Internet DRAFT - draft-iab-idn-encoding

draft-iab-idn-encoding






Network Working Group                                          D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                                J. Klensin
Expires: January 27, 2011
                                                             S. Cheshire
                                                                   Apple
                                                           July 26, 2010


      IAB Thoughts on Encodings for Internationalized Domain Names
                     draft-iab-idn-encoding-04.txt

Abstract

   This document explores issues with Internationalized Domain Names
   (IDNs) that result from the use of various encoding schemes such as
   UTF-8 and the ASCII-Compatible Encoding produced by the Punycode
   algorithm.  It focuses on the importance of agreeing on a canonical
   format and how complicated it ends up being as a result of using
   different encodings today.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 27, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  APIs . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   2.  Use of Non-DNS Protocols . . . . . . . . . . . . . . . . . . . 10
   3.  Use of Non-ASCII in DNS  . . . . . . . . . . . . . . . . . . . 11
     3.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  IAB Members at the time of publication . . . . . . . . . . . . 20
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23





























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1.  Introduction

   The goal of this document is to explore what can be learned from some
   current difficulties in implementing Internationalized Domain Names
   (IDNs).

   A domain name consists of a set of labels, conventionally written
   separated with dots.  An Internationalized Domain Name (IDN) is a
   domain name that contains one or more labels that, in turn, contain
   one or more non-ASCII characters.  Just as with plain ASCII domain
   names, each IDN label must be encoded using some mechanism before it
   can be transmitted in network packets, stored in memory, stored on
   disk, etc.  These encodings need to be reversible, but they need not
   store domain names the same way humans conventionally write them on
   paper.  For example, when transmitted over the network in DNS
   packets, domain name labels are *not* separated with dots.

   IDNA, discussed later in this document, is the standard that defines
   the use and coding of internationalized domain names for use on the
   public Internet.  It is described as "Internationalizing Domain Names
   in Applications (IDNA)" and is defined in several documents.
   Definitions for the current version and a roadmap of related
   documents appears in [IDNA2008-Defs].  An earlier version of IDNA
   [RFC3490] is now being phased out.  Except where noted, the two
   versions are approximately the same with regard to the issues
   discussed in this document.  However, some explanations appeared in
   the earlier documents that did not seem useful when the revision was
   created; they are quoted here from the documents in which they
   appear.  In addition, the terminology of the two version differs
   somewhat; this document reflects the terminology of the current
   version.

   Unicode [Unicode] is a list of characters (including non-spacing
   marks that are used to form some other characters), where each
   character is assigned an integer value, called a code point.  In
   simple terms a Unicode string is a string of integer code point
   values in the range 0 to 1,114,111 (10FFFF in base 16), which
   represent a string of Unicode characters.  These integer code points
   must be encoded using some mechanism before they can be transmitted
   in network packets, stored in memory, stored on disk, etc.  Some
   common ways of encoding these integer code point values in computer
   systems include UTF-8, UTF-16, and UTF-32.  In addition to the
   material below, those forms and the tradeoffs among them are
   discussed in Chapter 2 of The Unicode Standard [Unicode].

   UTF-8 is a mechanism for encoding a Unicode code point in a variable
   number of 8-bit octets, where an ASCII code point is preserved as-is.
   Those octets encode a string of integer code point values, which



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   represent a string of Unicode characters.  The authoritative
   definition of UTF-8 is in Sections 3.9 and 3.10 of The Unicode
   Standard [Unicode], but UTF-8 is also discussed in [RFC3629].
   Descriptions and formulae can also be found in Annex D of ISO/IEC
   10646-1 [10646].

   UTF-16 is a mechanism for encoding a Unicode code point in one or two
   16-bit integers, described in detail in Sections 3.9 and 3.10 of The
   Unicode Standard [Unicode].  A UTF-16 string encodes a string of
   integer code point values that represent a string of Unicode
   characters.

   UTF-32 (formerly UCS-4), also described in [Unicode] Sections 3.9 and
   3.10, is a mechanism for encoding a Unicode code point in a single
   32-bit integer.  A UTF-32 string is thus a string of 32-bit integer
   code point values, which represent a string of Unicode characters.

   Note that UTF-16 results in some all-zero octets when code points
   occur early in the Unicode sequence, and UTF-32 always has all-zero
   octets.

   IDNA specifies validity of a label, such as what characters it can
   contain, relationships among them, and so on, in Unicode terms.
   Valid labels can take either of two forms, with the appropriate one
   determined by particular protocols or by context.  One of those
   forms, called a U-label, is a direct representation of the Unicode
   characters using one of the encoding forms discussed above.  This
   document discusses UTF-8 strings in many places.  While all U-labels
   can be represented by UTF-8 strings, not all UTF-8 strings are valid
   U-labels (see Section 2.3.2 of [IDNA2008-Defs] for a discussion of
   these distinctions).  The other, called an A-label, uses a
   compressed, ASCII-compatible encoding (an "ACE" in IDNA and other
   terminology) produced by an algorithm called Punycode.  U-labels and
   A-labels are duals of each other: transformations from one to the
   other do not lose information.  The transformation mechanisms are
   specified in [IDNA2008-Protocol].

