Network Working Group                                         J. Klensin
Internet-Draft                                          December 7, 2008
Obsoletes: 3490, 3491
(if approved)
Updates: 3492 (if approved)
Intended status: Standards Track
Expires: June 10, 2009

    Internationalized Domain Names in Applications (IDNA): Protocol

Status of this Memo

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   This document supplies the protocol definition for a revised and
   updated specification for internationalized domain names (IDNs).  The
   rationale for these changes, the relationship to the older
   specification, and important terminology are provided in other
   documents.  This document specifies the protocol mechanism, called
   Internationalizing 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements and Applicability . . . . . . . . . . . . . . . .  5
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  DNS Resource Records . . . . . . . . . . . . . . . . .  6
       3.2.2.  Non-domain-name Data Types Stored in the DNS . . . . .  6
   4.  Registration Protocol  . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Proposed label . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Conversion to Unicode and Normalization  . . . . . . . . .  7
     4.3.  Permitted Character and Label Validation . . . . . . . . .  7
       4.3.1.  Input Format . . . . . . . . . . . . . . . . . . . . .  7
       4.3.2.  Rejection of Characters that are not Permitted . . . .  8
       4.3.3.  Label Validation . . . . . . . . . . . . . . . . . . .  8
       4.3.4.  Registration Validation Summary  . . . . . . . . . . .  9
     4.4.  Registry Restrictions  . . . . . . . . . . . . . . . . . .  9
     4.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 10
     4.6.  Insertion in the Zone  . . . . . . . . . . . . . . . . . . 10
   5.  Domain Name Lookup Protocol  . . . . . . . . . . . . . . . . . 10
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . . 11
     5.3.  Character Changes in Preprocessing or the User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.4.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 12
     5.5.  Validation and Character List Testing  . . . . . . . . . . 12
     5.6.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 14
     5.7.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 14
   6.  Name Server Considerations . . . . . . . . . . . . . . . . . . 14
     6.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 14
     6.2.  DNSSEC Authentication of IDN Domain Names  . . . . . . . . 15
     6.3.  Root and other DNS Server Considerations . . . . . . . . . 15
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 16
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Summary of Major Changes from IDNA2003  . . . . . . . 19
   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 20
     B.1.  Changes between Version -00 and -01 of
           draft-ietf-idnabis-protocol  . . . . . . . . . . . . . . . 20
     B.2.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 20
     B.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 21
     B.4.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 21

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     B.5.  Version -05  . . . . . . . . . . . . . . . . . . . . . . . 21
     B.6.  Version -06  . . . . . . . . . . . . . . . . . . . . . . . 21
     B.7.  Version -07  . . . . . . . . . . . . . . . . . . . . . . . 22
     B.8.  Version -08  . . . . . . . . . . . . . . . . . . . . . . . 22
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23

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1.  Introduction

   This document supplies the protocol definition for a revised and
   updated specification for internationalized domain names.  Essential
   definitions and terminology for understanding this document and a
   road map of the collection of documents that make up IDNA2008 appear
   in [IDNA2008-Defs].  Appendix A discusses the relationship between
   this specification and the earlier version of IDNA (referred to here
   as "IDNA2003") and the rationale for these changes, along with
   considerable explanatory material and advice to zone administrators
   who support IDNs is provided in another documents, notably

   IDNA works by allowing applications to use certain 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 depend on changes to any infrastructure.  In
   particular, IDNA does not depend on any changes to DNS servers,
   resolvers, or protocol elements, because the ASCII name service
   provided by the existing DNS is entirely sufficient for IDNA.

   IDNA is applied only to DNS labels.  Standards for combining labels
   into fully-qualified domain names and parsing labels out of those
   names are covered in the base DNS standards [RFC1034] [RFC1035] and
   their various updates.  An application may, of course, apply locally-
   appropriate conventions to the presentation forms of domain names as
   discussed in [IDNA2008-Rationale].

   While they share terminology, reference data, and some operations,
   this document describes two separate protocols, one for IDN
   registration (Section 4) and one for IDN lookup (Section 5).

