFC starts from a fact about Unicode and ISO/IEC 10646: they contain many characters that look similar. 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 does it prohibit some characters because they look like others.
Security on the Internet partly relies on the DNS, so 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, and 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 that name.
The RFC also warns about scale and about old assumptions. 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.
Two different scripts can contain characters that look identical on screen, and this RFC cannot tell them apart. So a name that looks the same to a person may not be the same name to a resolver, which is a risk for anyone comparing them by eye.