   Punycode [RFC3492] is thus a mechanism for encoding a Unicode string
   in an ASCII-compatible encoding, i.e., using only letters, digits,
   and hyphens from the ASCII character set.  When a Unicode label that
   is valid under the IDNA rules (a U-label) is encoded with Punycode
   for IDNA purposes, it is prefixed with "xn--"; the result is called
   an A-label.  The prefix convention assumes that no other DNS labels
   (at least no other DNS labels in IDNA-aware applications) are allowed
   to start with these four characters.  Consequently, when A-label
   encoding is assumed, any DNS labels beginning with "xn--" now have a
   different meaning (the Punycode encoding of a label containing one or
   more non-ASCII characters) or no defined meaning at all (in the case



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   of labels that are not IDNA-compliant, i.e., are not well-formed
   A-labels).

   ISO-2022-JP [RFC1468] is a mechanism for encoding a string of ASCII
   and Japanese characters, where an ASCII character is preserved as-is.
   ISO-2022-JP is stateful: special sequences are used to switch between
   character coding tables.  As a result, if there are lost or mangled
   characters in a character stream, it is extremely difficult to
   recover the original stream after such a lost character encoding
   shift.

   Comparison of Unicode strings is not as easy as comparing, for
   example, ASCII strings.  First, there are a multitude of ways of
   representing a string of Unicode characters.  Second, in many
   languages and scripts, the actual definition of "same" is very
   context-dependent.  Because of this, comparison of two Unicode
   strings must take into account how the Unicode strings are encoded.
   Regardless of the encoding, however, comparison cannot simply be done
   by comparing the encoded Unicode strings byte by byte.  The only time
   that is possible is when the strings both are mapped into some
   canonical format and encoded the same way.

   RFC 2130 [RFC2130] reports on an IAB-sponsored workshop on character
   sets and encodings.  This document adds to that discussion and
   focuses on the importance of agreeing on a canonical format and how
   complicated it ends up being as a result of using different encodings
   today.

   Different applications, APIs, and protocols use different encoding
   schemes today.  Historically, many of them were originally defined to
   use only ASCII.  Internationalizing  Domain Names in Applications
   (IDNA) [IDNA2008-Defs] defined a mechanism that required changes to
   applications, but in attempt not to change APIs or servers, specified
   that the A-label format is to be used in many contexts.  In some ways
   this could be seen as not changing the existing APIs, in the sense
   that the strings being passed to and from the APIs were still
   apparently ASCII strings.  In other ways it was a very profound
   change to the existing APIs, because while those strings were still
   syntactically valid ASCII strings, they no longer meant the same
   thing as they used to.  What looked like a plain ASCII string to one
   piece of software or library could be seen by another piece of
   software or library (with the application of out-of-band information)
   to be in fact an encoding of a Unicode string.

   Section 1.3 of the original IDNA specification [RFC3490] states:






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      The IDNA protocol is contained completely within applications.  It
      is not a client-server or peer-to-peer protocol: everything is
      done inside the application itself.  When used with a DNS resolver
      library, IDNA is inserted as a "shim" between the application and
      the resolver library.  When used for writing names into a DNS
      zone, IDNA is used just before the name is committed to the zone.

   Figure 1 depicts a simplistic architecture that a naive reader might
   assume from the paragraph quoted above.  (A variant of this same
   picture appears in Section 6 of the IDNA specification [RFC3490]
   further strengthening this assumption.)

    +-----------------------------------------+
    |Host                                     |
    |             +-------------+             |
    |             | Application |             |
    |             +------+------+             |
    |                    |                    |
    |               +----+----+               |
    |               |   DNS   |               |
    |               | Resolver|               |
    |               | Library |               |
    |               +----+----+               |
    |                    |                    |
    +-----------------------------------------+
                         |
                _________|_________
               /                   \
              /                     \
             /                       \
            |         Internet        |
             \                       /
              \                     /
               \___________________/

                          Simplistic Architecture

                                 Figure 1

   There are, however, two problems with this simplistic architecture
   that cause it to differ from reality.

   First, resolver APIs on Operating Systems (OSs) today (MacOS,
   Windows, Linux, etc.) are not DNS-specific.  They typically provide a
   layer of indirection so that the application can work independent of
   the name resolution mechanism, which could be DNS, mDNS
   [I-D.cheshire-dnsext-multicastdns], LLMNR [RFC4795], NetBIOS-over-TCP
   [RFC1001][RFC1002], etc/hosts file [RFC0952], NIS [NIS], or anything



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   else.  For example, "Basic Socket Interface Extensions for IPv6"
   [RFC3493] specifies the getaddrinfo() API and contains many phrases
   like "For example, when using the DNS" and "any type of name
   resolution service (for example, the DNS)".  Importantly, DNS is
   mentioned only as an example, and the application has no knowledge as
   to whether DNS or some other protocol will be used.