1.1.  Discussion Forum

   [[anchor3: RFC Editor: please remove this section.]]

   This work is being discussed in the IETF IDNABIS WG and on the
   mailing list

2.  Terminology

   General terminology applicable to IDNA, but with meanings familiar to
   those who have worked with Unicode or other character set standards
   and the DNS, appears in [IDNA2008-Defs].  Terminology that is an
   integral, normative, part of the IDNA definition, including the
   definitions of "ACE", appears in that document as well.  Familiarity
   with the terminology materials in that document is assumed for

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   reading this one.  The reader of this document is assumed to be
   familiar with DNS-specific terminology as defined in RFC 1034

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

3.  Requirements and Applicability

3.1.  Requirements

   IDNA conformance means adherence to the following requirements:

   1.  Whenever a domain name is put into an IDN-unaware domain name
       slot (see Section 2 and [IDNA2008-Defs]), it MUST contain only
       ASCII characters (i.e., must be either an A-label or an LDH-
       label), or must be a label associated with a DNS application that
       is not subject to either IDNA or the historical recommendations
       for "hostname"-style names [RFC1034].

   2.  Comparison of labels MUST be done on 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, when a
       pair of putative A-labels are compared, the comparison MUST use
       an ASCII case-insensitive comparison (as with all comparisons of
       ASCII DNS labels).  Comparisons on putative U-labels must test
       that the two strings are identical, without case-folding or other
       intermediate steps.  Note that it is not necessary to verify that
       labels are valid in order to compare them.  In many cases,
       verification of validity (that the strings actually are A-labels
       or U-labels) may be important for other reasons and SHOULD be

   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.

3.2.  Applicability

   IDNA is applicable to all domain names in all domain name slots
   except where it is explicitly excluded.  It is not applicable to
   domain name slots which do not use the LDH syntax rules.

   This implies that IDNA is applicable to many protocols that predate

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   IDNA.  Note that IDNs occupying domain name slots in those older
   protocols MUST be in A-label form until and unless those protocols
   and implementations of them are upgraded to be IDN-aware.  IDNs
   actually appearing in DNS queries or responses MUST be A-labels.

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.

   There are currently no other exclusions on the applicability of IDNA
   to DNS resource records.  Applicability depends entirely on the
   CLASS, and not on the TYPE except as noted below.  This will remain
   true, even as new types are defined, unless there is a compelling
   reason for a new type that requires type-specific rules.  The special
   naming conventions applicable to SRV records are examples of type-
   specific rules that are incompatible with IDNA coding.  Hence the
   first two labels (the ones required to start in "_") on a record with
   TYPE SRV MUST NOT be A-labels or U-labels (while it would be possible
   to write a non-ASCII string with a leading underscore, conversion to
   an A-label would be impossible without loss of information because
   the underscore is not a letter, digit, or hyphen and is consequently
   DISALLOWED in IDNs).  Of course, those 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 ( would be
   represented as  IDNA specifically 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 procedure for registering an IDN.  The
   procedure is implementation independent; any sequence of steps that
   produces exactly the same result for all labels is considered a valid

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   Note that, while the registration and lookup protocols (Section 5)
   are very similar in most respects, they are different and
   implementers should carefully follow the steps they are implementing.

4.1.  Proposed label

   The registrant submits a request for an IDN.  The user typically
   produces the request string by the keyboard entry of a character
   sequence in the local native character set (which might, of course,
   be Unicode).

4.2.  Conversion to Unicode and Normalization

   Some system routine, or a localized front-end to the IDNA process,
   ensures that the proposed label is a Unicode string or converts it to
   one as appropriate.  Independent of its source form, the string MUST
   be in Unicode Normalization Form C (NFC [Unicode-UAX15]) before
   further processing in this protocol.

