rfc3696.txt

Network Working Group                                         J. Klensin
Request for Comments: 3696                                 February 2004
Category: Informational


    Application Techniques for Checking and Transformation of Names

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   Many Internet applications have been designed to deduce top-level
   domains (or other domain name labels) from partial information.  The
   introduction of new top-level domains, especially non-country-code
   ones, has exposed flaws in some of the methods used by these
   applications.  These flaws make it more difficult, or impossible, for
   users of the applications to access the full Internet.  This memo
   discusses some of the techniques that have been used and gives some
   guidance for minimizing their negative impact as the domain name
   environment evolves.  This document draws summaries of the applicable
   rules together in one place and supplies references to the actual
   standards.





















Klensin                      Informational                      [Page 1]

RFC 3696          Checking and Transformation of Names     February 2004


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Restrictions on domain (DNS) names . . . . . . . . . . . . . .  3
   3.  Restrictions on email addresses  . . . . . . . . . . . . . . .  5
   4.  URLs and URIs  . . . . . . . . . . . . . . . . . . . . . . . .  7
       4.1.  URI syntax definitions and issues  . . . . . . . . . . .  7
       4.2.  The HTTP URL . . . . . . . . . . . . . . . . . . . . . .  8
       4.3.  The MAILTO URL . . . . . . . . . . . . . . . . . . . . .  9
       4.4.  Guessing domain names in web contexts  . . . . . . . . . 11
   5.  Implications of internationalization . . . . . . . . . . . . . 11
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 14
       9.2.  Informative References . . . . . . . . . . . . . . . . . 15
   10. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
   11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   Designers of user interfaces to Internet applications have often
   found it useful to examine user-provided values for validity before
   passing them to the Internet tools themselves.  This type of test,
   most commonly involving syntax checks or application of other rules
   to domain names, email addresses, or "web addresses" (URLs or,
   occasionally, extended URI forms (see Section 4)) may enable better-
   quality diagnostics for the user than might be available from the
   protocol itself.  Local validity tests on values are also thought to
   improve the efficiency of back-office processing programs and to
   reduce the load on the protocols themselves.  Certainly, they are
   consistent with the well-established principle that it is better to
   detect errors as early as possible.

   The tests must, however, be made correctly or at least safely.  If
   criteria are applied that do not match the protocols, users will be
   inconvenienced, addresses and sites will effectively become
   inaccessible to some groups, and business and communications
   opportunities will be lost.  Experience in recent years indicates
   that syntax tests are often performed incorrectly and that tests for
   top-level domain names are applied using obsolete lists and
   conventions.  We assume that most of these incorrect tests are the
   result of the inability to conveniently locate exact definitions for
   the criteria to be applied.  This document draws summaries of the
   applicable rules together in one place and supplies references to the





Klensin                      Informational                      [Page 2]

RFC 3696          Checking and Transformation of Names     February 2004


   actual standards.  It does not add anything to those standards; it
   merely draws the information together into a form that may be more
   accessible.

   Many experts on Internet protocols believe that tests and rules of
   these sorts should be avoided in applications and that the tests in
   the protocols and back-office systems should be relied on instead.
   Certainly implementations of the protocols cannot assume that the
   data passed to them will be valid.  Unless the standards specify
   particular behavior, this document takes no position on whether or
   not the testing is desirable.  It only identifies the correct tests
   to be made if tests are to be applied.

   The sections that follow discuss domain names, email addresses, and
   URLs.

2.  Restrictions on domain (DNS) names

   The authoritative definitions of the format and syntax of domain
   names appear in RFCs 1035 [RFC1035], 1123 [RFC1123], and 2181
   [RFC2181].

