Skip to main content

Domain Name System (DNS) Case Insensitivity Clarification

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4343.
Author Donald E. Eastlake 3rd
Last updated 2020-01-21 (Latest revision 2005-07-20)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 4343 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Margaret Cullen
Send notices to
INTERNET-DRAFT                                    Donald E. Eastlake 3rd
Updates RFC 1034, 1035                             Motorola Laboratories
Expires January 2006                                           July 2005

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

                         Donald E. Eastlake 3rd

Status of This Document

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Distribution of this document is unlimited. Comments should be sent
   to the DNSEXT working group at

   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-

   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 a "work in progress."

   The list of current Internet-Drafts can be accessed at

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

Copyright Notice

   Copyright (C) The Internet Society (2005). All Rights Reserved.


   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 updates RFCs 1034 and 1035.

D. Eastlake 3rd                                                 [Page 1]

INTERNET-DRAFT                                    DNS Case Insensitivity


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

Table of Contents

      Status of This Document....................................1
      Copyright Notice...........................................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
      3. Name Lookup, Label Types, and CLASS.....................4
      3.1 Original DNS Label Types...............................5
      3.2 Extended Label Type Case Insensitivity Considerations..5
      3.3 CLASS 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
      5. Internationalized Domain Names..........................7
      6. Security Considerations.................................8

      Copyright and Disclaimer...................................9
      Normative References.......................................9
      Informative References....................................10

      Changes Between Draft Version.............................11
      -02 to -03 Changes........................................11
      -03 to -04 Changes........................................11
      -04 to -05 Changes........................................11
      -05 to -06 Changes........................................12

      Author's Address..........................................13
      Expiration and File Name..................................13

D. Eastlake 3rd                                                 [Page 2]

INTERNET-DRAFT                                    DNS Case Insensitivity

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] that are treated in a
   case insensitive fashion. This document clarifies the meaning of
   "case insensitive" for the DNS. This clarification updates RFCs 1034
   and 1035 [STD 13].

   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
   part of the DNS name space. For example,

   Case varied alternatives to the above would be DNS names like

   However, the individual octets of which DNS names consist are not
   limited to valid ASCII character codes. They are 8-bit bytes and all
   values are allowed. Many applications, however, interpret them as
   ASCII characters.

2.1 Escaping Unusual DNS Label Octets

   In Master Files [STD 13] and other human readable and writable ASCII
   contexts, an escape is needed for the byte value for period (0x2E,
   ".") and all octet values outside of the inclusive range of 0x21
   ("!")  to 0x7E ("~"). That is to say, 0x2E and all octet values in
   the two inclusive ranges 0x00 to 0x20 and 0x7F to 0xFF.

D. Eastlake 3rd                                                 [Page 3]

INTERNET-DRAFT                                    DNS Case Insensitivity

   One typographic convention for octets that do not correspond to an
   ASCII printing graphic is to use a back-slash followed by the value
   of the octet as an unsigned integer represented by exactly three
   decimal digits.

   The same convention can be used for printing ASCII characters so that
   they will be treated as a normal label character.  This includes the
   back-slash character used in this convention itself which can be
   expressed as \092 or \\ and the special label separator period (".")
   which can be expressed as and \046 or \.  respectively. It is
   advisable to avoid using a backslash to quote an immediately
   following non-printing ASCII character code to avoid implementation

   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.

2.2 Example Labels with Escapes

   The first example below shows embedded spaces and a period (".")
   within a label. The second one show a 5-octet label where the second
   octet has all bits zero, the third is a backslash, and the fourth
   octet has all bits one.

   and   a\000\\\255z.example.

3. Name Lookup, Label Types, and CLASS

   The original DNS 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 range.

   (Historical Note: the terms "upper case" and "lower case" were
   invented after movable type.  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

D. Eastlake 3rd                                                 [Page 4]

INTERNET-DRAFT                                    DNS Case Insensitivity

   terms were "majuscule" and "minuscule".)

   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.