   As a local implementation choice, the implementation MAY choose to
   map some forbidden characters to permitted characters (for instance
   mapping uppercase characters to lowercase ones), displaying the
   result to the user, and allowing processing to continue.  This should
   be done very conservatively to prevent interoperability problems with
   lookup applications that do not follow exactly the same rules.  In
   particular, it is strongly recommended that, to avoid any possible
   ambiguity, entities responsible for zone files ("registries") accept
   registrations only for A-labels (to be converted to U-labels by the
   registry as discussed above) or U-labels actually produced from
   A-labels, not forms expected to be converted by some other process.

4.3.  Permitted Character and Label Validation

4.3.1.  Input Format

   [[anchor8: Note in -07 -- this section was formerly the second
   paragraph of Section 4.1.  It may need additional work; suggestions

   The registry MAY permit submission of labels in A-label form.  If it
   does so, it MUST perform a conversion to a U-label, perform the steps
   and tests described below, and verify that the A-label produced by
   the step in Section 4.5 matches the one provided as input.  If, for
   some reason, it does not, the registration MUST be rejected.  If the
   conversion to a U-label is not performed, the registry MUST verify
   that the A-label is superficially valid, i.e., that it does not

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   violate any of the rules of Punycode [RFC3492] encoding such as the
   prohibition on trailing hyphen-minus, appearance of non-basic
   characters before the delimiter, and so on.  Invalid strings that
   appear to be A-labels MUST NOT be placed in DNS zones.

4.3.2.  Rejection of Characters that are not Permitted

   The candidate Unicode string is checked to verify that characters
   that IDNA does not permit do not appear in it.  Those characters are
   identified in the "DISALLOWED" and "UNASSIGNED" lists that are
   specified in [IDNA2008-Tables] and described informally in
   [IDNA2008-Rationale].  Characters that are either DISALLOWED or
   UNASSIGNED MUST NOT be part of labels to be processed for
   registration in the DNS.

4.3.3.  Label Validation

   The proposed label (in the form of a Unicode string, i.e., a putative
   U-label) is then examined, performing tests that require examination
   of more than one character.  Rejection of Hyphen Sequences in U-labels

   The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
   the third and fourth character positions when the label is considered
   in "on the wire" order.  Leading Combining Marks

   The first character of the string (when the label is considered in
   "on the wire" order) is examined to verify that it is not a combining
   mark (or combining character) (see The Unicode Standard, Section 2.11
   [Unicode] for an exact definition).  If it is a combining mark, the
   string MUST NOT be registered.  Contextual Rules

   Each code point is checked for its identification as a character
   requiring contextual processing for registration (the list of
   characters appears as the combination of CONTEXTJ and CONTEXTO in
   [IDNA2008-Tables] as do the contextual rules themselves).  If that
   indication appears, the table of contextual rules is checked for a
   rule for that character.  If no rule is found, the proposed label is
   rejected and MUST NOT be installed in a zone file.  If one is found,
   it is applied (typically as a test on the entire label or on adjacent
   characters within the label).  If the application of the rule does
   not conclude that the character is valid in context, the proposed
   label MUST BE rejected.  (See the IANA Considerations: IDNA Context

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   Registry section of [IDNA2008-Tables].)

   These contextual rules are required to support the use of characters
   that could be used, under other conditions, to produce misleading
   labels or to cause unacceptable ambiguity in label matching and
   interpretation.  For example, labels containing invisible ("zero-
   width") characters may be permitted in context with characters whose
   presentation forms are significantly changed by the presence or
   absence of the zero-width characters, while other labels in which
   zero-width characters appear may be rejected.  Labels Containing Characters Written Right to Left

   Special tests are required for strings containing characters that are
   normally written from right to left.  The criteria for classifying
   characters in terms of directionality are identified in the "Bidi"
   document [IDNA2008-BIDI] in this series.  That document also
   describes conditions for strings that contain one or more of those
   characters to be U-labels.  The tests for those conditions, specified
   there, are applied.  Strings that contain right to left characters
   that do not conform to the IDNA Bidi rules MUST NOT be inserted as
   labels in zone files.