   Second, even with the DNS protocol, private name spaces (sometimes
   including private uses of the DNS), do not necessarily use the same
   character set encoding scheme as the public Internet name space.

   We will discuss each of the above issues in subsequent sections.  For
   reference, Figure 2 depicts a more realistic architecture on typical
   hosts today (which don't have IDNA inserted as a shim immediately
   above the DNS resolver library).  More generally, the host may be
   attached to one or more local networks, each of which may or may not
   be connected to the public Internet and may or may not have a private
   name space.

































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    +-----------------------------------------+
    |Host                                     |
    |             +-------------+             |
    |             | Application |             |
    |             +------+------+             |
    |                    |                    |
    |             +------+------+             |
    |             |   Generic   |             |
    |             |    Name     |             |
    |             |  Resolution |             |
    |             |     API     |             |
    |             +------+------+             |
    |                    |                    |
    |   +-----+------+---+--+-------+-----+   |
    |   |     |      |      |       |     |   |
    | +-+-++--+--++--+-++---+---++--+--++-+-+ |
    | |DNS||LLMNR||mDNS||NetBIOS||hosts||...| |
    | +---++-----++----++-------++-----++---+ |
    |                                         |
    +-----------------------------------------+
                         |
                   ______|______
                  /             \
                 /               \
                /      local      \
                \     network     /
                 \               /
                  \_____________/
                         |
                _________|_________
               /                   \
              /                     \
             /                       \
            |         Internet        |
             \                       /
              \                     /
               \___________________/

                          Realistic Architecture

                                 Figure 2

1.1.  APIs

   Section 6.2 of the original IDNA specification [RFC3490] states
   (where ToASCII and ToUnicode below refer to conversions using the
   Punycode algorithm):




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      It is expected that new versions of the resolver libraries in the
      future will be able to accept domain names in other charsets than
      ASCII, and application developers might one day pass not only
      domain names in Unicode, but also in local script to a new API for
      the resolver libraries in the operating system.  Thus the ToASCII
      and ToUnicode operations might be performed inside these new
      versions of the resolver libraries.

   Resolver APIs such as getaddrinfo() and its predecessor
   gethostbyname() were defined to accept "char *" arguments, meaning
   they accept a string of bytes, terminated with a NULL (0) byte.
   Because of the use of a NULL octet as a string terminator, this is
   sufficient for ASCII strings (including A-labels) and even
   ISO-2022-JP and UTF-8 strings (unless an implementation artificially
   precludes them), but not UTF-16 or UTF-32 strings because a NULL
   octet could appear in the the middle of strings using these
   encodings.  Several operating systems historically used in Japan will
   accept (and expect) ISO-2022-JP strings in such APIs.  Some platforms
   used worldwide also have new versions of the APIs (e.g.,
   GetAddrInfoW() on Windows) that accept other encoding schemes such as
   UTF-16.

   It is worth noting that an API using "char *" arguments can
   distinguish between conventional ASCII "host name" labels, A-labels,
   ISO-2022-JP, and UTF-8 labels in names if the coding is known to be
   one of those four, and the label is intact (no lost or mangled
   characters).  If a stateful encoding like ISO-2022-JP is used,
   applications extracting labels from text must take special
   precautions to be sure that the appropriate state-setting characters
   are included in the string passed to the API.

   An example method for distinguishing among such codings is as
   follows:
   o  if the label contains an ESC (0x1B) byte the label is ISO-2022-JP;
      otherwise,
   o  if any byte in the label has the high bit set, the label is UTF-8;
      otherwise,
   o  if the label starts with "xn--" then it is presumed to be an
      A-label; otherwise,
   o  the label is ASCII.
   Again this assumes that ASCII labels never start with "xn--", and
   also that UTF-8 strings never contain an ESC character.  Also the
   above is merely an illustration; UTF-8 can be detected and
   distinguished from other 8-bit encodings with good accuracy [MJD].

   It is more difficult or impossible to distinguish the ISO 8859
   character sets from each other, because they differ in up to about 90
   characters which have exactly the same encodings, and a short string



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   is very unlikely to contain enough characters to allow a receiver to
   deduce the character set.  Similarly, it is not possible in general
   to distinguish between ISO-2022-JP and any other encoding based on
   ISO 2022 code table switching.

   Although it is possible (as in the example above) to distinguish some
   encodings when not explicitly specified, it is cleaner to have the
   encodings specified explicitly, such as specifying UTF-16 for
   GetAddrInfoW(), or specifying explicitly which APIs expect UTF-8
   strings.