   Any characters, or combination of bits (as octets), are permitted in
   DNS names.  However, there is a preferred form that is required by
   most applications.  This preferred form has been the only one
   permitted in the names of top-level domains, or TLDs.  In general, it
   is also the only form permitted in most second-level names registered
   in TLDs, although some names that are normally not seen by users obey
   other rules.  It derives from the original ARPANET rules for the
   naming of hosts (i.e., the "hostname" rule) and is perhaps better
   described as the "LDH rule", after the characters that it permits.
   The LDH rule, as updated, provides that the labels (words or strings
   separated by periods) that make up a domain name must consist of only
   the ASCII [ASCII] alphabetic and numeric characters, plus the hyphen.
   No other symbols or punctuation characters are permitted, nor is
   blank space.  If the hyphen is used, it is not permitted to appear at
   either the beginning or end of a label.  There is an additional rule
   that essentially requires that top-level domain names not be all-
   numeric.

   When it is necessary to express labels with non-character octets, or
   to embed periods within labels, there is a mechanism for keying them
   in that utilizes an escape sequence.  RFC 1035 [RFC1035] should be
   consulted if that mechanism is needed (most common applications,
   including email and the Web, will generally not permit those escaped
   strings).  A special encoding is now available for non-ASCII
   characters, see the brief discussion in Section 5.




Klensin                      Informational                      [Page 3]

RFC 3696          Checking and Transformation of Names     February 2004


   Most internet applications that reference other hosts or systems
   assume they will be supplied with "fully-qualified" domain names,
   i.e., ones that include all of the labels leading to the root,
   including the TLD name.  Those fully-qualified domain names are then
   passed to either the domain name resolution protocol itself or to the
   remote systems.  Consequently, purported DNS names to be used in
   applications and to locate resources generally must contain at least
   one period (".") character.  Those that do not are either invalid or
   require the application to supply additional information.  Of course,
   this principle does not apply when the purpose of the application is
   to process or query TLD names themselves.  The DNS specification also
   permits a trailing period to be used to denote the root, e.g.,
   "a.b.c" and "a.b.c." are equivalent, but the latter is more explicit
   and is required to be accepted by applications.  This convention is
   especially important when a TLD name is being referred to directly.
   For example, while ".COM" has become the popular terminology for
   referring to that top-level domain, "COM." would be strictly and
   technically correct in talking about the DNS, since it shows that
   "COM" is a top-level domain name.

   There is a long history of applications moving beyond the "one or
   more periods" test in an attempt to verify that a valid TLD name is
   actually present.  They have done this either by applying some
   heuristics to the form of the name or by consulting a local list of
   valid names.  The historical heuristics are no longer effective.  If
   one is to keep a local list, much more effort must be devoted to
   keeping it up-to-date than was the case several years ago.

   The heuristics were based on the observation that, since the DNS was
   first deployed, all top-level domain names were two, three, or four
   characters in length.  All two-character names were associated with
   "country code" domains, with the specific labels (with a few early
   exceptions) drawn from the ISO list of codes for countries and
   similar entities [IS3166].  The three-letter names were "generic"
   TLDs, whose function was not country-specific, and there was exactly
   one four-letter TLD, the infrastructure domain "ARPA."  [RFC1591].
   However, these length-dependent rules were conventions, rather than
   anything on which the protocols depended.

   Before the mid-1990s, lists of valid top-level domain names changed
   infrequently.  New country codes were gradually, and then more
   rapidly, added as the Internet expanded, but the list of generic
   domains did not change at all between the establishment of the "INT."
   domain in 1988 and ICANN's allocation of new generic TLDs in 2000.
   Some application developers responded by assuming that any two-letter
   domain name could be valid as a TLD, but the list of generic TLDs was
   fixed and could be kept locally and tested.  Several of these
   assumptions changed as ICANN started to allocate new top-level



Klensin                      Informational                      [Page 4]

RFC 3696          Checking and Transformation of Names     February 2004


   domains: one two-letter domain that does not appear in the ISO 3166-1
   table [ISO.3166.1988] was tentatively approved, and new domains were
   created with three, four, and even six letter codes.