4.3.4.  Registration Validation Summary

   Strings that contain at least one non-ASCII character, have been
   produced by the steps above, whose contents pass the above tests, and
   are 63 or fewer characters long in ACE form (see Section 4.5), are

   To summarize, tests are made in Section 4.3 for invalid characters,
   invalid combinations of characters, and for labels that are invalid
   even if the characters they contain are valid individually.

4.4.  Registry Restrictions

   Registries at all levels of the DNS, not just the top level, are
   expected to establish policies about the labels that may be
   registered, and for the processes associated with that action.  While
   exact policies are not specified as part of IDNA2008 and it is
   expected that different registries may specify different policies,
   there SHOULD be policies.  Even a trivial policy (e.g., "anything can
   be registered in this zone that can be represented as an A-label -
   U-label pair") has value because it provides notice to users and
   applications implementers that the registry cannot be relied upon to
   provide even minimal user-protection restrictions.  These per-
   registry policies and restrictions are an essential element of the
   IDNA registration protocol even for registries (and corresponding

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   zone files) deep in the DNS hierarchy.  As discussed in
   [IDNA2008-Rationale], such restrictions have always existed in the
   DNS.  That document also contains a discussion and recommendations
   about possible types of rules.

   The string produced by the above steps 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.

4.5.  Punycode Conversion

   The resulting U-label is converted to an A-label.  The A-label, more
   precisely defined elsewhere, 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 much, of
   course, conform to the length limits imposed by the DNS.  This
   document updates RFC 3492 only to the extent of replacing the
   reference to the discussion of the ACE prefix.  The ACE prefix is now
   specified in this document rather than as part of RFC 3490 or
   Nameprep [RFC3491] but is the same in both sets of documents.

   The failure conditions identified in the Punycode encoding procedure
   cannot occur if the input is a U-label as determined by the steps

4.6.  Insertion in the Zone

   The A-label is registered in the DNS by insertion into a zone.

5.  Domain Name Lookup Protocol

   Lookup is conceptually different from registration and different
   tests are applied on the client.  Although some validity checks are
   necessary to avoid serious problems with the protocol (see
   Section 5.5ff.), 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 wild
   cards 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, typically by
   typing it or clicking on, or copying and pasting, a resource
   identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the
   domain name is extracted.  Alternately, some process not directly

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   involving the user may read the string from a file or obtain it in
   some other way.  Processing in this step and the next two are local
   matters, to be accomplished prior to actual invocation of IDNA, but
   at least the two steps in Section 5.2 and Section 5.3 must be
   accomplished in some way.

5.2.  Conversion to Unicode

   The string is converted from the local character set into Unicode, if
   it is not already Unicode.  The exact nature of this conversion is
   beyond the scope of this document, but may involve normalization as
   described in Section 4.2.  The result MUST be a Unicode string in NFC

5.3.  Character Changes in Preprocessing or the User Interface

   The Unicode string MAY then be processed to prevent confounding of
   user expectations.  For instance, it might be reasonable, at this
   step, to convert all upper case characters to lower case, if this
   makes sense in the user's environment, but even this should be
   approached with caution due to some edge cases: in the long term, it
   is probably better for users to understand IDNs strictly in lower-
   case, U-label, form.  More generally, preprocessing may be useful to
   smooth the transition from IDNA2003, especially for direct user
   input, but with similar cautions.  In general, IDNs appearing in
   files and those transmitted across the network as part of protocols
   are expected to be in either ASCII form (including A-labels) or to
   contain U-labels, rather than being in forms requiring mapping or
   other conversions.

   Other examples of processing for localization might be applied,
   especially to direct user input, at this point.  They include
   interpreting various characters as separating domain name components
   from each other (label separators) because they either look like
   periods or are used to separate sentences, mapping halfwidth or
   fullwidth East Asian characters to the common form permitted in
   labels, or giving special treatment to characters whose presentation
   forms are dependent only on placement in the label.  Such
   localization changes are also outside the scope of this

   Recommendations for preprocessing for global contexts (i.e., when
   local considerations do not apply or cannot be used) and for maximum
   interoperability with labels that might have been specified under
   liberal readings of IDNA2003 are given in [IDNA2008-Rationale].  It
   is important to note that the intent of these specifications is that
   labels in application protocols, files, or links are intended to be
   in U-label or A-label form.  Preprocessing MUST NOT map a character

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   that is valid in a label as specified elsewhere in this document or
   in [IDNA2008-Tables] into another character.  Excessively liberal use
   of preprocessing, especially to strings stored in files, poses a
   threat to consistent and predictable behavior for the user even if
   not to actual interoperability.