2.  Use of Non-DNS Protocols

   As noted earlier, typical name resolution libraries are not DNS-
   specific.  Furthermore, some protocols are defined to use encoding
   forms other than IDNA A-labels.  For example, mDNS
   [I-D.cheshire-dnsext-multicastdns] specifies that UTF-8 be used.
   Indeed, the IETF policy on character sets and languages [RFC2277]
   (which followed the IAB-sponsored workshop [RFC2130]) states:

      Protocols MUST be able to use the UTF-8 charset, which consists of
      the ISO 10646 coded character set combined with the UTF-8
      character encoding scheme, as defined in [10646] Annex R
      (published in Amendment 2), for all text.  Protocols MAY specify,
      in addition, how to use other charsets or other character encoding
      schemes for ISO 10646, such as UTF-16, but lack of an ability to
      use UTF-8 is a violation of this policy; such a violation would
      need a variance procedure ([BCP9] section 9) with clear and solid
      justification in the protocol specification document before being
      entered into or advanced upon the standards track.  For existing
      protocols or protocols that move data from existing datastores,
      support of other charsets, or even using a default other than
      UTF-8, may be a requirement.  This is acceptable, but UTF-8
      support MUST be possible.

   Applications that convert an IDN to A-label form before calling
   getaddrinfo() will result in name resolution failures if the Punycode
   name is directly used in such protocols.  Having libraries or
   protocols to convert from A-labels to the encoding scheme defined by
   the protocol (e.g., UTF-8) would require changes to APIs and/or
   servers, which IDNA was intended to avoid.

   As a result, applications that assume that non-ASCII names are
   resolved using the public DNS and blindly convert them to A-labels
   without knowledge of what protocol will be selected by the name
   resolution library, have problems.  Furthermore, name resolution
   libraries often try multiple protocols until one succeeds, because



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   they are defined to use a common name space.  For example, the hosts
   file, DNS, and NetBIOS-over-TCP are all defined to be able to share a
   common syntax (e.g., see ([RFC0952], [RFC1001] section 11.1.1, and
   [RFC1034] section 2.1).  This means that when an application passes a
   name to be resolved, resolution may in fact be attempted using
   multiple protocols, each with a potentially different encoding
   scheme.  For this to work successfully, the name must be converted to
   the appropriate encoding scheme only after the choice is made to use
   that protocol.  In general, this cannot be done by the application
   since the choice of protocol is not made by the application.


3.  Use of Non-ASCII in DNS

   A common misconception is that DNS only supports names that can be
   expressed using letters, digits, and hyphens.

   This misconception originally stemmed from the definition in 1985 of
   an "Internet host name" (and net, gateway, and domain name) for use
   in the "hosts" file [RFC0952].  An Internet host name was defined
   therein as including only letters, digits, and hyphens, where upper
   and lower case letters were to be treated as identical.  The DNS
   specification [RFC1034] section 3.5 entitled "Preferred name syntax"
   then repeated this definition in 1987, saying that this "syntax will
   result in fewer problems with many applications that use domain names
   (e.g., mail, TELNET)".

   The confusion was thus left as to whether the "preferred" name syntax
   was a mandatory restriction in DNS, or merely "preferred".

   The definition of an Internet host name was updated in 1989
   ([RFC1123] section 2.1) to allow names starting with a digit (to
   support IPv4 addresses in dotted-decimal form).  Section 6.1 of
   "Requirements for Internet Hosts -- Application and Support"
   [RFC1123] discusses the use of DNS (and the hosts file) for resolving
   host names to IP addresses and vice versa.  This led to confusion as
   to whether all names in DNS are "host names", or whether a "host
   name" is merely a special case of a DNS name.

   By 1997, things had progressed to a state where it was necessary to
   clarify these areas of confusion.  "Clarifications to the DNS
   Specification" [RFC2181] section 11 states:

      The DNS itself places only one restriction on the particular
      labels that can be used to identify resource records.  That one
      restriction relates to the length of the label and the full name.
      The length of any one label is limited to between 1 and 63 octets.
      A full domain name is limited to 255 octets (including the



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      separators).  The zero length full name is defined as representing
      the root of the DNS tree, and is typically written and displayed
      as ".".  Those restrictions aside, any binary string whatever can
      be used as the label of any resource record.  Similarly, any
      binary string can serve as the value of any record that includes a
      domain name as some or all of its value (SOA, NS, MX, PTR, CNAME,
      and any others that may be added).  Implementations of the DNS
      protocols must not place any restrictions on the labels that can
      be used.

   Hence, it clarified that the restriction to letters, digits, and
   hyphens does not apply to DNS names in general, nor to records that
   include "domain names".  Hence the "preferred" name syntax described
   in the original DNS specification [RFC1034] is indeed merely
   "preferred", not mandatory.

   Since there is no restriction even to ASCII, let alone letter-digit-
   hyphen use, DNS is in conformance with the IETF requirement to allow
   UTF-8 [RFC2277].

   Using UTF-16 or UTF-32 encoding, however, would not be ideal for use
   in DNS packets or "char *" APIs because existing software already
   uses ASCII, and UTF-16 and UTF-32 strings can contain all-zero octets
   that existing software will interpret as the end of the string.  To
   use UTF-16 or UTF-32 one would need some way of knowing whether the
   string was encoded using ASCII, UTF-16, or UTF-32, and indeed for
   UTF-16 or UTF-32 whether it was big-endian or little-endian encoding.
   In contrast, UTF-8 works well because any 7-bit ASCII string is also
   a UTF-8 string representing the same characters.

