Network Working Group                                           S. Woolf
Internet-Draft                         Internet Systems Consortium, Inc.
Intended status: Informational                                    J. Yao
Expires: April 28, 2011                                           X. Lee
                                                        October 25, 2010

           Problem Statement: DNS Resolution of Aliased Names


   This document attempts to describe a set of issues that arises from
   the desire to treat a set or group of names as "aliases" of each
   other, "bundled," "variants," or "the same," which is problematic in
   terms of corresponding behavior for DNS labels.

   With the emergence of internationalized domain names, among other
   potential use cases, two or more names that users will regard as
   having identical meaning will sometimes require corresponding
   behavior in the DNS.  It's not clear how to accommodate these
   requirements for behavior of such names in DNS resolution; in
   particular, it's not clear when they are best accommodated in
   registry practices for generating names for lookup in the DNS,
   existing DNS protocol elements and behavior, or some set of protocol
   elements or behavior not yet defined.  This document attempts to
   describe some of these cases and the behavior of some of the possible
   solutions discussed to date.

   NOTE: Even more than usual, version -02 of this document is a "work
   in progress".  Additional updates may be expected between the date of
   this document and the DNSEXT meeting in Beijing, and can be found at

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

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

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 28, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  What this document does  . . . . . . . . . . . . . . . . .  5
     1.2.  What this document does not do . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Registration of Domain Name Variants . . . . . . . . . . .  7
     2.2.  Identical DNS Resolution for Bundled DNS Names . . . . . .  7
     2.3.  Character Variants . . . . . . . . . . . . . . . . . . . .  8
       2.3.1.  An example: Simplified and Traditional Chinese . . . .  8
       2.3.2.  An example: Greek  . . . . . . . . . . . . . . . . . .  9
       2.3.3.  An Example: Arabic . . . . . . . . . . . . . . . . . .  9
   3.  Possible Solutions . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Mapping or Redirection of Domain Names . . . . . . . . . . 10
       3.1.1.  Mapping itself . . . . . . . . . . . . . . . . . . . . 10
       3.1.2.  Mapping its descendants  . . . . . . . . . . . . . . . 10
       3.1.3.  Mapping itself and its descendants . . . . . . . . . . 10
     3.2.  Zone Clone . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.1.  draft-yao-dnsext-identical-resolution: Version 00  . . . . 13
     7.2.  draft-yao-dnsext-identical-resolution: Version 01  . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   As the Internet and the DNS have evolved beyond their original realms
   of use, a set of needs and expectations has appeared about how DNS
   labels behave that is informed significantly by common human
   assumptions about how "names" or "words" work.  One aspect of this is
   the notion or expectation that multiple sets of names may be similar
   to a human user, and expected to behave "the same" or as "aliases" of
   one another.  The DNS was designed with the implicit expectation that
   names would be based on ASCII characters, and the "similarity" or
   "sameness" property doesn't seem to arise terribly often in the names
   people originally wanted to use in the DNS; thus the requirements of
   identical resolution of "aliased" or "bundled" names hasn't figured
   prominently as an attribute that needed to be accommodated in the
   generation or lookup of DNS names.  However, with the standardization
   of internationalized domain names protocols (ref: IDNA and IDNAbis),
   more and more internationalized domain name labels [RFC3490] are
   appearing in DNS zones.  In some cases, these labels [RFC3743] are
   accompanied by the expectation that they are "equivalent" or should
   behave "the same," often because these labels are derived from names
   or strings that users consider "the same" in some languages.
   Accordingly, Internet users hope for such labels to behave in DNS
   contexts as they expect the corresponding human constructs to behave.

   The general issues of what "the same" means, or of defining
   "variants" in human scripts as codified in Unicode (or anywhere else)
   are well outside the scope of the DNS or the expertise of most of the
   people who work on it.  They are matters for philosophers and
   applications developers, respectively.  However, to the extent that
   these issues can be specified as involving the resolution of names in
   the DNS, it's reasonable to describe those expectations and attempt
   to accomodate them.