   Because these transformations are local, it is important that domain
   names that might be passed between systems (e.g., in IRIs) be
   U-labels or A-labels and not forms that might be accepted locally as
   a consequence of this step.  This step is not standardized as part of
   IDNA, and is not further specified here.

5.4.  A-label Input

   If the input to this procedure appears to be an A-label (i.e., it
   starts in "xn--"), the lookup application MAY attempt to convert it
   to a U-label and apply the tests of Section 5.5 and the conversion of
   Section 5.6 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 also Section 6.1.

   That conversion and testing 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 make tests 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.

5.5.  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 [IDNA2008-Rationale], the lookup check is more liberal
   than the registration one.  Putative labels with any of the following
   characteristics MUST BE rejected prior to DNS lookup:

   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 [IDNA2008-Tables].

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   o  Labels that are not in NFC form as defined in [Unicode-UAX15].

   o  Labels containing prohibited code points, i.e., those that are
      assigned to the "DISALLOWED" category in the permitted character
      table [IDNA2008-Tables].

   o  Labels containing code points that are identified in
      [IDNA2008-Tables] as "CONTEXTJ", i.e., requiring exceptional
      contextual rule processing on lookup, but that do not conform to
      that rule.  Note that this implies that a rule much be defined,
      not missing: a character that requires a contextual rule but for
      which the rule is missing is treated in this step as having failed
      to conform to the rule.

   o  Labels containing code points that are identified in in
      [IDNA2008-Tables] 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 exists (although they MAY make such tests to give better
      information to the user).

   o  Labels whose first character is a combining mark (see

   In addition, the application SHOULD apply the following test.  The
   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.  In any event, lookup
   applications should avoid attempting to resolve labels that are
   invalid under that test.

   o  Verification that the string is compliant with the requirements
      for right to left characters, specified in [IDNA2008-BIDI].

   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.  A lookup application that
   declines to process a string that conforms to the rules above and
   does not look it up in the DNS is not in conformance with this

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5.6.  Punycode Conversion

   The validated string, a U-label, is converted to an A-label using the
   Punycode algorithm with the ACE prefix added.

5.7.  DNS Name Resolution

   The A-label is looked up in the DNS, using normal DNS resolver

6.  Name Server Considerations

   [[anchor15: Note in draft: If we really want this document to contain
   only information that is necessary to proper implementation of IDNA
   by implementers who are familiar with the DNS, the material in this
   section is either tutorial, explanatory, or totally unnecessary.
   Should some or all of it be moved back to Rationale?]]

6.1.  Processing Non-ASCII Strings

   Existing DNS servers do not know the IDNA rules for handling non-
   ASCII forms of IDNs, and therefore need to be shielded from them.
   All existing channels through which names can enter a DNS server
   database (for example, master files (as described in RFC 1034) and
   DNS update messages [RFC2136]) are IDN-unaware because they predate
   IDNA.  Other sections of this document provide the needed shielding
   by ensuring that internationalized domain names entering DNS server
   databases through such channels have already been converted to their
   equivalent ASCII A-label forms.

   Because of the design of the algorithms in Section 4 and Section 5 (a
   domain name containing only ASCII codepoints can not be converted to
   an A-label), there can not be more than one A-label form for any
   given U-label.

   As specified in RFC 2181 [RFC2181], the DNS protocol explicitly
   allows domain labels to contain octets beyond the ASCII range
   (0000..007F), and this document does not change that.  Note, however,
   that there is no defined interpretation of octets 0080..00FF as
   characters.  If labels containing these octets are returned to
   applications, unpredictable behavior could result.  The A-label form,
   which cannot contain those characters, is the only standard
   representation for internationalized labels in the DNS protocol.