   As of the first quarter of 2003, the list of valid, non-country,
   top-level domains was .AERO, .BIZ, .COM, .COOP, .EDU, .GOV, .INFO,
   .INT, .MIL, .MUSEUM, .NAME, .NET, .ORG, .PRO, and .ARPA.  ICANN is
   expected to expand that list at regular intervals, so the list that
   appears here should not be used in testing.  Instead, systems that
   filter by testing top-level domain names should regularly update
   their local tables of TLDs (both "generic" and country-code-related)
   by polling the list published by IANA [DomainList].  It is
   likely that the better strategy has now become to make the "at least
   one period" test, to verify LDH conformance (including verification
   that the apparent TLD name is not all-numeric), and then to use the
   DNS to determine domain name validity, rather than trying to maintain
   a local list of valid TLD names.

   A DNS label may be no more than 63 octets long.  This is in the form
   actually stored; if a non-ASCII label is converted to encoded
   "punycode" form (see Section 5), the length of that form may restrict
   the number of actual characters (in the original character set) that
   can be accommodated.  A complete, fully-qualified, domain name must
   not exceed 255 octets.

   Some additional mechanisms for guessing correct domain names when
   incomplete information is provided have been developed for use with
   the web and are discussed in Section 4.4.

3.  Restrictions on email addresses

   Reference documents: RFC 2821 [RFC2821] and RFC 2822 [RFC2822]

   Contemporary email addresses consist of a "local part" separated from
   a "domain part" (a fully-qualified domain name) by an at-sign ("@").
   The syntax of the domain part corresponds to that in the previous
   section.  The concerns identified in that section about filtering and
   lists of names apply to the domain names used in an email context as
   well.  The domain name can also be replaced by an IP address in
   square brackets, but that form is strongly discouraged except for
   testing and troubleshooting purposes.

   The local part may appear using the quoting conventions described
   below.  The quoted forms are rarely used in practice, but are
   required for some legitimate purposes.  Hence, they should not be
   rejected in filtering routines but, should instead be passed to the
   email system for evaluation by the destination host.




Klensin                      Informational                      [Page 5]

RFC 3696          Checking and Transformation of Names     February 2004


   The exact rule is that any ASCII character, including control
   characters, may appear quoted, or in a quoted string.  When quoting
   is needed, the backslash character is used to quote the following
   character.  For example

      Abc\@def@example.com

   is a valid form of an email address.  Blank spaces may also appear,
   as in

      Fred\ Bloggs@example.com

   The backslash character may also be used to quote itself, e.g.,

      Joe.\\Blow@example.com

   In addition to quoting using the backslash character, conventional
   double-quote characters may be used to surround strings.  For example

      "Abc@def"@example.com

      "Fred Bloggs"@example.com

   are alternate forms of the first two examples above.  These quoted
   forms are rarely recommended, and are uncommon in practice, but, as
   discussed above, must be supported by applications that are
   processing email addresses.  In particular, the quoted forms often
   appear in the context of addresses associated with transitions from
   other systems and contexts; those transitional requirements do still
   arise and, since a system that accepts a user-provided email address
   cannot "know" whether that address is associated with a legacy
   system, the address forms must be accepted and passed into the email
   environment.

   Without quotes, local-parts may consist of any combination of
   alphabetic characters, digits, or any of the special characters

      ! # $ % & ' * + - / = ?  ^ _ ` . { | } ~

   period (".") may also appear, but may not be used to start or end the
   local part, nor may two or more consecutive periods appear.  Stated
   differently, any ASCII graphic (printing) character other than the
   at-sign ("@"), backslash, double quote, comma, or square brackets may
   appear without quoting.  If any of that list of excluded characters
   are to appear, they must be quoted.  Forms such as

      user+mailbox@example.com




Klensin                      Informational                      [Page 6]

RFC 3696          Checking and Transformation of Names     February 2004


      customer/department=shipping@example.com

      $A12345@example.com

      !def!xyz%abc@example.com

      _somename@example.com

   are valid and are seen fairly regularly, but any of the characters
   listed above are permitted.  In the context of local parts,
   apostrophe ("'") and acute accent ("`") are ordinary characters, not
   quoting characters.  Some of the characters listed above are used in
   conventions about routing or other types of special handling by some
   receiving hosts.  But, since there is no way to know whether the
   remote host is using those conventions or just treating these
   characters as normal text, sending programs (and programs evaluating
   address validity) must simply accept the strings and pass them on.

