Internet Engineering Task Force (IETF) J. Klensin
Request for Comments: 5891 August 2010
Obsoletes: 3490, 3491
Category: Standards Track
Internationalized Domain Names in Applications (IDNA): Protocol
This document is the revised protocol definition for
Internationalized Domain Names (IDNs). The rationale for changes,
the relationship to the older specification, and important
terminology are provided in other documents. This document specifies
the protocol mechanism, called Internationalized Domain Names in
Applications (IDNA), for registering and looking up IDNs in a way
that does not require changes to the DNS itself. IDNA is only meant
for processing domain names, not free text.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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This document supplies the protocol definition for Internationalized
Domain Names in Applications (IDNA), with the version specified here
known as IDNA2008. Essential definitions and terminology for
understanding this document and a road map of the collection of
documents that make up IDNA2008 appear in a separate Definitions
document [RFC5890]. Appendix A discusses the relationship between
this specification and the earlier version of IDNA (referred to here
as "IDNA2003"). The rationale for these changes, along with
considerable explanatory material and advice to zone administrators
who support IDNs, is provided in another document, known informally
in this series as the "Rationale document" [RFC5894].
IDNA works by allowing applications to use certain ASCII [ASCII]
string labels (beginning with a special prefix) to represent
non-ASCII name labels. Lower-layer protocols need not be aware of
this; therefore, IDNA does not change any infrastructure. In
particular, IDNA does not depend on any changes to DNS servers,
resolvers, or DNS protocol elements, because the ASCII name service
provided by the existing DNS can be used for IDNA.
IDNA applies only to a specific subset of DNS labels. The base DNS
standards [RFC1034] [RFC1035] and their various updates specify how
to combine labels into fully-qualified domain names and parse labels
out of those names.
This document describes two separate protocols, one for IDN
registration (Section 4) and one for IDN lookup (Section 5). These
two protocols share some terminology, reference data, and operations.
As mentioned above, terminology used as part of the definition of
IDNA appears in the Definitions document [RFC5890]. It is worth
noting that some of this terminology overlaps with, and is consistent
with, that used in Unicode or other character set standards and the
DNS. Readers of this document are assumed to be familiar with the
associated Definitions document and with the DNS-specific terminology
in RFC 1034 [RFC1034].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
3. Requirements and Applicability
IDNA makes the following requirements:
1. Whenever a domain name is put into a domain name slot that is not
IDNA-aware (see Section 22.214.171.124 of the Definitions document
[RFC5890]), it MUST contain only ASCII characters (i.e., its
labels must be either A-labels or NR-LDH labels), unless the DNS
application is not subject to historical recommendations for
"hostname"-style names (see RFC 1034 [RFC1034] and
2. Labels MUST be compared using equivalent forms: either both
A-label forms or both U-label forms. Because A-labels and
U-labels can be transformed into each other without loss of
information, these comparisons are equivalent (however, in
practice, comparison of U-labels requires first verifying that
they actually are U-labels and not just Unicode strings). A pair
of A-labels MUST be compared as case-insensitive ASCII (as with
all comparisons of ASCII DNS labels). U-labels MUST be compared
as-is, without case folding or other intermediate steps. While
it is not necessary to validate labels in order to compare them,
successful comparison does not imply validity. In many cases,
not limited to comparison, validation may be important for other
reasons and SHOULD be performed.
3. Labels being registered MUST conform to the requirements of
Section 4. Labels being looked up and the lookup process MUST
conform to the requirements of Section 5.
IDNA applies to all domain names in all domain name slots in
protocols except where it is explicitly excluded. It does not apply
to domain name slots that do not use the LDH syntax rules as
described in the Definitions document [RFC5890].
Because it uses the DNS, IDNA applies to many protocols that were
specified before it was designed. IDNs occupying domain name slots
in those older protocols MUST be in A-label form until and unless
those protocols and their implementations are explicitly upgraded to
be aware of IDNs and to accept the U-label form. IDNs actually
appearing in DNS queries or responses MUST be A-labels.
IDNA-aware protocols and implementations MAY accept U-labels,
A-labels, or both as those particular protocols specify. IDNA is not
defined for extended label types (see RFC 2671 [RFC2671], Section 3).
