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INTERNET-DRAFT                                    Donald E. Eastlake 3rd
Clarifies STD0013                                  Motorola Laboratories
Expires August 2003                                        February 2003

       Domain Name System (DNS) Case Insensitivity Clarification
       ------ ---- ------ ----- ---- ------------- -------------

                         Donald E. Eastlake 3rd

Status of This Document

   Distribution of this document is unlimited. Comments should be sent
   to the DNSEXT working group at namedroppers@ops.ietf.org.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   Domain Name System (DNS) names are "case insensitive". This document
   explains exactly what that means and provides a clear specification
   of the rules. This clarification should not have any interoperability

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

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   The contributions to this document of Rob Austein, Olafur
   Gudmundsson, Daniel J. Anderson, Alan Barrett, Dana, Andrew Main, and
   Scott Seligman are gratefully acknowledged.

Table of Contents

      Status of This Document....................................1

      Table of Contents..........................................2

      1. Introduction............................................3
      2. Case Insensitivity of DNS Labels........................3
      2.1 Escaping Unusual DNS Label Octets......................3
      2.2 Example Labels with Escapes............................4
      2.3 Name Lookup Case Insensitivity.........................4
      2.4 Original DNS Label Types...............................5
      3. Additional DNS Case Insensitivity Considerations........5
      3.1 CLASS Case Insensitivity Considerations................5
      3.2 Extended Label Type Case Insensitivity Considerations..5
      4. Case on Input and Output................................6
      4.1 DNS Output Case Preservation...........................6
      4.2 DNS Input Case Preservation............................6
      4.3 Wildcard Matching......................................7
      5. Security Considerations.................................7

      Normative References.......................................9
      Informative References.....................................9

      Author's Address..........................................10
      Expiration and File Name..................................10

D. Eastlake 3rd                                                 [Page 2]

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1. Introduction

   The Domain Name System (DNS) is the global hierarchical replicated
   distributed database system for Internet addressing, mail proxy, and
   other information. Each node in the DNS tree has a name consisting of
   zero or more labels [STD 13][RFC 1591, 2606] which have been
   specified as being treated in a case insensitive fashion. This
   document clarifies the meaning of "case insensitive" for this

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

2. Case Insensitivity of DNS Labels

   DNS was specified in the era of [ASCII]. DNS names were expected to
   look like most host names or Internet email address right halves (the
   part after the at-sign ("@")) or be numeric as in the in-addr.arpa
   part of the DNS name space. For example,


   Case varied alternatives to the above would be DNS names like


   The individual octets of which DNS names consist are not limited to
   valid ASCII character codes. They are defined as 8-bit bytes and all
   values are allowed. Most applications, however, interprete them
   as ASCII characters.

2.1 Escaping Unusual DNS Label Octets

   An escape is needed for all octet values outside of the inclusive
   range of 0x21 ("!") to 0x7E ("~"). That is to say, all octet values in
   the two inclusive ranges 0x00 to 0x20 and 0x7F to 0xFF.

   One typographic convention for octets that do not correspond to an
   ASCII printing graphic is to show them as a back-slash followed by the

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   value of the octet as an unsigned integer represented by exactly three
   decimal digits.  The same convention can be used for printing ASCII
   characters.  This includes the back-slash character used in this
   convention itself and the special label separator period (".") which
   can be expressed as \092 and \046 respectively.

   A back-slash followed by only one or two decimal digits is
   undefined. A back-slash followed by four decimal digits produces two
   octets, the first octet having the value of the first three digits
   considered as a decimal number and the second octet being the
   character code for the fourth decimal digit.

   Octets, other than those corresponding to the ASCII digits 0 through
   9, can also be protected from recognition, so that they will be
   treated as a normal label character, by a second convention:
   preceding them with a back-slash. This is the most commonly used
   technique for protecting back slash ("\") and period ("."). However,
   it is advisable to avoid using this on other than printing ASCII
   characters to avoid implementation difficulties.

2.2 Example Labels with Escapes

   The first example below shows embedded spaces and a period (".")
   within a label. The second one show a 4 octet label where the second
   octet has all bits zero and the third octet has all bits one.

   or  a\000\255z.example.