   In addition to restrictions on syntax, there is a length limit on
   email addresses.  That limit is a maximum of 64 characters (octets)
   in the "local part" (before the "@") and a maximum of 255 characters
   (octets) in the domain part (after the "@") for a total length of 320
   characters.  Systems that handle email should be prepared to process
   addresses which are that long, even though they are rarely
   encountered.

4.  URLs and URIs

4.1.  URI syntax definitions and issues

   The syntax for URLs (Uniform Resource Locators) is specified in
   [RFC1738].  The syntax for the more general "URI" (Uniform Resource
   Identifier) is specified in [RFC2396].  The URI syntax is extremely
   general, with considerable variations permitted according to the type
   of "scheme" (e.g., "http", "ftp", "mailto") that is being used.
   While it is possible to use the general syntax rules of RFC 2396 to
   perform syntax checks, they are general enough --essentially only
   specifying the separation of the scheme name and "scheme specific
   part" with a colon (":") and excluding some characters that must be
   escaped if used-- to provide little significant filtering or
   validation power.

   The following characters are reserved in many URIs -- they must be
   used for either their URI-intended purpose or must be encoded.  Some
   particular schemes may either broaden or relax these restrictions
   (see the following sections for URLs applicable to "web pages" and
   electronic mail), or apply them only to particular URI component
   parts.



Klensin                      Informational                      [Page 7]

RFC 3696          Checking and Transformation of Names     February 2004


      ; / ? : @ & = + $ , ?

   In addition, control characters, the space character, the double-
   quote (") character, and the following special characters

      < > # %

   are generally forbidden and must either be avoided or escaped, as
   discussed below.

   The colon after the scheme name, and the percent sign used to escape
   characters, are specifically reserved for those purposes, although
   ":" may also be used elsewhere in some schemes.

   When it is necessary to encode these, or other, characters, the
   method used is to replace it with a percent-sign ("%") followed by
   two hexidecimal digits representing its octet value.  See section
   2.4.1 of [RFC2396] for an exact definition.  Unless it is used as a
   delimiter of the URI scheme itself, any character may optionally be
   encoded this way; systems that are testing URI syntax should be
   prepared for these encodings to appear in any component of the URI
   except the scheme name itself.

   A "generic URI" syntax is specified and is more restrictive, but
   using it to test URI strings requires that one know whether or not
   the particular scheme in use obeys that syntax.  Consequently,
   applications that intend to check or validate URIs should normally
   identify the scheme name and then apply scheme-specific tests.  The
   rules for two of those -- HTTP [RFC1738] and MAILTO [RFC2368] URLs --
   are discussed below, but the author of an application which intends
   to make very precise checks, or to reject particular syntax rather
   than just warning the user, should consult the relevant scheme-
   definition documents for precise syntax and relationships.

4.2.  The HTTP URL

   Absolute HTTP URLs consist of the scheme name, a host name (expressed
   as a domain name or IP address), and optional port number, and then,
   optionally, a path, a search part, and a fragment identifier.  These
   are separated, respectively, by a colon and the two slashes that
   precede the host name, a colon, a slash, a question mark, and a hash
   mark ("#").  So we have

      http://host:port/path?search#fragment

      http://host/path/

      http://host/path#fragment



Klensin                      Informational                      [Page 8]

RFC 3696          Checking and Transformation of Names     February 2004


      http://host/path?search

      http://host

   and other variations on that form.  There is also a "relative" form,
   but it almost never appears in text that a user might, e.g., enter
   into a form.  See [RFC2616] for details.

   The characters

      / ; ?

   are reserved within the path and search parts and must be encoded;
   the first of these may be used unencoded, and is often used within
   the path, to designate hierarchy.

4.3.  The MAILTO URL

   MAILTO is a URL type whose content is an email address.  It can be
   used to encode any of the email address formats discussed in Section
   3 above.  It can also support multiple addresses and the inclusion of
   headers (e.g., Subject lines) within the body of the URL.  MAILTO is
   authoritatively defined in RFC 2368 [RFC2368]; anyone expecting to
   accept and test multiple addresses or mail header or body formats
   should consult that document carefully.