3.2.1. DNS Resource Records
IDNA applies only to domain names in the NAME and RDATA fields of DNS
resource records whose CLASS is IN. See the DNS specification
[RFC1035] for precise definitions of these terms.
The application of IDNA to DNS resource records depends entirely on
the CLASS of the record, and not on the TYPE except as noted below.
This will remain true, even as new TYPEs are defined, unless a new
TYPE defines TYPE-specific rules. Special naming conventions for SRV
records (and "underscore labels" more generally) are incompatible
with IDNA coding as discussed in the Definitions document [RFC5890],
especially Section 126.96.36.199. Of course, underscore labels may be part
of a domain that uses IDN labels at higher levels in the tree.
3.2.2. Non-Domain-Name Data Types Stored in the DNS
Although IDNA enables the representation of non-ASCII characters in
domain names, that does not imply that IDNA enables the
representation of non-ASCII characters in other data types that are
stored in domain names, specifically in the RDATA field for types
that have structured RDATA format. For example, an email address
local part is stored in a domain name in the RNAME field as part of
the RDATA of an SOA record (e.g., email@example.com would be
represented as hostmaster.example.com). IDNA does not update the
existing email standards, which allow only ASCII characters in local
parts. Even though work is in progress to define
internationalization for email addresses [RFC4952], changes to the
email address part of the SOA RDATA would require action in, or
updates to, other standards, specifically those that specify the
format of the SOA RR.
4. Registration Protocol
This section defines the model for registering an IDN. The model is
implementation independent; any sequence of steps that produces
exactly the same result for all labels is considered a valid
Note that, while the registration (this section) and lookup protocols
(Section 5) are very similar in most respects, they are not
identical, and implementers should carefully follow the steps
described in this specification.
4.1. Input to IDNA Registration
Registration processes, especially processing by entities (often
called "registrars") who deal with registrants before the request
actually reaches the zone manager ("registry") are outside the scope
of this definition and may differ significantly depending on local
needs. By the time a string enters the IDNA registration process as
described in this specification, it MUST be in Unicode and in
Normalization Form C (NFC [Unicode-UAX15]). Entities responsible for
zone files ("registries") MUST accept only the exact string for which
registration is requested, free of any mappings or local adjustments.
They MAY accept that input in any of three forms:
1. As a pair of A-label and U-label.
2. As an A-label only.
3. As a U-label only.
The first two of these forms are RECOMMENDED because the use of
A-labels avoids any possibility of ambiguity. The first is normally
preferred over the second because it permits further verification of
user intent (see Section 4.2.1).
4.2. Permitted Character and Label Validation
4.2.1. Input Format
If both the U-label and A-label forms are available, the registry
MUST ensure that the A-label form is in lowercase, perform a
conversion to a U-label, perform the steps and tests described below
on that U-label, and then verify that the A-label produced by the
step in Section 4.4 matches the one provided as input. In addition,
the U-label that was provided as input and the one obtained by
conversion of the A-label MUST match exactly. If, for some reason,
these tests fail, the registration MUST be rejected.
If only an A-label was provided and the conversion to a U-label is
not performed, the registry MUST still verify that the A-label is
superficially valid, i.e., that it does not violate any of the rules
of Punycode encoding [RFC3492] such as the prohibition on trailing
hyphen-minus, the requirement that all characters be ASCII, and so
on. Strings that appear to be A-labels (e.g., they start with
"xn--") and strings that are supplied to the registry in a context
reserved for A-labels (such as a field in a form to be filled out),
but that are not valid A-labels as described in this paragraph, MUST
NOT be placed in DNS zones that support IDNA.
If only an A-label is provided, the conversion to a U-label is not
performed, but the superficial tests described in the previous
paragraph are performed, registration procedures MAY, and usually
will, bypass the tests and actions in the balance of Section 4.2 and
in Sections 4.3 and 4.4.
4.2.2. Rejection of Characters That Are Not Permitted
The candidate Unicode string MUST NOT contain characters that appear
in the "DISALLOWED" and "UNASSIGNED" lists specified in the Tables
4.2.3. Label Validation
The proposed label (in the form of a Unicode string, i.e., a string
that at least superficially appears to be a U-label) is then examined
using tests that require examination of more than one character.
Character order is considered to be the on-the-wire order. That
order may not be the same as the display order.
