INTERNET-DRAFT Donald E. Eastlake 3rd
Clarifies STD0013 Motorola Laboratories
Expires August 2003 February 2003
Domain Name System (DNS) Case Insensitivity Clarification
------ ---- ------ ----- ---- ------------- -------------
<draft-ietf-dnsext-insensitive-01.txt>
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
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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.
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Acknowledgements
The contributions to this document of Rob Austein, Olafur
Gudmundsson, Daniel J. Anderson, Alan Barrett, and Andrew Main 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
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
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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
application.
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.
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.
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.
Donald\032E\.\032Eastlake\0323rd.example.
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
range.
(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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"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.
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
document.
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 it the BINARY type [RFC
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2673].)
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 creted. However, no change is
made in the name label on nodes that already exist is 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 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 wild card
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
1999.
[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-01.txt.
D. Eastlake 3rd [Page 10]