   If a private name space is defined to use UTF-8 (and not other
   encodings such as UTF-16 or UTF-32), there's no need for a mechanism
   to know whether a string was encoded using ASCII or UTF-8, because
   (for any string that can be represented using ASCII) the
   representations are exactly the same.  In other words, for any string
   that can be represented using ASCII it doesn't matter whether it is
   interpreted as ASCII or UTF-8 because both encodings are the same,
   and for any string that can't be represented using ASCII, it's
   obviously UTF-8.  In addition, unlike UTF-16 and UTF-32, ASCII and
   UTF-8 are both byte-oriented encodings so the question of big-endian
   or little-endian encoding doesn't apply.

   While implementations of the DNS protocol must not place any
   restrictions on the labels that can be used, applications that use
   the DNS are free to impose whatever restrictions they like, and many
   have.  The above rules permit a domain name label that contains
   unusual characters, such as embedded spaces which many applications
   would consider a bad idea.  For example, the original specification



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   in [RFC0821] of the SMTP protocol [RFC5321] constrains the character
   set usable in email addresses.  There is now an effort underway to
   permit SMTP to support internationalized email addresses via an
   extension.

   Shortly after the DNS Clarifications [RFC2181] and IETF character
   sets and languages policy [RFC2277] were published, the need for
   internationalized names within private name spaces (i.e., within
   enterprises) arose.  The current (and past, predating IDNA and the
   prefixed ACE conventions) practice within enterprises that support
   other languages is to put UTF-8 names in their internal DNS servers
   in a private name space.  For example, "Using the UTF-8 Character Set
   in the Domain Name System" [I-D.skwan-utf8-dns-00] was first written
   in 1997, and was then widely deployed in Windows.  The use of UTF-8
   names in DNS was similarly implemented and deployed in MacOS, simply
   by virtue of the fact that applications blindly passed UTF-8 strings
   to the name resolution APIs, and the name resolution APIs blindly
   passed those UTF-8 strings to the DNS servers, and the DNS servers
   correctly answered those queries, and from the user's point of view
   everything worked properly without any special new code being
   written, except that ASCII is matched case-insensitively whereas
   UTF-8 is not (although some enterprise DNS servers reportedly attempt
   to do case-insensitive matching on UTF-8 within private name spaces).
   Within a private name space, and especially in light of the IETF
   UTF-8 policy [RFC2277], it was reasonable to assume within a private
   name space that binary strings were encoded in UTF-8.

   As implied earlier, there are also issues with mapping strings to
   some canonical form, independent of the encoding.  Such issues are
   not discussed in detail in this document.  They are discussed to some
   extent in, for example, Section 3 of [RFC5198], and are left as
   opportunities for elaboration in other documents.

   A few years after UTF-8 was already in use in private name spaces in
   DNS, the strategy of using a reserved prefix and an ASCII-compatible
   Encoding (ACE) was developed for IDNA.  That strategy included the
   Punycode algorithm, which began to be developed (during the period
   from 2002 [I-D.ietf-idn-punycode-00] to 2003 [RFC3492]) for use in
   the public DNS name space.  There were a number of reasons for this.
   One such reason the prefixed ACE strategy was selected for the public
   DNS name space had to do with the fact that other encodings such as
   ISO 8859-1 were also in use in DNS and the various encodings were not
   necessarily distinguishable from each other.  Another reason had to
   do with concerns about whether the details of IDNA, including the use
   of the Punycode algorithm, were an adequate solution to the problems
   that were posed.  If either the Punycode algorithm or fundamental
   aspects of character handling were wrong, and had to be changed to
   something incompatible, it would be possible to switch to a new



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   prefix or adopt another model entirely.  Only the part of the public
   DNS namespace that starts a label with "xn--" would be polluted.

   Today the algorithm is seen as being about as good as it can
   realistically be, so moving to a different encoding (UTF-8 as
   suggested in this document) that can be viewed as "native" would not
   be as risky as it would have been in 2002.

   In any case, the publication of [RFC3492] and the dependencies on it
   in [IDNA2008-Protocol] and the earlier [RFC3490] thus resulted in
   having to use different encodings for different name spaces (where
   UTF-8 for private name spaces was already deployed).  Hence,
   referring back to Figure 2, a different encoding scheme may be in use
   on the Internet vs. a local network.

   In general a host may be connected to zero or more networks using
   private name spaces, plus potentially the public name space.
   Applications that convert a U-label form IDN to an A-label before
   calling getaddrinfo() will incur name resolution failures if the name
   is actually registered in a private name space in some other encoding
   (e.g., UTF-8).  Having libraries or protocols convert from A-labels
   to the encoding used by a private name space (e.g., UTF-8) would
   require changes to APIs and/or servers, which IDNA was intended to
   avoid.

   Also, a fully-qualified domain name (FQDN) to be resolved may be
   obtained directly from an application, or it may be composed by the
   DNS resolver itself from a single label obtained from an application
   by using a configured suffix search list, and the resulting FQDN may
   use multiple encodings in different labels.  For more information on
   the suffix search list, see section 6 of "Common DNS Implementation
   Errors and Suggested Fixes" [RFC1536], the DHCP Domain Search Option
   [RFC3397], and section 4 of "DNS Configuration options for DHCPv6"
   [RFC3646].