   In accepting text for, or validating, a MAILTO URL, it is important
   to note that, while it can be used to encode any valid email address,
   it is not sufficient to copy an email address into a MAILTO URL since
   email addresses may include a number of characters that are invalid
   in, or have reserved uses for, URLs.  Those characters must be
   encoded, as outlined in Section 4.1 above, when the addresses are
   mapped into the URL form.  Conversely, addresses in MAILTO URLs
   cannot, in general, be copied directly into email contexts, since few
   email programs will reverse the decodings (and doing so might be
   interpreted as a protocol violation).

   The following characters may appear in MAILTO URLs only with the
   specific defined meanings given.  If they appear in an email address
   (i.e., for some other purpose), they must be encoded:

      :       The colon in "mailto:"

      < > # " % { } | \ ^ ~ `

      These characters are "unsafe" in any URL, and must always be
      encoded.




Klensin                      Informational                      [Page 9]

RFC 3696          Checking and Transformation of Names     February 2004


   The following characters must also be encoded if they appear in a
   MAILTO URL

      ? & =
         Used to delimit headers and their values when these are encoded
         into URLs.

   Some examples may be helpful:

   +-------------------------+-----------------------------+-----------+
   |      Email address      |         MAILTO URL          |   Notes   |
   +-------------------------+-----------------------------+-----------+
   |     Joe@example.com     |  mailto:joe@example.com     |     1     |
   |                         |                             |           |
   |  user+mailbox@example   |         mailto:             |     2     |
   |          .com           |  user%2Bmailbox@example     |           |
   |                         |          .com               |           |
   |                         |                             |           |
   |  customer/department=   |  mailto:customer%2F         |     3     |
   |  shipping@example.com   | department=shipping@example |           |
   |                         |          .com               |           |
   |                         |                             |           |
   |   $A12345@example.com   |  mailto:$A12345@example     |     4     |
   |                         |          .com               |           |
   |                         |                             |           |
   |  !def!xyz%abc@example   |  mailto:!def!xyz%25abc      |     5     |
   |          .com           |       @example.com          |           |
   |                         |                             |           |
   |  _somename@example.com  |  mailto:_somename@example   |     4     |
   |                         |          .com               |           |
   +-------------------------+-----------------------------+-----------+

                                  Table 1

   Notes on Table

   1.  No characters appear in the email address that require escaping,
       so the body of the MAILTO URL is identical to the email address.

   2.  There is actually some uncertainty as to whether or not the "+"
       characters requires escaping in MAILTO URLs (the standards are
       not precisely clear).  But, since any character in the address
       specification may optionally be encoded, it is probably safer to
       encode it.

   3.  The "/" character is generally reserved in URLs, and must be
       encoded as %2F.




Klensin                      Informational                     [Page 10]

RFC 3696          Checking and Transformation of Names     February 2004


   4.  Neither the "$" nor the "_" character are given any special
       interpretation in MAILTO URLs, so need not be encoded.

   5.  While the "!" character has no special interpretation, the "%"
       character is used to introduce encoded sequences and hence it
       must always be encoded.

4.4.  Guessing domain names in web contexts

   Several web browsers have adopted a practice that permits an
   incomplete domain name to be used as input instead of a complete URL.
   This has, for example, permitted users to type "microsoft" and have
   the browser interpret the input as "http://www.microsoft.com/".
   Other browser versions have gone even further, trying to build DNS
   names up through a series of heuristics, testing each variation in
   turn to see if it appears in the DNS, and accepting the first one
   found as the intended domain name.  Still, others automatically
   invoke search engines if no period appears or if the reference fails.
   If any of these approaches are to be used, it is often critical that
   the browser recognize the complete list of TLDs.  If an incomplete
   list is used, complete domain names may not be recognized as such and
   the system may try to turn them into completely different names.  For
   example, "example.aero" is a fully-qualified name, since "AERO." is a
   TLD name.  But, if the system doesn't recognize "AERO" as a TLD name,
   it is likely to try to look up "example.aero.com" and
   "www.example.aero.com" (and then fail or find the wrong host), rather
   than simply looking up the user-supplied name.