   There is some existing technology defined in the DNS for behavior
   that can be described as one name behaving "the same" as another.
   For a single node in the DNS tree, CNAME can be used to map one name
   as an "alias" to another, "canonical" name.  If there is a need to
   map a subtree of the DNS-- a zone, or a domain and its subdomains--
   to another domain, DNAME has been defined to allow this behavior.
   However, there is no way currently defined to do both, as CNAME is
   required to be the record at its node in the tree.  Behavior that
   combines their characteristics is not currently defined in the DNS.

   If existing protocol does not meet the zone administrator's need to
   be able to treat one label, name, or zone as "the same" as another,
   there are also administrative mechanisms available for manipulating
   databases underlying the generation and resolution of DNS names.
   Registry operators have many mechanisms for working around DNS

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   protocol in order to get behavior they want for names in DNS zones,
   and management of "aliases" is no exception.  However, it is not
   clear how much of the user and operator requirements for "aliases"
   can be met by mechanisms for provisioning DNS zones, at acceptable
   cost.  Concerns have been raised about this approach particularly at
   large scales.

1.1.  What this document does

   Attempts to think about "aliases" or similar concepts as applied to
   the DNS have been difficult, both because use cases have been unclear
   and because terminology for describing and distinguishing them has
   not been readily available.  This document attempts to provide both
   brief descriptions of identified use cases, and a rough organization
   for how to think about behavior in the DNS that might correspond to
   the requirements derived from them as a way of evaluating proposed
   solutions.  This includes existing and additional possible solutions.

   As a departure point, we attempt to be rigorous about distinguishing
   DNS "labels" from "words" (a human construct) and "strings" (which we
   use here as machine-readable constructs that nonetheless may not
   conform to DNS label constraints, such as IDNA U-labels).

   We also review existing constructs (CNAME, DNAME) and proposed new
   ones ("BNAME," "zone clones").

1.2.  What this document does not do

   This document makes no attempt to solve or even describe
   "translation" of one name into another in the DNS, which is likely to
   be impossible.  "Translation" in general, or even the particular
   problem of determining when or why two DNS labels (or even FQDNs)
   should be considered "the same", are simply not in scope for the DNS
   protocol.  We pre-suppose those decisions are made elsewhere and that
   the DNS needs to deliver behavior in conformance with that external

   Accordingly, this document makes no comment on policy regarding when
   two names are "the same," what restrictions should be placed on their
   generation or use outside those imposed by the DNS protocol, or the
   ability of one approach over another to instantiate what a given user
   regards as "the same" for a language, script, culture, or community.

1.3.  Terminology

   All the basic terms used in this specification are defined in the
   documents [RFC1034], [RFC1035], [RFC2672] and [RFC3490].

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   We also note that there is a wide variety of terminology in use to
   describe the issues we attempt to treat in this document, and no
   consensus on which apply under what conditions.  Terms for "a set of
   domain names that somehow need to be treated as similar" include
   "bundle," "variants," or "clones".  As uniformity of terms is one of
   the goals of any work on this topic in the DNS, we try not to add to
   the confusion in the problem statement but can't claim to have
   finalized a recommendation in early versions of this document.

2.  Problem Statement

   From the point of view of the DNS, a number of attributes suggest
   themselves as important dimensions for evaluating what "the same"
   might mean.

   One question is exactly what it is that's to be defined as "the
   same"?  Are the end results to be identical, and if so from what
   perspective: that of the recursive resolver?  The application?  The
   human consumer of content?  Is it enough that lookups on the FQDN
   portion of an email address result in the same A or AAAA records, or
   does some intermediate mapping need to be maintained between MX
   records in the resolution chain?  What about the FQDN portion of a
   URL handed back to an application, or in resolution processes that
   include multiple lookups of records that may include FQDNs?  Do there
   need to be general rules specified for the handling of FQDNs in RDATA
   of present and future RRtypes?

   Another question is the behavior of multiple names with respect to
   one another: is it enough to define one as "canonical" or
   "preferred," with the others considered as "variants" that are
   transformed to the "preferred" form?  Or is there a real need for
   multiple names to be "equivalent", interchangeable, with none
   considered "preferred" over the others?  (We note here that no
   requirement for complete interchangeability or identity has been
   articulated, except anecdotally, and such equivalence would be
   extremely difficult to define in the DNS.)

