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INTERNET-DRAFT Donald E. Eastlake 3rd
Clarifies STD0013 Motorola Laboratories
Expires December 2004 July 2004
Domain Name System (DNS) Case Insensitivity Clarification
------ ---- ------ ----- ---- ------------- -------------
<draft-ietf-dnsext-insensitive-04.txt>
Donald E. Eastlake 3rd
Status of This Document
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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
http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-
Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
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
consequences.
D. Eastlake 3rd [Page 1]
INTERNET-DRAFT DNS Case Insensitivity
Acknowledgements
The contributions to this document of Rob Austein, Olafur
Gudmundsson, Daniel J. Anderson, Alan Barrett, Marc Blanchet, Dana,
Andreas Gustafsson, Andrew Main, and Scott Seligman are gratefully
acknowledged.
Table of Contents
Status of This Document....................................1
Abstract...................................................1
Acknowledgements...........................................2
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.................................7
Copyright and Disclaimer...................................9
Normative References.......................................9
Informative References....................................10
-02 to -03 Changes........................................10
-03 to -04 Changes........................................11
Author's Address..........................................11
Expiration and File Name..................................11
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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,
foo.example.net.
aol.com.
www.gnu.ai.mit.edu.
or 69.2.0.192.in-addr.arpa.
Case varied alternatives to the above would be DNS names like
Foo.ExamplE.net.
AOL.COM.
WWW.gnu.AI.mit.EDU.
or 69.2.0.192.in-ADDR.ARPA.
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.
One typographic convention for octets that do not correspond to an
D. Eastlake 3rd [Page 3]
INTERNET-DRAFT DNS Case Insensitivity
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
difficulties.
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.
Donald\032E\.\032Eastlake\0323rd.example.
and a\000\\\255z.example.
3. Name Lookup, Label Types, and CLASS
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
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
terms were "majuscule" and "minuscule".)
D. Eastlake 3rd [Page 4]
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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.
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 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].)
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.
D. Eastlake 3rd [Page 5]
INTERNET-DRAFT DNS Case Insensitivity
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 input 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 may retain
the case first input for such a label or allow new input to override
the old case or even maintain separate copies preserving the input
case.
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", might or might
not be changed to "BAR" or the actual input case could be preserved.
Thus later retrieval of data stored under "xyz.bar.example" in this
case can easily return data with "xyz.BAR.example". The same
D. Eastlake 3rd [Page 6]
INTERNET-DRAFT DNS Case Insensitivity
considerations apply when inputting multiple data records with owner
names differing only in case. For example, if an "A" record is stored
as the first resourced record 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.
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.
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
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 8 of [RFC 2535].
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
D. Eastlake 3rd [Page 7]
INTERNET-DRAFT DNS Case Insensitivity
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 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.
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
1996.
[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 2535] - D. Eastlake, "Domain Name System Security Extensions",
March 1999.
[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.
[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
[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
1999.
[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 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
2003.
[UNICODE] - The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/unicode/standard/standard.html>.
-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.
D. Eastlake 3rd [Page 10]
INTERNET-DRAFT DNS Case Insensitivity
-03 to -04 Changes
The following changes were made between draft version -03 and -04:
1. Change to conform to the new IPR, Copyright, etc., notice
requirements.
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.
Author's Address
Donald E. Eastlake 3rd
Motorola Laboratories
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1 508-786-7554 (w)
+1 508-634-2066 (h)
EMail: Donald.Eastlake@motorola.com
Expiration and File Name
This draft expires December 2004.
Its file name is draft-ietf-dnsext-insensitive-04.txt.
D. Eastlake 3rd [Page 11]
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