   As noted in [RFC1536] section 6, the community has had bad
   experiences (e.g., [RFC1535]) with "searching" for domain names by
   trying multiple variations or appending different suffixes.  Such
   searching can yield inconsistent results depending on the order in
   which alternatives are tried.  Nonetheless, the practice is
   widespread and must be considered.

   The practice of searching for names, whether by the use of a suffix
   search list or by searching in different namespaces can yield
   inconsistent results.  For example, even when a suffix search list is
   only used when an application provides a name containing no dots, two
   clients with different configured suffix search lists can get
   different answers, and the same client could get different answers at



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   different times if it changes its configuration (e.g., when moving to
   another network).  A deeper discussion of this topic is outside the
   scope of this document.

3.1.  Examples

   Some examples of cases that can happen in existing implementations
   today (where {non-ASCII} below represents some user-entered non-ASCII
   string) are:
   1.  User types in {non-ASCII}.{non-ASCII}.com, and the application
       passes it, in the form of a UTF-8 string, to getaddrinfo or
       gethostbyname or equivalent.
       *  The DNS resolver passes the (UTF-8) string unmodified to a DNS
          server.
   2.  User types in {non-ASCII}.{non-ASCII}.com, and the application
       passes it to a name resolution API that accepts strings in some
       other encoding such as UTF-16, e.g., GetAddrInfoW on Windows.
       *  The name resolution API decides to pass the string to DNS (and
          possibly other protocols).
       *  The DNS resolver converts the name from UTF-16 to UTF-8 and
          passes the query to a DNS server.
   3.  User types in {non-ASCII}.{non-ASCII}.com, but the application
       first converts it to A-label form such that the name that is
       passed to name resolution APIs is (say) xn--e1afmkfd.xn--
       80akhbyknj4f.com.
       *  The name resolution API decides to pass the string to DNS (and
          possibly other protocols).
       *  The DNS resolver passes the string unmodified to a DNS server.
       *  If the name is not found in DNS, the name resolution API
          decides to try another protocol, say mDNS.
       *  The query goes out in mDNS, but since mDNS specified that
          names are to be registered in UTF-8, the name isn't found
          since it was encoded as an A-label in the query.
   4.  User types in {non-ASCII}, and the application passes it, in the
       form of a UTF-8 string, to getaddrinfo or equivalent.
       *  The name resolution API decides to pass the string to DNS (and
          possibly other protocols).
       *  The DNS resolver will append suffixes in the suffix search
          list, which may contain UTF-8 characters if the local network
          uses a private name space.
       *  Each FQDN in turn will then be sent in a query to a DNS
          server, until one succeeds.
   5.  User types in {non-ASCII}, but the application first converts it
       to an A-label, such that the name that is passed to getaddrinfo
       or equivalent is (say) xn--e1afmkfd.
       *  The name resolution API decides to pass the string to DNS (and
          possibly other protocols).




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       *  The DNS stub resolver will append suffixes in the suffix
          search list, which may contain UTF-8 characters if the local
          network uses a private name space, resulting in (say) xn--
          e1afmkfd.{non-ASCII}.com
       *  Each FQDN in turn will then be sent in a query to a DNS
          server, until one succeeds.
       *  Since the private name space in this case uses UTF-8, the
          above queries fail, since the A-label version of the name was
          not registered in that name space.
   6.  User types in {non-ASCII1}.{non-ASCII2}.{non-ASCII3}.com, where
       {non-ASCII3}.com is a public name space using IDNA and A-labels,
       but {non-ASCII2}.{non-ASCII3}.com is a private name space using
       UTF-8, which is accessible to the user.  The application passes
       the name, in the form of a UTF-8 string, to getaddrinfo or
       equivalent.
       *  The name resolution API decides to pass the string to DNS (and
          possibly other protocols).
       *  The DNS resolver tries to locate the authoritative server, but
          fails the lookup because it cannot find a server for the UTF-8
          encoding of {non-ASCII3}.com, even though it would have access
          to the private name space.  (To make this work, the private
          name space would need to include the UTF-8 encoding of {non-
          ASCII3}.com.)

   When users use multiple applications, some of which do A-label
   conversion prior to passing a name to name resolution APIs, and some
   of which do not, odd behavior can result which at best violates the
   principle of least surprise, and at worst can result in security
   vulnerabilities.

   First consider two competing applications, such as web browsers, that
   are designed to achieve the same task.  If the user types the same
   name into each browser, one may successfully resolve the name (and
   hence access the desired content) because the encoding scheme was
   correct, while the other may fail name resolution because the
   encoding scheme was incorrect.  Hence the issue can incent users to
   switch to another application (which in some cases means switching to
   an IDNA application, and in other cases means switching away from an
   IDNA application).