3.1 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 (or
   compression) 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 herein and, as stated above, can actually contain arbitrary
   byte values. Indirect labels are, in effect, replaced by the name to
   which they point which is then treated with the case insensitivity
   rules in this document.

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
   2673] which is now Experimental [RFC 3363].)

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

3.3 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. With the
   original design of DNS, it was intended that a recursive DNS resolver

D. Eastlake 3rd                                                 [Page 5]

INTERNET-DRAFT                                    DNS Case Insensitivity

   be able to handle new CLASSes that were unknown at the time of its
   implementation. This requires uniform handling of label case
   insensitivity. Should it become desireable, for example, to allocate
   a CLASS with "case sensitive ASCII labels" for example, it would be
   necessary to allocate a new label type for these labels.

4. Case on Input and Output

   While ASCII label comparisons are case insensitive, [STD 13] says
   case MUST be preserved on output, and preserved when convenient on
   input. However, this means less than it would appear since the
   preservation of case on output is NOT required when output is
   optimized by the use of indirect labels, as explained below.

4.1 DNS Output Case Preservation

   [STD 13] views the DNS namespace as a node tree. ASCII output is as
   if a name was marshaled 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, thus "preserving" case.  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, for example the QNAME, is the "same" as the remainder of
   the name being optimized, the case insensitive comparison specified
   above is done. Thus such optimization may easily destroy the output
   preservation of case. This type of optimization is commonly called
   "name compression".

4.2 DNS Input Case Preservation

   Originally, DNS data came from an ASCII Master File as defined in
   [STD 13] or a zone transfer.  DNS Dynamic update and incremental zone
   transfers [RFC 1995] have been added as a source of DNS data [RFC
   2136, 3007]. When a node in the DNS name tree is created by any of
   such inputs, no case conversion is done. Thus the case of ASCII
   labels is preserved if they are for nodes being created. However,
   when a name label is input for a node that already exist in DNS data
   being held, the situation is more complex. Implementations are free
   to retain the case first loaded for such a label or allow new input
   to override the old case or even maintain separate copies preserving

D. Eastlake 3rd                                                 [Page 6]

INTERNET-DRAFT                                    DNS Case Insensitivity

   the input case.

   For example, if data with owner name "" is loaded 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", might or might
   not be changed to "BAR" in the DNS stored data or the actual input
   case could be preserved.  Thus later retrieval of data stored under
   "" in this case can return all data with
   "xyz.BAR.example" or all data with "" or even, when
   more than one RR is being returned, a mixture of these two cases.
   This last case is unlikely because optimization of answer length
   through indirect labels tends to cause only copy of the name tail
   ("bar.example" or "BAR.example") to be used for all returned RRs.
   Note that none of this has any effect on the number of completeness
   of the RR set returned, only on the case of the names in the RR set

   The same considerations apply when inputting multiple data records
   with owner names differing only in case. For example, if an "A"
   record is the first resourced record stored under owner name
   "xyz.BAR.example" and then a second "A" record is stored under
   "XYZ.BAR.example", the second MAY be stored with the first (lower
   case initial label) name or the second MAY override the first so that
   only an upper case initial label is retained or both capitalizations
   MAY be kept in the DNS stored data. In any case, a retrieval with
   either capitalization will retrieve all RRs with either

   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
   unpredictable output capitalization.

5. Internationalized Domain Names

   A scheme has been adopted for "internationalized domain names" and
   "internationalized labels" as described in [RFC 3490, 3454, 3491, and
   3492]. It makes most of [UNICODE] available through a separate
   application level transformation from internationalized domain name
   to DNS domain name and from DNS domain name to internationalized
   domain name. Any case insensitivity that internationalized domain
   names and labels have varies depending on the script and is handled
   entirely as part of the transformation described in [RFC 3454] and
   [RFC 3491] which should be seen for further details.  This is not a
   part of the DNS as standardized in STD 13.