   In addition, the tree structure of the DNS requires that we consider
   the behavior of "identical" names across multiple zones in the
   hierarchy.  Are mappings to be maintained in names more than a level,
   or two, deep?  If so, with what characteristics, and what
   characteristics are required for scalability?

   A further question arises with respect to how applications should
   interact with alias-specific DNS behavior.  A basic requirement would
   seem to be "First, do no harm," or in other words, any extensions to
   DNS protocol in support of the desired "alias" behavior should not

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   interfere with applications that expect to do such interpretation on
   their own.  This concern is based in the expectation that DNS is
   simple and predictable, operating strictly as infrastructure under
   the process of creating "the user experience," not as part of it.

2.1.  Registration of Domain Name Variants

   The introduction of IDN has provided a forcing function for defining
   how "variants" might behave as DNS names.  It's generally conceded
   that recognition and careful management of cases where multiple names
   are associated together as "variants" in the expectation or
   preference of users are important; without such management of grouped
   domain names, security risks may be increased and the quality of user
   experience may be compromised.  [RFC3743] developed by JET (Joint
   Engineering Team) gives one possible solution of how to manage
   registration of a domain name, intended to be applied to the script
   and usage common across Chinese, Japanese, and Korean users.
   [RFC3743] proposed an algorithm which will allocate a group of names,
   consisting of a domain name and its variants, to the same domain
   holder.  It means that the domain holder will get control of the
   domain name and its variants.  [RFC4290] suggests the practice in
   [RFC3743] to be used in registrations of internationalized domain
   names.  But [RFC3743] and [RFC4290] do not define how, exactly, these
   bundles of names are to be treated by the registry or the DNS in
   order to obtain the desired "identical" behavior.  [RFC4690] said
   that the "variant" model introduced in [RFC3743] and [RFC4290] can be
   used by a registry to prevent the most negative consequences of
   possible confusion, by ensuring either that both names are registered
   to the same party in a given domain or that one of them is completely
   prohibited.  The principles described in [RFC3743], [RFC4290] and
   [RFC4690] have been accepted by many registries.  But the technical
   details of how to guarantee that a bundle of domain names are
   "identical" in the DNS remain unspecified.

2.2.  Identical DNS Resolution for Bundled DNS Names

   To some extent, the desired behavior can be described: "identical DNS
   resolution" means that the process of resolving two domain names will
   end with the same result, in most cases the same IP address.  In the
   history of DNS protocol development, there have been two attempts to
   specify such "identical resolution" behavior:CNAME[RFC1034] which
   maps or redirects itself, and DNAME[RFC2672] which maps or redirects
   its descendants.  In the case of bundles or groups of names, however,
   some operators have asserted they need identical DNS resolution at
   all levels' domain names, including the domain name itself and its
   descendants.  As alluded to above, registries are left with ad hoc
   provisioning and database management mechanisms for managing variant
   names, with some help from existing DNS protocol mechanisms for

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   mapping labels or FQDNs to each other.  However, some are finding the
   existing mechanisms to have unsatisfactory limitations; they are
   seeking more guidance on the use of existing mechanisms, and perhaps
   the addition of new ones in the DNS protocol.

2.3.  Character Variants

   Many defined scripts as used in many different languages have
   "character variants" included.  There is no uniform definition of
   variants, and in fact their characteristics differ widely, but it's
   possible to define some.  For example, the definition of variant
   characters in the JET Guidelines [RFC3743], intended for use with the
   CJK language/script communities, is roughly this: One conceptual
   character can be identified with several different Code Points in
   character sets for computer use.  In UNICODE definitions of some
   scripts, including Han (chinese), some characters can be identified
   as "compatibility variants" of another character, which usually
   implies that the first can be remapped to the second without the loss
   of any meaning.  In this document, variant characters are two or more
   characters that may be similar in appearance or identical in meaning
   (similarity in appearance is not required by the definition but often

   With the introduction of IDNs in the DNS, perhaps most prominently in
   the root zone, decisions about how to deal with IDN variants is a
   significant challenge ahead of us.  We describe here a couple of
   examples, Chinese and Greek; [STW: ....and additional cases (Arabic,
   Cyrillic, etc.)should be described in future versions by experts in
   those scripts and languages].