   Next consider two separate applications where one is designed to be
   launched from the other, for example a web browser launching a media
   player application when the link to a media file is clicked.  If both
   types of content (web pages and media files in this example) are
   hosted at the same IDN in a private name space, but one application
   converts to A-labels before calling name resolution APIs and the
   other does not, the user may be able to access a web page, click on
   the media file causing the media player to launch and attempt to



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   retrieve the media file, which will then fail because the IDN
   encoding scheme was incorrect.  Or even worse, if an attacker was
   able to register the same name in the other encoding scheme, may get
   the content from the attacker's machine.  This is similar to a normal
   phishing attack, except that the two names represent exactly the same
   Unicode characters.


4.  Recommendations

   On many platforms, the name resolution library will automatically use
   a variety of protocols to search a variety of name spaces which might
   be using UTF-8 or other encodings.  In addition, even when only the
   DNS protocol is used, in many operational environments, a private DNS
   name space using UTF-8 is also deployed and is automatically searched
   by the name resolution library.

   As explained earlier, using multiple canonical formats, and multiple
   encodings in different protocols or even in different places in the
   same namespace creates problems.  Because of this, and the fact that
   both IDNA A-labels and UTF-8 are in use as encoding mechanisms for
   domain names today, we recommend the following.

   It is inappropriate for an application that calls a general-purpose
   name resolution library to convert a name to an A-label unless the
   application is absolutely certain that, in all environments where the
   application might be used, only the global DNS that uses IDNA
   A-labels actually will be used to resolve the name.

   Instead, conversion to A-label form, UTF-8, or any other encoding,
   should be done only by an entity that knows which protocol will be
   used (e.g., the DNS resolver, or getaddrinfo upon deciding to pass
   the name to DNS), rather than by general applications that call
   protocol-independent name resolution APIs.  (Of course, it is still
   necessary for applications to convert to whatever form those APIs
   expect.)  Similarly, even when DNS is used, the conversion to
   A-labels should be done only by an entity that knows which name space
   will be used.

   That is, a more intelligent DNS resolver would be more liberal in
   what it would accept from an application and be able to query for
   both a name in A-label form (e.g., over the Internet) and a UTF-8
   name (e.g., over a corporate network with a private name space) in
   case the server only recognized one.  However, we might also take
   into account that the various resolution behaviors discussed earlier
   could also occur with record updates (e.g., with Dynamic Update
   [RFC2136]), resulting in some names being registered in a local
   network's private name space by applications doing conversion to



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   A-labels, and other names being registered using UTF-8.  Hence a name
   might have to be queried with both encodings to be sure to succeed
   without changes to DNS servers.

   Similarly, a more intelligent stub resolver would also be more
   liberal in what it would accept from a response as the value of a
   record (e.g., PTR) in that it would accept either UTF-8 (U-labels in
   the case of IDNA) or A-labels and convert them to whatever encoding
   is used by the application APIs to return strings to applications.

   Indeed the choice of conversion within the resolver libraries is
   consistent with the quote from section 6.2 of the original IDNA
   specification [RFC3490] stating that conversion using the Punycode
   algorithm (i.e., to A-labels) "might be performed inside these new
   versions of the resolver libraries".

   That said, some application-layer protocols (e.g., [RFC5731]) are
   defined to use A-labels rather than simply using UTF-8 as recommended
   by the IETF character sets and languages policy [RFC2277].  In this
   case, an application may receive a string containing A-labels and
   want to pass it to name resolution APIs.  Again the recommendation
   that a resolver library be more liberal in what it would accept from
   an application would mean that such a name would be accepted and re-
   encoded as needed, rather than requiring the application to do so.

   It is important that any APIs used by applications to pass names
   specify what encoding(s) the API uses.  For example, GetAddrInfoW()
   on Windows specifies that it accepts UTF-16.  In contrast, the
   original specification of getaddrinfo() [RFC3493] did not, and hence
   platforms vary in what they use (e.g., MacOS uses UTF-8 whereas
   Windows uses Windows code pages).

   Finally, the question remains about what, if anything, a DNS server
   should do to handle cases where some existing applications or hosts
   do IDNA queries using A-labels within the local network using a
   private name space, and other existing applications or hosts send
   UTF-8 queries.  It is undesirable to store different records for
   different encodings of the same name, since this introduces the
   possibility for inconsistency between them.  Instead, a new DNS
   server serving a private name space using UTF-8 could potentially
   treat encoding-conversion in the same way as case-insensitive
   comparison which a DNS server is already required to do, as long the
   DNS server has some way to know what the encoding is.  Two encodings
   are, in this sense, two representations of the same name, just as two
   case-different strings are.  However, whereas case comparison of non-
   ASCII characters is complicated by ambiguities (as explained in the
   IAB's Review and Recommendations for Internationalized Domain Names
   [RFC4690]), encoding conversion between A-labels and U-labels is



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   unambiguous.