188.8.131.52. Hyphen Restrictions
The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
the third and fourth character positions and MUST NOT start or end
with a "-" (hyphen).
184.108.40.206. Leading Combining Marks
The Unicode string MUST NOT begin with a combining mark or combining
character (see The Unicode Standard, Section 2.11 [Unicode] for an
220.127.116.11. Contextual Rules
The Unicode string MUST NOT contain any characters whose validity is
context-dependent, unless the validity is positively confirmed by a
contextual rule. To check this, each code point identified as
CONTEXTJ or CONTEXTO in the Tables document [RFC5892] MUST have a
non-null rule. If such a code point is missing a rule, the label is
invalid. If the rule exists but the result of applying the rule is
negative or inconclusive, the proposed label is invalid.
18.104.22.168. Labels Containing Characters Written Right to Left
If the proposed label contains any characters from scripts that are
written from right to left, it MUST meet the Bidi criteria [RFC5893].
4.2.4. Registration Validation Requirements
Strings that contain at least one non-ASCII character, have been
produced by the steps above, whose contents pass all of the tests in
Section 4.2.3, and are 63 or fewer characters long in
ASCII-compatible encoding (ACE) form (see Section 4.4), are U-labels.
To summarize, tests are made in Section 4.2 for invalid characters,
invalid combinations of characters, for labels that are invalid even
if the characters they contain are valid individually, and for labels
that do not conform to the restrictions for strings containing
4.3. Registry Restrictions
In addition to the rules and tests above, there are many reasons why
a registry could reject a label. Registries at all levels of the
DNS, not just the top level, are expected to establish policies about
label registrations. Policies are likely to be informed by the local
languages and the scripts that are used to write them and may depend
on many factors including what characters are in the label (for
example, a label may be rejected based on other labels already
registered). See the Rationale document [RFC5894], Section 3.2, for
further discussion and recommendations about registry policies.
The string produced by the steps in Section 4.2 is checked and
processed as appropriate to local registry restrictions. Application
of those registry restrictions may result in the rejection of some
labels or the application of special restrictions to others.
4.4. Punycode Conversion
The resulting U-label is converted to an A-label (defined in Section
22.214.171.124 of the Definitions document [RFC5890]). The A-label is the
encoding of the U-label according to the Punycode algorithm [RFC3492]
with the ACE prefix "xn--" added at the beginning of the string. The
resulting string must, of course, conform to the length limits
imposed by the DNS. This document does not update or alter the
Punycode algorithm specified in RFC 3492 in any way. RFC 3492 does
make a non-normative reference to the information about the value and
construction of the ACE prefix that appears in RFC 3490 or Nameprep
[RFC3491]. For consistency and reader convenience, IDNA2008
effectively updates that reference to point to this document. That
change does not alter the prefix itself. The prefix, "xn--", is the
same in both sets of documents.
With the exception of the maximum string length test on Punycode
output, the failure conditions identified in the Punycode encoding
procedure cannot occur if the input is a U-label as determined by the
steps in Sections 4.1 through 4.3 above.
4.5. Insertion in the Zone
The label is registered in the DNS by inserting the A-label into a
5. Domain Name Lookup Protocol
Lookup is different from registration and different tests are applied
on the client. Although some validity checks are necessary to avoid
serious problems with the protocol, the lookup-side tests are more
permissive and rely on the assumption that names that are present in
the DNS are valid. That assumption is, however, a weak one because
the presence of wildcards in the DNS might cause a string that is not
actually registered in the DNS to be successfully looked up.
5.1. Label String Input
The user supplies a string in the local character set, for example,
by typing it, clicking on it, or copying and pasting it from a
resource identifier, e.g., a Uniform Resource Identifier (URI)
[RFC3986] or an Internationalized Resource Identifier (IRI)
[RFC3987], from which the domain name is extracted. Alternately,
some process not directly involving the user may read the string from
a file or obtain it in some other way. Processing in this step and
the one specified in Section 5.2 are local matters, to be
accomplished prior to actual invocation of IDNA.
5.2. Conversion to Unicode
The string is converted from the local character set into Unicode, if
it is not already in Unicode. Depending on local needs, this
conversion may involve mapping some characters into other characters
as well as coding conversions. Those issues are discussed in the
mapping-related sections (Sections 4.2, 4.4, 6, and 7.3) of the
Rationale document [RFC5894] and in the separate Mapping document
[IDNA2008-Mapping]. The result MUST be a Unicode string in NFC form.