2.3.1.  An example: Simplified and Traditional Chinese

   For example, if the IDN TLD "China"(U+4E2D U+56FD) and its variant
   (U+4E2D U+570B) are put into the root, the first one (U+4E2D U+56FD)
   can be considered the "original" IDN TLD and the second one (U+4E2D
   U+570B) can be considered the IDN TLD "variant".  Ideally, it should
   be possible to treat the original IDN TLD and its IDN TLD variant as
   "identical" for purposes of DNS resolution, in a way similar to the
   case mapping most DNS users take for granted, in which the uppercase
   "A" is the variant of lowercase "a".  For example, the string ".COM"
   can be considered a variant of ".com", and the corresponding DNS
   labels are treated as identical.  However, for the historical reasons
   already discussed, and technical reasons having to do with the
   underpinnings of the IDNAbis protocol (ref: IDNAbis rationale),
   there's no generalization of the "case" mapping available for
   situations where it might be useful for IDNs.  In addition, it's
   perilous because DNS rules around "case insensitivity" and "case
   preservation" are not intuitively consistent.

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   At this writing, four Chinese script IDN TLDs are in the root,
   including two pairs comprising a Traditional Chinese name and its
   Simplified counterpart, which should allow future versions of this
   document to include those operators' experience of managing
   maintenance of those names as "the same."

2.3.2.  An example: Greek

   In Greek, almost every word has the "tonos" accent sign, but where it
   is placed (on which character) can vary.  Further, some words end in
   a final sigma, which is represented differently to sigma appearing
   elsewhere in the word.  If a registry wishes to be able to enforce
   the association among all of the domain names that correspond to a
   "word" in Greek, with all its possible Unicode strings, some
   mechanism must be used to enumerate the "variant" names and tie them
   together.  This makes sense from the human factors perspective, as
   depending on how the user types something, results may include a
   different domain to what was expected, although the user may have the
   firm belief that "the same word" was input in multiple cases.

   As an example, the domain names "" and "xn--" appear to a native speaker/reader of Greek to represent
   "the same word," in a sense very much like the case insensitivity
   that native users of Latin script take for granted in the DNS.

2.3.3.  An Example: Arabic

   [STW: [to be added]

3.  Possible Solutions

   Currently, there are several possible mechanisms to support identical
   DNS resolution of "bundled" or "variant" names as "aliases" in the
   DNS.  Existing mechanisms in the DNS include CNAME and DNAME.  In
   addition, as described briefly above, registry operators have a great
   many techniques for applying policy to what names can be registered,
   and provisioning technology to how they are instantiated in the DNS,
   in support of keeping "variant" names behaving similarly to each
   other, or in preventing the use of such variants as might be
   considered confusing or dangerous.

   In addition, there are new proposals for DNS protocol to support
   "aliases" in the DNS as part of the desired behavior of "variant"
   names: Names direction[BNAME], and "Zone clone".

   All of the solutions have their advantages and disadvantages.  In
   particular, there are a couple of limitations they all share.  Every

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   mechanism existing or proposed to support "aliases" in the DNS
   requires that one name be designated as the "canonical" name
   ("preferred" in the terminology of the JET variant mechanism) and any
   others bundled with it are to be considered "variants" or "aliases".
   The only known way to enforce a symmetrical or equivalent association
   is via careful registry provisioning within and across domains.  In
   addition, the different "alias" mechanisms differ in subtle ways that
   have to be carefully reviewed against the desired behavior of the DNS
   in support of different types of "variants".

3.1.  Mapping or Redirection of Domain Names

3.1.1.  Mapping itself

   It was recognized as part of the original specification of the DNS
   that a host can have many names; in fact this expectation predates
   the DNS, referring to the earlier specification of host names.  In
   the simplest case for "aliases", Internet users need these multiple
   names to be resolved to the same IP address by a DNS server.  The
   CNAME record [RFC1034], where "CNAME" is an abbreviation for
   "Canonical Name", is a way to designate aliases of the "real" or
   canonical name of a host.  In some cases, CNAME can be used to
   produce the necessary association a bundle of variant domain names.
   But the CNAME only maps itself, not its descendants; in fact it is
   defined to not have descendants, as it is the only name at a node in
   the DNS tree and can't exist at the same name as delegation.  In the
   case of IDN variants, however, it is often desirable that the name
   map both itself and its descendants.

