|Title||Domain Name System (DNS) Case Insensitivity Clarification
Network Working Group D. Eastlake 3rd
Request for Comments: 4343 Motorola Laboratories
Updates: 1034, 1035, 2181 January 2006
Category: Standards Track
Domain Name System (DNS) Case Insensitivity Clarification
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
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, 1035, and 2181.
Table of Contents
1. Introduction ....................................................2
2. Case Insensitivity of DNS Labels ................................2
2.1. Escaping Unusual DNS Label Octets ..........................2
2.2. Example Labels with Escapes ................................3
3. Name Lookup, Label Types, and CLASS .............................3
3.1. Original DNS Label Types ...................................4
3.2. Extended Label Type Case Insensitivity Considerations ......4
3.3. CLASS Case Insensitivity Considerations ....................4
4. Case on Input and Output ........................................5
4.1. DNS Output Case Preservation ...............................5
4.2. DNS Input Case Preservation ................................5
5. Internationalized Domain Names ..................................6
6. Security Considerations .........................................6
7. Acknowledgements ................................................7
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 [STD13, RFC1591, RFC2606] that are treated in
a case insensitive fashion. This document clarifies the meaning of
"case insensitive" for the DNS. This clarification updates RFCs
1034, 1035 [STD13], and [RFC2181].
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 [RFC2119].
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 to be numeric, as in the in-addr.arpa
part of the DNS name space. For example,
Case-varied alternatives to the above [RFC3092] would be DNS names
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 [STD13] 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 from 0x21
("!") to 0x7E ("~"). That is to say, 0x2E and all octet values in
the two inclusive ranges from 0x00 to 0x20 and from 0x7F to 0xFF.
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
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 \. 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 shows 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.
3. Name Lookup, Label Types, and CLASS
According to the original DNS design decision, comparisons on name
lookup for DNS queries should be case insensitive [STD13]. That is
to say, a lookup string octet with a value in the inclusive range
from 0x41 to 0x5A, the uppercase ASCII letters, MUST match the
identical value and also match the corresponding value in the
inclusive range from 0x61 to 0x7A, the lowercase ASCII letters. A
lookup string octet with a lowercase ASCII letter value MUST
similarly match the identical value and also match the corresponding
value in the uppercase ASCII letter range.
(Historical note: The terms "uppercase" and "lowercase" 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".)
One way to implement this rule would be to subtract 0x20 from all
octets in the inclusive range from 0x61 to 0x7A before comparing
octets. 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 [STD13] had only two types: ASCII labels,
with a length 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 [RFC2671] so that additional label type numbers
would be available. (The only such type defined so far is the BINARY
type [RFC2673], which is now Experimental [RFC3363].)
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 [STD13] and [RFC2929], DNS has an additional axis for
data location called CLASS. The only CLASS in global use at this
time is the "IN" (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
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 desirable, for example, to allocate
a CLASS with "case sensitive ASCII labels", 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, [STD13] 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
[STD13] views the DNS namespace as a node tree. ASCII output is as
if a name were 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 are 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
4.2. DNS Input Case Preservation
Originally, DNS data came from an ASCII Master File as defined in
[STD13] or a zone transfer. DNS Dynamic update and incremental zone
transfers [RFC1995] have been added as a source of DNS data [RFC2136,
RFC3007]. 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 exists in DNS data being
held, the situation is more complex. Implementations are free to
retain the case first loaded for such a label, to allow new input to
override the old case, or even to maintain separate copies preserving
the input case.
For example, if data with owner name "foo.bar.example" [RFC3092] 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. Thus,
later retrieval of data stored under "xyz.bar.example" in this case
can use "xyz.BAR.example" in all returned data, use "xyz.bar.example"
in all returned data, or even, when more than one RR is being
returned, use a mixture of these two capitalizations. This last case
is unlikely, as optimization of answer length through indirect labels
tends to cause only one 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 or completeness of the RR set
returned, only on the case of the names in the RR set returned.
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 resource 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, the second MAY override the first so that
only an uppercase 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 [RFC3490, RFC3454,
RFC3491, and RFC3492]. 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 [RFC3454] and [RFC3491], which should be seen for
further details. This is not a part of the DNS as standardized in
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 that 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 database
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
uppercase or lower case. An example of a canonical form for domain
names (and also a canonical ordering for them) appears in Section 6
of [RFC4034]. See also [RFC3597].
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 Responsible Person
(RP) [RFC1183] 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.
The contributions to this document by Rob Austein, Olafur
Gudmundsson, Daniel J. Anderson, Alan Barrett, Marc Blanchet, Dana,
Andreas Gustafsson, Andrew Main, Thomas Narten, and Scott Seligman
are gratefully acknowledged.
[ASCII] ANSI, "USA Standard Code for Information Interchange",
X3.4, American National Standards Institute: New York,
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS
UPDATE)", RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security
Extensions", RFC 4034, March 2005.
[STD13] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[ISO-8859-1] International Standards Organization, Standard for
Character Encodings, Latin-1.
[ISO-8859-2] International Standards Organization, Standard for
Character Encodings, Latin-2.
[RFC1183] Everhart, C., Mamakos, L., Ullmann, R., and P.
Mockapetris, "New DNS RR Definitions", RFC 1183, October
[RFC1591] Postel, J., "Domain Name System Structure and
Delegation", RFC 1591, March 1994.
[RFC2606] Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC2929] Eastlake 3rd, D., Brunner-Williams, E., and B. Manning,
"Domain Name System (DNS) IANA Considerations", BCP 42,
RFC 2929, September 2000.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC3092] Eastlake 3rd, D., Manros, C., and E. Raymond, "Etymology
of "Foo"", RFC 3092, 1 April 2001.
[RFC3363] 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,
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
[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.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.
[UNICODE] The Unicode Consortium, "The Unicode Standard",
Donald E. Eastlake 3rd
155 Beaver Street
Milford, MA 01757 USA
Phone: +1 508-786-7554 (w)
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