D. Eastlake 3rd                                                 [Page 7]

INTERNET-DRAFT                                    DNS Case Insensitivity

6. 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 a case sensitive index into some data base
   or file 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 mapping 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 6 of [RFC
   4034]. See also [RFC 3597].

   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
   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.

D. Eastlake 3rd                                                 [Page 8]

INTERNET-DRAFT                                    DNS Case Insensitivity

Copyright and Disclaimer

   Copyright (C) The Internet Society (2005).  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

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 1995] - M. Ohta, "Incremental Zone Transfer in DNS", August

   [RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", 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 3007] - B. Wellington, "Secure Domain Name System (DNS) Dynamic
   Update", November 2000.

   [RFC 3597] - Andreas Gustafsson, "Handling of Unknown DNS RR Types",
   draft-ietf-dnsext-unknown-rrs-05.txt, March 2003.

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

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

D. Eastlake 3rd                                                 [Page 9]

INTERNET-DRAFT                                    DNS Case Insensitivity

Informative References

   [ISO 8859-1] - International Standards Organization, Standard for
   Character Encodings, Latin-1.

   [ISO 8859-2] - International Standards Organization, Standard for
   Character Encodings, Latin-2.

   [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.

   [RFC 3092] - D. Eastlake 3rd, C. Manros, E. Raymond, "Etymology of
   Foo", 1 April 2001.

   [RFC 3363] - Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
   Hain, "Representing Internet Protocol version 6 (IPv6) Addresses in
   the Domain Name System (DNS)", RFC 3363, August 2002.

   [RFC 3454] - P. Hoffman, M. Blanchet, "Preparation of
   Internationalized String ("stringprep")", December 2002.

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

   [RFC 3491] - P. Hoffman, M. Blanchet, "Nameprep: A Stringprep Profile
   for Internationalized Domain Names (IDN)", March 2003.

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

   [UNICODE] - The Unicode Consortium, "The Unicode Standard",

D. Eastlake 3rd                                                [Page 10]

INTERNET-DRAFT                                    DNS Case Insensitivity

Changes Between Draft Version

   RFC Editor: The following summaries of changes between draft versions
   are to be removed before publication.

-02 to -03 Changes

   The following changes were made between draft version -02 and -03:

   1. Add internationalized domain name section and references.

   2. Change to indicate that later input of a label for an existing DNS
   name tree node may or may not be normalized to the earlier input or
   override it or both may be preserved.

   3. Numerous minor wording changes.

-03 to -04 Changes

   The following changes were made between draft versions -03 and -04:

   1. Change to conform to the new IPR, Copyright, etc., notice

   2. Change in some section headers for clarity.

   3. Drop section on wildcards.

   4. Add emphasis on loss of case preservation due to name compression.

   5. Add references to RFCs 1995 and 3092.

-04 to -05 Changes

   The following changes were made between draft versions -04 and -05:

   1. More clearly state that this draft updates RFCs 1034, 1035 [STD

   2. Add informative references to ISO 8859-1 and ISO 8859-2.

   3. Fix hyphenation and capitalization nits.

D. Eastlake 3rd                                                [Page 11]

INTERNET-DRAFT                                    DNS Case Insensitivity

-05 to -06 Changes

   The following changes were made between draft version -05 and -06.

   1. Add notation to the RFC Editor that the draft version change
   summaries are to be removed before RFC publication.

   2. Additional text explaining why labe case insensitivity is CLASS

   3. Changes and additional text clarifying that the fact that
   inconsistent case in data loaded into DNS may result in
   unpredicatable or inconsistent case in DNS storage but has no effect
   on the completeness of RR sets retrieved.

   4. Add reference to [RFC 3363] and update reference to [RFC 2535] to
   be to [RFC 4034].

D. Eastlake 3rd                                                [Page 12]

INTERNET-DRAFT                                    DNS Case Insensitivity

Author's Address

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

   Telephone:   +1 508-786-7554 (w)


Expiration and File Name

   This draft expires January 2006.

   Its file name is draft-ietf-dnsext-insensitive-06.txt.

D. Eastlake 3rd                                                [Page 13]