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6.2.  DNSSEC Authentication of IDN Domain Names

   DNS Security (DNSSEC) [RFC2535] is a method for supplying
   cryptographic verification information along with DNS messages.
   Public Key Cryptography is used in conjunction with digital
   signatures to provide a means for a requester of domain information
   to authenticate the source of the data.  This ensures that it can be
   traced back to a trusted source, either directly or via a chain of
   trust linking the source of the information to the top of the DNS

   IDNA specifies that all internationalized domain names served by DNS
   servers that cannot be represented directly in ASCII MUST use the
   A-label form.  Conversion to A-labels MUST be performed prior to a
   zone being signed by the private key for that zone.  Because of this
   ordering, it is important to recognize that DNSSEC authenticates a
   domain name containing A-labels or conventional LDH-labels, not
   U-labels.  In the presence of DNSSEC, no form of a zone file or query
   response that contains a U-label may be signed or the signature

   One consequence of this for sites deploying IDNA in the presence of
   DNSSEC is that any special purpose proxies or forwarders used to
   transform user input into IDNs must be earlier in the lookup flow
   than DNSSEC authenticating nameservers for DNSSEC to work.

6.3.  Root and other DNS Server Considerations

   IDNs in A-label form will generally be somewhat longer than current
   domain names, so the bandwidth needed by the root servers is likely
   to go up by a small amount.  Also, queries and responses for IDNs
   will probably be somewhat longer than typical queries historically,
   so EDNS0 [RFC2671] support may be more important (otherwise, queries
   and responses may be forced to go to TCP instead of UDP).

7.  Security Considerations

   The general security principles and issues for IDNA appear in
   [IDNA2008-Defs] with additional explanation in [IDNA2008-Rationale].
   The comments below are specific to the registration and loopup
   protocols specified in this document, but should be read in the
   context of the material in the first of those documents and the
   definitions and specifications, identified there, on which this one

   This memo describes procedures for registering and looking up labels
   that are not compatible with the preferred syntax described in the

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   base DNS specifications (STD13 [RFC1034] [RFC1035] and Host
   Requirements [RFC1123]) because they contain non-ASCII characters.
   These procedures depend on the use of a special ASCII-compatible
   encoding form that contains only characters permitted in host names
   by those earlier specifications.  The encoding used is Punycode
   [RFC3492].  No security issues such as string length increases or new
   allowed values are introduced by the encoding process or the use of
   these encoded values, apart from those introduced by the ACE encoding

   Domain names (or portions of them) are sometimes compared against a
   set of domains to be given special treatment if a match occurs, e.g.,
   treated as more privileged than others or blocked in some way.  In
   such situations, it is especially important that the comparisons be
   done properly, as specified in Requirement 2 of Section 3.1.  For
   labels already in ASCII form (i.e., are LDH-labels or A-labels), the
   proper comparison reduces to the same case-insensitive ASCII
   comparison that has always been used for ASCII labels.

   The introduction of IDNA means that any existing labels that start
   with the ACE prefix would be construed as A-labels, at least until
   they failed one of the relevant tests, whether or not that was the
   intent of the zone administrator or registrant.  There is no evidence
   that this has caused any practical problems since RFC 3490 was
   adopted, but the risk still exists in principle.

8.  IANA Considerations

   IANA actions for this version of IDNA are specified in
   [IDNA2008-Tables] and discussed informally in [IDNA2008-Rationale].
   The components of IDNA described in this document do not require any
   IANA actions.

9.  Contributors

   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 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 or omissions.

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10.  Acknowledgements

   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, [RFC4690], and [IDNA2008-Rationale] were particularly

   Specific textual changes were incorporated into this document after
   suggestions from the other contributors, Stephane Bortzmeyer, Vint
   Cerf, Mark Davis, Paul Hoffman, Kent Karlsson, Erik van der Poel,
   Marcos Sanz, Andrew Sullivan, Ken Whistler, and other WG
   participants.  Special thanks are due to Paul Hoffman for permission
   to extract material from his Internet-Draft to form the basis for
   Appendix A

11.  References

11.1.  Normative References

              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right-to-left scripts", July 2008, <https://

              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              November 2008, <

              Faltstrom, P., "The Unicode Codepoints and IDNA",
              July 2008, <

              A version of this document is available in HTML format at

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and

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              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [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.