2.3 Name Lookup Case Insensitivity

   The design decision was made that comparisons on name lookup for DNS
   queries should be case insensitive [STD 13]. That is to say, a lookup
   string octet with a value in the inclusive range of 0x41 to 0x5A, the
   upper case ASCII letters, MUST match the identical value and also
   match the corresponding value in the inclusive range 0x61 to 0x7A,
   the lower case ASCII letters. And a lookup string octet with a lower
   case ASCII letter value MUST similarly match the identical value and
   also match the corresponding value in the upper case ASCII letter

   (Historical Note: the terms "upper case" and "lower case" were
   invented after movable type became wide spread for printing.  The
   terms originally referred to the two font trays for storing, in
   partitioned areas, the different physical type elements.  Before
   movable type, the nearest equivalent terms were "majuscule" and

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   One way to implement this rule would be, when comparing octets, to
   subtract 0x20 from all octets in the inclusive range 0x61 to 0x7A
   before the comparison. Such an operation is commonly known as "case
   folding" but implementation via case folding is not required. Note
   that the DNS case insensitivity does NOT correspond to the case
   folding specified in iso-8859-1 or iso-8859-2. For example, the
   octets 0xDD (\221) and 0xFD (\253) do NOT match although in other
   contexts where they are interpreted as the upper and lower case
   version of "Y" with an acute accent, they might.

2.4 Original DNS Label Types

   DNS labels in wire encoded names have a type associated with them.
   The original DNS standard [RFC 1035] had only two types. ASCII
   labels, with a length of from zero to 63 octets and indirect labels
   which consist of an offset pointer to a name location elsewhere in
   the wire encoding on a DNS message. (The ASCII label of length zero
   is reserved for use as the name of the root node of the name tree.)
   ASCII labels follow the ASCII case conventions described above.
   Indirect labels are, in effect, replaced by the name to which they
   point which is then treated with the case insensitivity rules in this

3. Additional DNS Case Insensitivity Considerations

   This section clarifies the effect of DNS CLASS and extended Label
   Type on case insensitivity.

3.1 CLASS Case Insensitivity Considerations

   As described in [STD 13] and [RFC 2929], DNS has an additional axis
   for data location called CLASS. The only CLASS in global use at this
   time is the "IN" or Internet CLASS.

   The handling of DNS label case is not CLASS dependent.

3.2 Extended Label Type Case Insensitivity Considerations

   DNS was extended by [RFC 2671] to have additional label type numbers
   available. (The only such type defined so far is the BINARY type [RFC

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   The ASCII case insensitivity conventions, or case folding, only apply
   to ASCII labels, that is to say, label type 0x0, whether appearing
   directly or invoked by indirect labels.

4. Case on Input and Output

   While ASCII label comparisons are case insensitive, case MUST be
   preserved on output, except when output is optimized by the use of
   indirect labels, and preserved when possible on input.

4.1 DNS Output Case Preservation

   [STD 13] views the DNS namespace as a node tree. ASCII output is as
   if a name was marshalled by taking the label on the node whose name
   is to be output, converting it to a typographically encoded ASCII
   string, walking up the tree outputting each label encountered, and
   preceding all labels but the first with a period ("."). Wire output
   follows the same sequence but each label is wire encoded and no
   periods inserted. No "case conversion" or "case folding" is done
   during such output operations.  However, to optimize output, indirect
   labels may be used to point to names elsewhere in the DNS answer. In
   determining whether the name to be pointed to is the "same" as the
   remainder of the name being optimized, the case insensitive
   comparison specified above is done. Thus such optimization MAY
   destroy the output preservation of case. This type of optimization is
   commonly called "name compression".

4.2 DNS Input Case Preservation

   Originally, DNS input came from an ASCII Master File as defined in
   [STD 13]. DNS Dynamic update has been added as a source of DNS data
   [RFC 2136, 3007]. When a node in the DNS name tree is created by such
   input, no case conversion is done and the case of ASCII labels is
   preserved if they are for nodes being created. However, no change is
   made in the name label on nodes that already exist in the DNS data
   being augmented or updated. It is quite common for higher level nodes
   to already exist.