5.  Security Considerations

   Having applications convert names to prefixed ACE format (A-labels)
   before calling name resolution can result in security
   vulnerabilities.  If the name is resolved by protocols or in zones
   for which records are registered using other encoding schemes, an
   attacker can claim the A-label version of the same name and hence
   trick the victim into accessing a different destination.  This can be
   done for any non-ASCII name, even when there is no possible confusion
   due to case, language, or other issues.  Other types of confusion
   beyond those resulting simply from the choice of encoding scheme are
   discussed in "Review and Recommendations for IDNs" [RFC4690].

   Designers and users of encodings that represent Unicode strings in
   terms of ASCII should also consider whether trademark protection or
   phishing are issues, e.g., if one name would be encoded in a way that
   would be naturally associated with another organization or product.


6.  IANA Considerations

   [RFC Editor: please remove this section prior to publication.]

   This document has no IANA Actions.


7.  Acknowledgements

   The authors wish to thank Patrik Faltstrom, Martin Duerst, JFC
   Morfin, Ran Atkinson, S. Moonesamy, Paul Hoffman, and Stephane
   Bortzmeyer for their careful review and helpful suggestions.  It is
   also interesting to note that none of the first three individuals'
   names above can be spelled out and written correctly in ASCII text.
   Furthermore, one of the IAB member's names below (Andrei Robachevsky)
   cannot be written in the script as it appears on his birth
   certificate.












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8.  IAB Members at the time of publication

   Bernard Aboba
   Marcelo Bagnulo
   Ross Callon
   Spencer Dawkins
   Vijay Gill
   Russ Housley
   John Klensin
   Olaf Kolkman
   Danny McPherson
   Jon Peterson
   Andrei Robachevsky
   Dave Thaler
   Hannes Tschofenig


9.  References

9.1.  Normative References

   [10646]    International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS)".

              ISO/IEC Standard 10646, comprised of ISO/IEC 10646-1:2000,
              "Information technology -- Universal Multiple-Octet Coded
              Character Set (UCS) -- Part 1: Architecture and Basic
              Multilingual Plane", ISO/IEC 10646-2:2001, "Information
              technology -- Universal Multiple-Octet Coded Character Set
              (UCS) -- Part 2: Supplementary Planes" and ISO/IEC 10646-
              1:2000/Amd 1:2002, "Mathematical symbols and other
              characters".

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              5.1.0", 2008.

              defined by: The Unicode Standard, Version 5.0, Boston, MA,
              Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
              Unicode 5.1.0
              (http://www.unicode.org/versions/Unicode5.1.0/).

9.2.  Informative References

   [I-D.cheshire-dnsext-multicastdns]
              Cheshire, S. and M. Krochmal, "Multicast DNS",
              draft-cheshire-dnsext-multicastdns-11 (work in progress),
              March 2010.



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   [I-D.ietf-idn-punycode-00]
              Costello, A., "Punycode version 0.3.3",
              draft-ietf-idn-punycode-00 (work in progress), July 2002.

   [I-D.skwan-utf8-dns-00]
              Kwan, S. and J. Gilroy, "Using the UTF-8 Character Set in
              the Domain Name System", draft-skwan-utf8-dns-00 (work in
              progress), November 1997.

   [IDNA2008-Defs]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              January 2010, <https://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-defs/>.

   [IDNA2008-Protocol]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", January 2010, <https://
              datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.

   [MJD]      Duerst, M., "The Properties and Promizes of UTF-8", 11th
              International Unicode Conference, San Jose ,
              September 1997, <http://www.ifi.unizh.ch/mml/mduerst/
              papers/PDF/IUC11-UTF-8.pdf>.

   [NIS]      Sun Microsystems, "System and Network Administration",
              March 1990.

   [RFC0821]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
              RFC 821, August 1982.

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, October 1985.

   [RFC1001]  NetBIOS Working Group, "Protocol standard for a NetBIOS
              service on a TCP/UDP transport: Concepts and methods",
              STD 19, RFC 1001, March 1987.

   [RFC1002]  NetBIOS Working Group, "Protocol standard for a NetBIOS
              service on a TCP/UDP transport: Detailed specifications",
              STD 19, RFC 1002, March 1987.

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

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




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   [RFC1468]  Murai, J., Crispin, M., and E. van der Poel, "Japanese
              Character Encoding for Internet Messages", RFC 1468,
              June 1993.

   [RFC1535]  Gavron, E., "A Security Problem and Proposed Correction
              With Widely Deployed DNS Software", RFC 1535,
              October 1993.

   [RFC1536]  Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
              Miller, "Common DNS Implementation Errors and Suggested
              Fixes", RFC 1536, October 1993.

   [RFC2130]  Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
              Atkinson, R., Crispin, M., and P. Svanberg, "The Report of
              the IAB Character Set Workshop held 29 February - 1 March,
              1996", RFC 2130, April 1997.

   [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.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [RFC3397]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397,
              November 2002.

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

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,



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              December 2003.

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

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795,
              January 2007.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731, August 2009.


Authors' Addresses

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com


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

   Phone: +1 617 245 1457
   Email: john+ietf@jck.com


   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014

   Phone: +1 408 974 3207
   Email: cheshire@apple.com




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