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Versions: 00 01 02 03 RFC 3696

Network Working Group                                         J. Klensin
Internet-Draft                                              May 27, 2003
Expires: November 25, 2003

   User Interface Evaluation and Filtering of Internet Addresses and

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on November 25, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   Many internet applications have been designed to deduce top-level
   domains (or other domain name labels) from partial information.
   Whether or not this practice is desirable from an overall network
   standpoint, the designers of the applications believe that it leads
   to a better and more responsive user experience.  The introduction of
   new top level domains, expecially 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Restrictions on domain (DNS) names . . . . . . . . . . . . . .  4
   3.  Restrictions on email addresses  . . . . . . . . . . . . . . .  6
   4.  URLs and URIs  . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.1 URI syntax definitions and issues  . . . . . . . . . . . . . .  8
   4.2 The HTTP URL . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.3 The MAILTO URL . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.4 Guessing domain names in web contexts  . . . . . . . . . . . . 11
   5.  Implications of internationalization . . . . . . . . . . . . . 13
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
       Normative References . . . . . . . . . . . . . . . . . . . . . 17
       Non-normative References . . . . . . . . . . . . . . . . . . . 19
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19
       Intellectual Property and Copyright Statements . . . . . . . . 20

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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.  They are also thought to improve the efficiency
   of back-office processing programs and to reduce 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, perhaps by
   assuming that the syntax rules are the same for email addresses and
   URLs, 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 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 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

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2. Restrictions on domain (DNS) names

   The authoritative definitions of the format and syntax of domain
   names appear in RFCs 1035 [2], 1123 [3], and 2181 [6].

   Any characters, or combination of bits, is permitted in DNS names.
   However, there is a preferred form that is required by most
   applications.  That form has been the only form permitted in the
   names of top-level domains (those that appear rightmost when a full
   domain is written out, sometimes erroneously referred to as a
   "suffix"), 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 is known 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 only of 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.

   Internet protocols are designed to work well only when given
   "fully-qualified" domain names, i.e., ones that include all of the
   labels leading to the root, including the TLD name.  Consequently,
   proported 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.

   There is a long history of applications moving beyond the "one or
   more periods" test to trying 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 entitles [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].

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   These length-dependent rules were, however, 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 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 that 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
   domains: one two-letter domain that does not appear in the ISO 3166-1
   table 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 check the
   current list of TLDs (both "generic" and country-code-related)
   published by IANA at http://www.iana.org/domain-names.htm.  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.

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3. Restrictions on email addresses

   Reference documents: RFC 2821 [10] and RFC 2822 [11]

   Contemporary email addresses consist of a "local part" separated from
   a "domain part" by an at-sign ("@").  The syntax of the domain part
   corresponds to that in the previous section, and the same concerns
   about filtering and lists of names apply.  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 and should not be rejected.  Subject to
   the quoting constraints, any ASCII character, including control
   characters, may appear quoted, or in a quoted string.  The backslash
   character is used to quote the following character.  For example


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

      Fred\ Bloggs@example.com

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


   Conventional double-quote characters may be used to surround strings.
   For example


      "Fred Bloggs"@example.com

   are alternate forms of the first two examples above.  The quoted
   forms are rarely recommended, and are uncommon in practice.  The only
   important exception arises with addresses associated with transitions
   from other systems and contexts; those transitional requirements do
   still arise.

   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

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   local part, nor may two or more consecutive periods appear.  Forms
   such as






   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.

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4. URLs and URIs

4.1 URI syntax definitions and issues

   The syntax for URLs (Uniform Resource Locators) is specified in [4]
   The syntax for the more general "URI" (Uniform Resource Identifier)
   is specified in [8].  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

      ; / ? : @ & = + $ , ?

   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.

   When it is necessary to encode these, or other, characters, the
   method used is replace it with a percent-sign ("%") followed by two
   hexidecimal digits representing its octet value.   See section 2.4.1
   of [8] 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 [5] and MAILTO [7] URLs -- are
   discussed below, but the author of an application which intends to
   make very precise checks, or to reject particular syntax rather than

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   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
   preceed the host name, a colon, a slash, a question mark, and a hash
   mark ("#").  So we have






   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 [9] 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.


   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 inclusion of
   headers (e.g., Subject lines) within the body of the URL.   MAILTO is
   authoritatively defined in RFC 2368 [7]; 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

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   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 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

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

      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.c |                   1 |
   |                      |          om          |                     |
   |                      |                      |                     |
   | user+mailbox@example |        mailto:       |                   2 |
   |         .com         | user%2Bmailbox@examp |                     |
   |                      |        le.com        |                     |
   |                      |                      |                     |
   | customer/department= | mailto:customer%2Fde |                   3 |
   | shipping@example.com | partment=shipping@ex |                     |
   |                      |       ample.com      |                     |
   |                      |                      |                     |
   |  $A12345@example.com | mailto:$A12345@examp |                   4 |
   |                      |        le.com        |                     |
   |                      |                      |                     |
   | !def!xyz%abc@example | mailto:!def!xyz%25ab |                   5 |
   |         .com         |     c@example.com    |                     |
   |                      |                      |                     |
   | _somename@example.co | mailto:_somename@exa |                   4 |
   |           m          |       mple.com       |                     |

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                                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.

   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.  If this approach is 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

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   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.

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5. Implications of internationalization

   The IETF has adopted a series of standards ([12] - [14]) 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 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 both 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 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., [17]. 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

   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 of filters should
   watch that work closely.

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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 protoocl 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
   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.  Making the test
   incorrectly, or with obsolete information, can be extremely
   frustrating for potential correspondents or customers and may harm
   desired relationships.

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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.

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8. Acknowledgements

   ...to be supplied...

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Normative References

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

   [2]   Mockapetris, P., "Domain names - concepts and facilities", RFC
         1035, STD 13, November 1987.

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

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

   [5]   Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
         2.0", RFC 1866, November 1995.

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

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

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

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

   [10]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April

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

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

   [13]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile
         for Internationalized Domain Names (IDN)", RFC 3491, March

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

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         3492, March 2003.

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Non-normative References

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

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

   [17]  Seng, J., "Internationalized Domain Names Registration and
         Administration Guideline for Chinese, Japanese and Korean",
         draft-jseng-idn-admin-02 (work in progress), December 2002.

Author's Address

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

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

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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
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