   For example, if data with owner name "foo.bar.example" is input and
   then later data with owner name "xyz.BAR.example" is input, the name
   of the label on the "bar.example" node, i.e. "bar", is not changed to
   "BAR". Thus later retrieval of data stored under "xyz.bar.example" in

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   this case can easily result is obtaining data with "xyz.BAR.example".
   The same considerations apply when inputting multiple data records
   with owner names differing only in case. From the example above, if
   an "A" record is stored under owner name "xyz.BAR.example" and then a
   second "A" record under "XYZ.BAR.example", the second will be stored
   at the node with the first (lower case initial label) name.

   Note that the order of insertion into a server database of the DNS
   name tree nodes that appear in a Master File is not defined so that
   the results of inconsistent capitalization in a Master File are
   unpredicatable output capitalization.

4.3 Wildcard Matching

   There is one additional instance of note, which reflects the general
   rules that output case reflects input case unless there is
   conflicting capitalization in the DNS database or the output case is
   hidden by name compression. This is when a query matches a wildcard
   in the DNS database at a server. In that case, the answer SHOULD
   reflect the input case of the label or labels that matched the
   wildcard unless they are replaced by an indirect label which MAY
   point to a name with different capitalization.

5. Security Considerations

   The equivalence of certain DNS label types with case differences, as
   clarified in this document, can lead to security problems. For
   example, a user could be confused by believing two domain names
   differing only in case were actually different names.

   Furthermore, a domain name may be used in contexts other than the
   DNS.  It could be used as an index into some case sensitive data base
   system. Or it could be interpreted as binary data by some integrity
   or authentication code system. These problems can usually be handled
   by using a standardized or "canonical" form of the DNS ASCII type
   labels, that is, always map the ASCII letter value octets in ASCII
   labels to some specific pre-chosen case, either upper case or lower
   case. An example of a canonical form for domain names (and also a
   canonical ordering for them) appears in Section 8 of [RFC 2535]. See
   also [UNKRR].

   Finally, a non-DNS name may be stored into DNS with the false
   expectation that case will always be preserved. For example, although
   this would be quite rare, on a system with case sensitive email
   address local parts, an attempt to store two "RP" records that
   differed only in case would probably produce unexpected results that

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   might have security implications. That is because the entire email
   address, including the possibly case sensitive local or left hand
   part, is encoded into a DNS name in a readable fashion where the case
   of some letters might be changed on output as described above.

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Normative References

   [ASCII] - ANSI, "USA Standard Code for Information Interchange",
   X3.4, American National Standards Institute: New York, 1968.

   [RFC 1034, 1035] - See [STD 13].

   [RFC 2119] - "Key words for use in RFCs to Indicate Requirement
   Levels", S.  Bradner, March 1997.

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

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

   [RFC 3007] - B. Wellington, "Secure Domain Name System (DNS) Dynamic
   Update", November 2000.

   [STD 13]
       - P. Mockapetris, "Domain names - concepts and facilities", RFC
   1034, November 1987.
       - P. Mockapetris, "Domain names - implementation and
   specification", RFC 1035, November 1987.

   [UNKRR] - Andreas Gustafsson, "Handling of Unknown DNS RR Types",
   draft-ietf-dnsext-unknown-rrs-04.txt, September 2002.

Informative References

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

   [RFC 2606] - D. Eastlake, A. Panitz, "Reserved Top Level DNS Names",
   June 1999.

   [RFC 2929] - D. Eastlake, E. Brunner-Williams, B. Manning, "Domain
   Name System (DNS) IANA Considerations", September 2000.

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

   [RFC 2673] - M. Crawford, "Binary Labels in the Domain Name System",
   August 1999.

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Author's Address

   Donald E. Eastlake 3rd
   Motorola Laboratories
   155 Beaver Street
   Milford, MA 01757 USA

   Telephone:   +1 508-851-8280 (w)
                +1 508-634-2066 (h)
   EMail:       Donald.Eastlake@motorola.com

Expiration and File Name

   This draft expires August 2003.

   Its file name is draft-ietf-dnsext-insensitive-02.txt.

D. Eastlake 3rd                                                [Page 10]

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