5.3. A-label Input
If the input to this procedure appears to be an A-label (i.e., it
starts in "xn--", interpreted case-insensitively), the lookup
application MAY attempt to convert it to a U-label, first ensuring
that the A-label is entirely in lowercase (converting it to lowercase
if necessary), and apply the tests of Section 5.4 and the conversion
of Section 5.5 to that form. If the label is converted to Unicode
(i.e., to U-label form) using the Punycode decoding algorithm, then
the processing specified in those two sections MUST be performed, and
the label MUST be rejected if the resulting label is not identical to
the original. See Section 8.1 of the Rationale document [RFC5894]
for additional discussion on this topic.
Conversion from the A-label and testing that the result is a U-label
SHOULD be performed if the domain name will later be presented to the
user in native character form (this requires that the lookup
application be IDNA-aware). If those steps are not performed, the
lookup process SHOULD at least test to determine that the string is
actually an A-label, examining it for the invalid formats specified
in the Punycode decoding specification. Applications that are not
IDNA-aware will obviously omit that testing; others MAY treat the
string as opaque to avoid the additional processing at the expense of
providing less protection and information to users.
5.4. Validation and Character List Testing
As with the registration procedure described in Section 4, the
Unicode string is checked to verify that all characters that appear
in it are valid as input to IDNA lookup processing. As discussed
above and in the Rationale document [RFC5894], the lookup check is
more liberal than the registration one. Labels that have not been
fully evaluated for conformance to the applicable rules are referred
to as "putative" labels as discussed in Section 126.96.36.199 of the
Definitions document [RFC5890]. Putative U-labels with any of the
following characteristics MUST be rejected prior to DNS lookup:
o Labels that are not in NFC [Unicode-UAX15].
o Labels containing "--" (two consecutive hyphens) in the third and
fourth character positions.
o Labels whose first character is a combining mark (see The Unicode
Standard, Section 2.11 [Unicode]).
o Labels containing prohibited code points, i.e., those that are
assigned to the "DISALLOWED" category of the Tables document
o Labels containing code points that are identified in the Tables
document as "CONTEXTJ", i.e., requiring exceptional contextual
rule processing on lookup, but that do not conform to those rules.
Note that this implies that a rule must be defined, not null: a
character that requires a contextual rule but for which the rule
is null is treated in this step as having failed to conform to the
o Labels containing code points that are identified in the Tables
document as "CONTEXTO", but for which no such rule appears in the
table of rules. Applications resolving DNS names or carrying out
equivalent operations are not required to test contextual rules
for "CONTEXTO" characters, only to verify that a rule is defined
(although they MAY make such tests to provide better protection or
give better information to the user).
o Labels containing code points that are unassigned in the version
of Unicode being used by the application, i.e., in the UNASSIGNED
category of the Tables document.
This requirement means that the application must use a list of
unassigned characters that is matched to the version of Unicode
that is being used for the other requirements in this section. It
is not required that the application know which version of Unicode
is being used; that information might be part of the operating
environment in which the application is running.
In addition, the application SHOULD apply the following test.
o Verification that the string is compliant with the requirements
for right-to-left characters specified in the Bidi document
This test may be omitted in special circumstances, such as when the
lookup application knows that the conditions are enforced elsewhere,
because an attempt to look up and resolve such strings will almost
certainly lead to a DNS lookup failure except when wildcards are
present in the zone. However, applying the test is likely to give
much better information about the reason for a lookup failure --
information that may be usefully passed to the user when that is
feasible -- than DNS resolution failure information alone.
For all other strings, the lookup application MUST rely on the
presence or absence of labels in the DNS to determine the validity of
those labels and the validity of the characters they contain. If
they are registered, they are presumed to be valid; if they are not,
their possible validity is not relevant. While a lookup application
may reasonably issue warnings about strings it believes may be
problematic, applications that decline to process a string that
conforms to the rules above (i.e., does not look it up in the DNS)
are not in conformance with this protocol.
5.5. Punycode Conversion
The string that has now been validated for lookup is converted to ACE
form by applying the Punycode algorithm to the string and then adding
the ACE prefix ("xn--").
