Movatterモバイル変換

Network Working Group                                    D. Eastlake 3rdRequest for Comments: 4343                         Motorola LaboratoriesUpdates:1034,1035,2181                                   January 2006Category: Standards TrackDomain Name System (DNS) Case Insensitivity ClarificationStatus 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 Notice   Copyright (C) The Internet Society (2006).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 updates RFCs 1034, 1035, and 2181.Table of Contents1. Introduction ....................................................22. Case Insensitivity of DNS Labels ................................22.1. Escaping Unusual DNS Label Octets ..........................22.2. Example Labels with Escapes ................................33. Name Lookup, Label Types, and CLASS .............................33.1. Original DNS Label Types ...................................43.2. Extended Label Type Case Insensitivity Considerations ......43.3. CLASS Case Insensitivity Considerations ....................44. Case on Input and Output ........................................54.1. DNS Output Case Preservation ...............................54.2. DNS Input Case Preservation ................................55. Internationalized Domain Names ..................................66. Security Considerations .........................................67. Acknowledgements ................................................7   Normative References................................................7   Informative References..............................................8Eastlake 3rd                Standards Track                     [Page 1]

RFC 4343          DNS Case Insensitivity Clarification      January 20061.  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 [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,       foo.example.net.       aol.com.       www.gnu.ai.mit.edu.   or  69.2.0.192.in-addr.arpa.   Case-varied alternatives to the above [RFC3092] 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 [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.Eastlake 3rd                Standards Track                     [Page 2]

RFC 4343          DNS Case Insensitivity Clarification      January 2006   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 \.  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.         Donald\032E\.\032Eastlake\0323rd.example.   and   a\000\\\255z.example.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".)Eastlake 3rd                Standards Track                     [Page 3]

RFC 4343          DNS Case Insensitivity Clarification      January 2006   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.Eastlake 3rd                Standards Track                     [Page 4]

RFC 4343          DNS Case Insensitivity Clarification      January 20064.  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   "name compression".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 caseEastlake 3rd                Standards Track                     [Page 5]

RFC 4343          DNS Case Insensitivity Clarification      January 2006   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   capitalization.   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, andRFC3492].  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   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 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 DNSEastlake 3rd                Standards Track                     [Page 6]

RFC 4343          DNS Case Insensitivity Clarification      January 2006   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 inSection 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.7.  Acknowledgements   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.Normative References   [ASCII]      ANSI, "USA Standard Code for Information Interchange",                X3.4, American National Standards Institute: New York,                1968.   [RFC1995]    Ohta, M., "Incremental Zone Transfer in DNS",RFC 1995,                August 1996.   [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.Eastlake 3rd                Standards Track                     [Page 7]

RFC 4343          DNS Case Insensitivity Clarification      January 2006   [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.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.   [RFC1183]    Everhart, C., Mamakos, L., Ullmann, R., and P.                Mockapetris, "New DNS RR Definitions",RFC 1183, October                1990.   [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)",RFC2671, 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,                August 2002.Eastlake 3rd                Standards Track                     [Page 8]

RFC 4343          DNS Case Insensitivity Clarification      January 2006   [RFC3454]    Hoffman, P. and M. Blanchet, "Preparation of                Internationalized Strings ("stringprep")",RFC 3454,                December 2002.   [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)",RFC3491, 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",                <http://www.unicode.org/unicode/standard/standard.html>.Author's Address   Donald E. Eastlake 3rd   Motorola Laboratories   155 Beaver Street   Milford, MA 01757 USA   Phone: +1 508-786-7554 (w)   EMail: Donald.Eastlake@motorola.comEastlake 3rd                Standards Track                     [Page 9]

RFC 4343          DNS Case Insensitivity Clarification      January 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Eastlake 3rd                Standards Track                    [Page 10]