3.1.2.  Mapping its descendants

   In order to maintain the address-to-name mappings in a context of
   network renumbering, a DNAME record or Delegation Name record defined
   by [RFC2672] was invented to create an alias for all its subdomains.
   In contrast, the CNAME record creates an alias only of a single name
   (and not of its subdomains).  As with the CNAME record, the DNS
   lookup will continue by retrying the lookup with the new name.  If a
   DNS resolver sends a query without EDNS[EDNS0], or with EDNS version
   0, then a name server synthesizes a CNAME record to simulate the
   semantics of the DNAME record.  A DNAME record is very much like the
   CNAME record, but while the CNAME record only applies for one name,
   with a DNAME record one can create aliases for all the records for
   its subdomain.

3.1.3.  Mapping itself and its descendants

   Bundling of "variant" strings or names as domain names, possibly
   along with other use cases not yet identified, require the ability to

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   map a whole tree of the domain space to another domain.  The current
   DNS protocols do not support this function.  A new DNS resource
   record [BNAME] has been proposed to deal with this problem.

   The advantage of BNAME is that it would enable a class of "aliasing"
   behavior that some operators find desirable, particularly in
   preference to some of the provisioning overhead they describe having
   to deploy to support potentially large numbers of "bundles" of
   variants at multiple levels of the DNS tree.  The disadvantage is
   that it may not provide the behavior people really want while
   requiring the time and resources to code and deploy any new DNS

   Alternatively, a proposal has been made that would leave CNAME as
   already specified, but eliminating the constraint that a CNAME must
   be alone at a node in the DNS tree.  This would avoid any coding and
   deployment overhead associated with new RRtypes, while obtaining the
   desired behavior.  Concerns expressed about it, however, include the
   possible (but not yet specified) effort required for backwards
   compatibility to avoid harm to implementations that expect, and use,
   the old behavior.

3.2.  Zone Clone

   The proposal of "zone clone" or "dns shadow", is an alternative
   solution for a higher level of support than the DNS currently
   provides for "alias" behavior across zones.  In this scheme, a new
   RRtype, SHADOW, is specified; it can exist at a zone apex and can be
   used to define "clones" or "shadows" of the zone content so that
   records in the zone are reachable via lookups from multiple
   delegations.  This mechanism varies fundamentally from CNAME/DNAME/
   BNAME in that it creates a local copy on each cooperating
   authoritative server that has the original zone, reachable by the
   names specified in the SHADOW RR.  Its scope, then, is the zone as
   maintained by an authoritative server rather than a single RRset
   (even one corresponding to a delegation).

   This scheme has the advantage that it allows a SHADOW zone to be used
   in all the same contexts as the canonical or underlying zone,
   including contexts where a CNAME or DNAME (or, presumably, a BNAME)
   cannot appear, such as in the RDATA of certain RRtypes.  Of the
   proposed DNS protocol mechanisms, it probably comes closest to the
   behavior some have requested as "equivalence," where none of the
   bundled or SHADOW names is canonical or preferred over the others.
   It does implicate an unknown level of effort to implement and

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4.  IANA Considerations

   There are no obvious IANA considerations in this memo; we reiterate
   that the determination of which names are to be considered "the same"
   is explicitly out of scope.

5.  Security Considerations

   [STW: Looking for examples for this section.]

   Unsolved issues that will have to be considered in the definition of
   what "the same" means for the DNS include the implications for
   DNSSEC, and whether "identical" resolution includes DNSSEC validation
   in the expected "identical" behavior.