5.6. DNS Name Resolution
The A-label resulting from the conversion in Section 5.5 or supplied
directly (see Section 5.3) is combined with other labels as needed to
form a fully-qualified domain name that is then looked up in the DNS,
using normal DNS resolver procedures. The lookup can obviously
either succeed (returning information) or fail.
6. Security Considerations
Security Considerations for this version of IDNA are described in the
Definitions document [RFC5890], except for the special issues
associated with right-to-left scripts and characters. The latter are
discussed in the Bidi document [RFC5893].
In order to avoid intentional or accidental attacks from labels that
might be confused with others, special problems in rendering, and so
on, the IDNA model requires that registries exercise care and
thoughtfulness about what labels they choose to permit. That issue
is discussed in Section 4.3 of this document which, in turn, points
to a somewhat more extensive discussion in the Rationale document
7. IANA Considerations
IANA actions for this version of IDNA are specified in the Tables
document [RFC5892] and discussed informally in the Rationale document
[RFC5894]. The components of IDNA described in this document do not
require any IANA actions.
While the listed editor held the pen, the original versions of this
document represent the joint work and conclusions of an ad hoc design
team consisting of the editor and, in alphabetic order, Harald
Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. This document
draws significantly on the original version of IDNA [RFC3490] both
conceptually and for specific text. This second-generation version
would not have been possible without the work that went into that
first version and especially the contributions of its authors Patrik
Faltstrom, Paul Hoffman, and Adam Costello. While Faltstrom was
actively involved in the creation of this version, Hoffman and
Costello were not and should not be held responsible for any errors
This revision to IDNA would have been impossible without the
accumulated experience since RFC 3490 was published and resulting
comments and complaints of many people in the IETF, ICANN, and other
communities (too many people to list here). Nor would it have been
possible without RFC 3490 itself and the efforts of the Working Group
that defined it. Those people whose contributions are acknowledged
in RFC 3490, RFC 4690 [RFC4690], and the Rationale document [RFC5894]
were particularly important.
Specific textual changes were incorporated into this document after
suggestions from the other contributors, Stephane Bortzmeyer, Vint
Cerf, Lisa Dusseault, Paul Hoffman, Kent Karlsson, James Mitchell,
Erik van der Poel, Marcos Sanz, Andrew Sullivan, Wil Tan, Ken
Whistler, Chris Wright, and other WG participants and reviewers
including Martin Duerst, James Mitchell, Subramanian Moonesamy, Peter
Saint-Andre, Margaret Wasserman, and Dan Winship who caught specific
errors and recommended corrections. Special thanks are due to Paul
Hoffman for permission to extract material to form the basis for
Appendix A from a draft document that he prepared.
10.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
for Internationalized Domain Names for Applications
(IDNA)", RFC 5893, August 2010.
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", September 2009,
10.2. Informative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968. ANSI
X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
Resnick, P. and P. Hoffman, "Mapping Characters in
Internationalized Domain Names for Applications (IDNA)",
Work in Progress, April 2010.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
and Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
5.0", 2007. Boston, MA, USA: Addison-Wesley. ISBN
0-321-48091-0. This printed reference has now been
updated online to reflect additional code points. For
code points, the reference at the time this document was
published is to Unicode 5.2.
Appendix A. Summary of Major Changes from IDNA2003
1. Update base character set from Unicode 3.2 to Unicode version
2. Separate the definitions for the "registration" and "lookup"
3. Disallow symbol and punctuation characters except where special
exceptions are necessary.
4. Remove the mapping and normalization steps from the protocol and
have them, instead, done by the applications themselves,
possibly in a local fashion, before invoking the protocol.
5. Change the way that the protocol specifies which characters are
allowed in labels from "humans decide what the table of code
points contains" to "decision about code points are based on
Unicode properties plus a small exclusion list created by
6. Introduce the new concept of characters that can be used only in
7. Allow typical words and names in languages such as Dhivehi and
Yiddish to be expressed.
8. Make bidirectional domain names (delimited strings of labels,
not just labels standing on their own) display in a less
surprising fashion, whether they appear in obvious domain name
contexts or as part of running text in paragraphs.
9. Remove the dot separator from the mandatory part of the
10. Make some currently valid labels that are not actually IDNA
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
Phone: +1 617 245 1457