   As discussed in Section 2 above, there are dangers associated with
   software that attempts to "know" the list of top-level domain names
   locally and take advantage of that knowledge.  These name-guessing
   heuristics are another example of that situation: if the lists are
   up-to-date and used carefully, the systems in which they are embedded
   may provide an easier, and more attractive, experience for at least
   some users.  But finding the wrong host, or being unable to find a
   host even when its name is precisely known, constitute bad
   experiences by any measure.

   More generally, there have been bad experiences with attempts to
   "complete" domain names by adding additional information to them.
   These issues are described in some detail in RFC 1535 [RFC1535].

5.  Implications of internationalization

   The IETF has adopted a series of proposals ([RFC3490] - [RFC3492])
   whose purpose is to permit encoding internationalized (i.e., non-
   ASCII) names in the DNS.  The primary standard, and the group
   generically, are known as "IDNA".  The actual strings stored in the



Klensin                      Informational                     [Page 11]

RFC 3696          Checking and Transformation of Names     February 2004


   DNS are in an encoded form: the labels begin with the characters
   "xn--" followed by the encoded string.  Applications should be
   prepared to accept and process the encoded form (those strings are
   consistent with the "LDH rule" (see Section 2) so should not raise
   any separate issues) and the use of local, and potentially other,
   characters as appropriate to local systems and circumstances.

   The IDNA specification describes the exact process to be used to
   validate a name or encoded string.  The process is sufficiently
   complex that shortcuts or heuristics, especially for versions of
   labels written directly in Unicode or other coded character sets, are
   likely to fail and cause problems.  In particular, the strings cannot
   be validated with syntax or semantic rules of any of the usual sorts:
   syntax validity is defined only in terms of the result of executing a
   particular function.

   In addition to the restrictions imposed by the protocols themselves,
   many domains are implementing rules about just which non-ASCII names
   they will permit to be registered (see, e.g., [JET], [RegRestr]).
   This work is still relatively new, and the rules and conventions are
   likely to be different for each domain, or at least each language or
   script group.  Attempting to test for those rules in a client program
   to see if a user-supplied name might possibly exist in the relevant
   domain would almost certainly be ill-advised.

   One quick local test however, may be reasonable: as of the time of
   this writing, there should be no instances of labels in the DNS that
   start with two characters, followed by two hyphens, where the two
   characters are not "xn" (in, of course, either upper or lower case).
   Such label strings, if they appear, are probably erroneous or
   obsolete, and it may be reasonable to at least warn the user about
   them.

   There is ongoing work in the IETF and elsewhere to define
   internationalized formats for use in other protocols, including email
   addresses.  Those forms may or may not conform to existing rules for
   ASCII-only identifiers; anyone designing evaluators or filters should
   watch that work closely.

6.  Summary

   When an application accepts a string from the user and ultimately
   passes it on to an API for a protocol, the desirability of testing or
   filtering the text in any way not required by the protocol itself is
   hotly debated.  If it must divide the string into its components, or
   otherwise interpret it, it obviously must make at least enough tests
   to validate that process.  With, e.g., domain names or email
   addresses that can be passed on untouched, the appropriateness of



Klensin                      Informational                     [Page 12]

RFC 3696          Checking and Transformation of Names     February 2004


   trying to figure out which ones are valid and which ones are not
   requires a more complex decision, one that should include
   considerations of how to make exactly the correct tests and to keep
   information that changes and evolves up-to-date.  A test containing
   obsolete information, can be extremely frustrating for potential
   correspondents or customers and may harm desired relationships.

7.  Security Considerations

   Since this document merely summarizes the requirements of existing
   standards, it does not introduce any new security issues.  However,
   many of the techniques that motivate the document raise important
   security concerns of their own.  Rejecting valid forms of domain
   names, email addresses, or URIs often denies service to the user of
   those entities.  Worse, guessing at the user's intent when an
   incomplete address, or other string, is given can result in
   compromises to privacy or accuracy of reference if the wrong target
   is found and returned.  From a security standpoint, the optimum
   behavior is probably to never guess, but instead, to force the user
   to specify exactly what is wanted.  When that position involves a
   tradeoff with an acceptable user experience, good judgment should be
   used and the fact that it is a tradeoff recognized.