   Another area of possible peril includes SSL certificates, "Host"
   headers as seen by web servers, and other security-relevant data
   often associated with domain names.  It will have to be considered
   whether, and how, the "sameness" property maps into the expected
   behavior of security-related protocols that use domain names,
   particularly given that it's unlikely that all operators will ever
   use the same set of constructs (whether in the DNS or elsewhere) to
   signal whether different "names" are "the same" for purposes of the
   function of a particular application or protocol.

   In addition, there is a large cluster of security risks at the user
   and application levels that motivate significant portions of the
   interest in what it means to treat a set of names as "aliases" of
   each other.  One set of issues is around the expectation that two
   strings are seen as "different" by the user in some obvious way (such
   as visually) but need to be treated as "the same".  The potential for
   user confusion and subversion is not hard to imagine in cases where
   two visually distinct strings are nonetheless likely to be expected
   by the user to behave "the same" in some functional way.  This is the
   case we have attempted to address here.

   There is a separate but complementary set of issues that arise around
   cases where strings that look "the same" should nonetheless be
   treated as different-- the so-called "confusing visual similarity"
   problem.  The easy example is substituting the Unicode codepoint for
   a character in one script, or a string of them, for the Unicode
   codepoints for similar-looking characters in an altogether different
   script.  This has a different set of potential risks to users, and
   has not been discussed here.  It's often closely related to the
   "alias" issue we have attempted to deal with, however, which poses
   risks of its own to analysis of the either subject.

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6.  Acknowledgements

   Most of the ideas here and much of the text is taken from discussions
   on the DNSEXT and DNSOP WG mailing lists.  Particular help is
   acknowledged from the authors of the proposed solutions drafts, and
   from the many contributors to the IDNAbis work and its underpinnings.
   Special thanks at the intersection of DNS and IDNAbis is owed to
   Patrik Faltstrom, Cary Karp, John Klensin, Vaggelis Segredakis, and
   Andrew Sullivan for their patient explanations.

7.  Change History

   [[anchor20: RFC Editor: Please remove this section.]]

7.1.  draft-yao-dnsext-identical-resolution: Version 00

   o  Domain Name Identical Resolution Problem Statement (initial

7.2.  draft-yao-dnsext-identical-resolution: Version 01

   o  Expanded introduction
   o  Added Greek example
   o  Added some detail to descriptions of proposed solutions

8.  References

8.1.  Normative References

   [ASCII]    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange", ANSI X3.4-1968, 1968.

   [EDNS0]    Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

   [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.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,

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              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection",
              RFC 2672, August 1999.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, September 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, November 2003.

   [RFC3743]  Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
              Engineering Team (JET) Guidelines for Internationalized
              Domain Names (IDN) Registration and Administration for
              Chinese, Japanese, and Korean", RFC 3743, April 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4290]  Klensin, J., "Suggested Practices for Registration of
              Internationalized Domain Names (IDN)", RFC 4290,
              December 2005.

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.

8.2.  Informative References

   [BNAME]    Yao, J., Lee, X., and P. Vixie, "Bundle DNS Name
              Redirection", draft-yao-dnsext-bname-01.txt (work in
              progress), 12 2009.


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              Sury, O., "CNAME+DNAME Name Redirection",
              draft-sury-dnsext-cname-dname-00.txt (work in progress),
              4 2010.

              Yao, J. and X. Lee, "IDN TLD Variants Implementation
              Guideline", draft-yao-dnsop-idntld-implementation-01.txt
              (work in progress), 11 2009.

              Rose, S. and W. Wijngaards, "Update to DNAME Redirection
              in the DNS", Internet-Draft ietf-dnsext-rfc2672bis-dname-
              17.txt, 6 2009.

   [SHADOW]   Vixie, P., "Use of DNS to Carry Configuration Metadata
              Concerning Automatic Replication of Zones",
              draft-vixie-dnsext-dnsshadow-00.txt (work in progress),
              2 2010.

Authors' Addresses

   Suzanne Woolf
   Internet Systems Consortium, Inc.
   950 Charter St.
   Redwood City, CA  94063

   Phone: +1 650 423 1333

   Jiankang YAO
   No.4 South 4th Street, Zhongguancun

   Phone: +86 10 58813007

   Xiaodong LEE
   No.4 South 4th Street, Zhongguancun

   Phone: +86 10 58813020

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