              The Unicode Consortium, "Unicode Character Database:
              PropertyValueAliases", March 2008, <http://

              The Unicode Consortium, "Unicode Technical Standard #18:
              Unicode Regular Expressions", May 2005,

              The Unicode Consortium, "Unicode Standard Annex #24:
              Unicode Script Property", February 2008,

              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", 2006,

11.2.  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.

              Klensin, J., Ed., "Internationalizing Domain Names for
              Applications (IDNA): Issues, Explanation, and Rationale",
              November 2008, <

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   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [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.

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              5.0", 2007.

              Boston, MA, USA: Addison-Wesley.  ISBN 0-321-48091-0

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"

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   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
        codepoints contains" to "decision about codepoints 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
        specific contexts.

   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
        labels invalid.

Appendix B.  Change Log

   [[anchor24: RFC Editor: Please remove this appendix.]]

B.1.  Changes between Version -00 and -01 of draft-ietf-idnabis-protocol

   o  Corrected discussion of SRV records.

   o  Several small corrections for clarity.

   o  Inserted more "open issue" placeholders.

B.2.  Version -02

   o  Rewrote the "conversion to Unicode" text in Section 5.2 as
      requested on-list.

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   o  Added a comment (and reference) about EDNS0 to the "DNS Server
      Conventions" section, which was also retitled.

   o  Made several editorial corrections and improvements in response to
      various comments.

   o  Added several new discussion placeholder anchors and updated some
      older ones.

B.3.  Version -03

   o  Trimmed change log, removing information about pre-WG drafts.

   o  Incorporated a number of changes suggested by Marcos Sanz in his
      note of 2008.07.17 and added several more placeholder anchors.

   o  Several minor editorial corrections and improvements.

   o  "Editor" designation temporarily removed because the automatic
      posting machinery does not accept it.

B.4.  Version -04

   o  Removed Contextual Rule appendices for transfer to Tables.

   o  Several changes, including removal of discussion anchors, based on
      discussions at IETF 72 (Dublin)

   o  Rewrote the preprocessing material (Section 5.3) somewhat.

B.5.  Version -05

   o  Updated part of the A-label input explanation (Section 5.4) per
      note from Erik van der Poel.

B.6.  Version -06

   o  Corrected a few typographical errors.

   o  Incorporated the material (formerly in Rationale) on the
      relationship between IDNA2003 and IDNA2008 as an appendix and
      pointed to the new definitions document.

   o  Text modified in several places to recognize the dangers of
      interaction between DNS wildcards and IDNs.

   o  Text added to be explicit about the handling of edge and failure
      cases in Punycode encoding and decoding.

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   o  Revised for consistency with the new Definitions document and to
      make the text read more smoothly.

B.7.  Version -07

   o  Multiple small textual and editorial changes and clarifications.

   o  Requirement for normalization clarified to apply to all cases and
      conditions for preprocessing further clarified.

   o  Substantive change to Section 4.3.1, turning a SHOULD to a MUST
      (see note from Mark Davis, 19 November, 2008 18:14 -0800).

B.8.  Version -08

   o  Added some references and altered text to improve clarity.

   o  Changed the description of CONTEXTJ/CONTEXTO to conform to that in
      Tables.  In other words, these are now treated as distinction
      categories (again), rather than as specially-flagged subsets of

   o  The discussion of label comparisons has been rewritten to make it
      more precise and to clarify that one does not need to verify that
      a string is a [valid] A-label or U-label in order to test it for
      equality with another string.  The WG should verify that the
      current text is what is desired.

   o  Other changes to reflect post-IETF discussions or editorial

Author's Address

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140

   Phone: +1 617 245 1457

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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