   Some characters have special or privileged meanings on some systems
   (i.e., ` on Unix).  Applications should be careful to escape those
   locally if necessary.  By the same token, they are valid, and should
   not be disallowed locally, or escaped when transmitted through
   Internet protocols, for such reasons if a remote site chooses to use
   them.

   The presence of local checking does not permit remote checking to be
   bypassed.  Note that this can apply to a single machine; in
   particular, a local MTA should not assume that a local MUA has
   properly escaped locally-significant special characters.

8.  Acknowledgements

   The author would like to express his appreciation for helpful
   comments from Harald Alvestrand, Eric A. Hall, and the RFC Editor,
   and for partial support of this work from SITA.  Responsibility for
   any errors remains, of course, with the author.

   The first Internet-Draft on this subject was posted in February 2003.
   The document was submitted to the RFC Editor on 20 June 2003,
   returned for revisions on 19 August, and resubmitted on 5 September
   2003.





Klensin                      Informational                     [Page 13]

RFC 3696          Checking and Transformation of Names     February 2004


9.  References

9.1.  Normative References

   [RFC1035]       Mockapetris, P., "Domain names - implementation and
                   specification", STD 13, RFC 1035, November 1987.

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

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

   [RFC1738]       Berners-Lee, T., Masinter, L. and M. McCahill,
                   "Uniform Resource Locators (URL)", RFC 1738, December
                   1994.

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

   [RFC2368]       Hoffman, P., Masinter, L. and J. Zawinski, "The
                   mailto URL scheme", RFC 2368, July 1998.

   [RFC2396]       Berners-Lee, T., Fielding, R. and L. Masinter,
                   "Uniform Resource Identifiers (URI): Generic Syntax",
                   RFC 2396, August 1998.

   [RFC2616]       Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                   Masinter, L., Leach, P. and T. Berners-Lee,
                   "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                   June 1999.

   [RFC2821]       Klensin, J., Ed., "Simple Mail Transfer Protocol",
                   RFC 2821, April 2001.

   [RFC2822]       Resnick, P., Ed., "Internet Message Format", RFC
                   2822, April 2001.

   [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.




Klensin                      Informational                     [Page 14]

RFC 3696          Checking and Transformation of Names     February 2004


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

   [ASCII]         American National Standards Institute (formerly
                   United States of America Standards Institute), "USA
                   Code for Information Interchange", ANSI X3.4-1968.
                   ANSI X3.4-1968 has been replaced by newer versions
                   with slight modifications, but the 1968 version
                   remains definitive for the Internet.

   [DomainList]    Internet Assigned Numbers Authority (IANA), Untitled
                   alphabetical list of current top-level domains.
                   http://data.iana.org/TLD/tlds-alpha-by-domain.txt
                   ftp://data.iana.org/TLD/tlds-alpha-by-domain.txt

9.2.  Informative References

   [ISO.3166.1988] International Organization for Standardization,
                   "Codes for the representation of names of countries,
                   3rd edition", ISO Standard 3166, August 1988.

   [JET]           Konishi, K., et al., "Internationalized Domain Names
                   Registration and Administration Guideline for
                   Chinese, Japanese and Korean", Work in Progress.

   [RFC1591]       Postel, J., "Domain Name System Structure and
                   Delegation", RFC 1591, March 1994.

   [RegRestr]      Klensin, J., "Registration of Internationalized
                   Domain Names: Overview and Method", Work in Progress,
                   February 2004.

10.  Author's Address

   John C Klensin
   1770 Massachusetts Ave, #322
   Cambridge, MA  02140
   USA

   Phone: +1 617 491 5735
   EMail: john-ietf@jck.com









Klensin                      Informational                     [Page 15]

RFC 3696          Checking and Transformation of Names     February 2004


11.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  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.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









Klensin                      Informational                     [Page 16]