INTERNET-DRAFT                                         Stuart Kwan
                                                       James Gilroy
                                                       Microsoft Corp.
                                                       November 1997
<draft-skwan-utf8-dns-00.txt>                          Expires May 1998


     Using the UTF-8 Character Set in the Domain Name System


Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
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Abstract

The Domain Name System standard specifies that names are represented
using the ASCII character encoding.  This document expands that
specification to allow the use of the UTF-8 character encoding, a
superset of ASCII and a translation of the UCS-2 character encoding.















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. Introduction

The Domain Name System standard [RFC1035] specifies that names are
represented using the ASCII character encoding.  This document expands
that specification to allow the use of the UTF-8 character encoding
[RFC2044], a superset of ASCII and a translation of the UCS-2
character encoding.

Interpreting names as ASCII-only limits the utility of DNS in an
international setting.  The UTF-8 character set includes characters
from most of the world's written languages, allowing a far greater
range of possible names and allowing names to use characters that are
relevant to a particular locality.  UTF-8 is the recommended character
set for protocols that are evolving beyond ASCII [RFC2130].

This document defines the technology for a richer character set in
DNS.  It does not define the policy for the characters allowed in a
name when used by a particular protocol.  Protocol authors are
encouraged to place no restrictions on characters allowed in a name.


. Protocol Description

A UTF-8-aware DNS server is a DNS server that can load and store DNS
names that contain UTF-8 characters.  Names are encoded in logical
order as opposed to visual order (see [UNICODE 2.0]).

Uniform downcasing permits UTF-8-aware DNS implementations to
interoperate with non-UTF-8-aware DNS implementations.  Any binary
string can be used in a DNS name [RFC2181], but names must be
compared with case-insensitivity [RFC1035].  A non-UTF-8-aware DNS
implementation is unable to perform a case-insensitive comparison
on a name containing UTF-8 characters.  However, if UTF-8 names are
downcased before transmission, then binary comparisons will provide
the desired result on non-UTF-8-aware servers without violating the
case-insensitivity requirement.

The DNS protocol standard states that original case should be
preserved when possible as data is entered into the system.  This
requirement is modified as follows:  a UTF-8-aware DNS server must
downcase all names containing UTF-8 characters in both record names
and record data before transmitting those names in any message.
A UTF-8-aware DNS client/resolver must downcase all names containing
UTF-8 characters before transmitting those names in any message.

For consistency, UTF-8-aware DNS servers must compare names that
contain UTF-8 characters byte-for-byte, as opposed to using Unicode
equivalency rules.



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Applications should take care when allowing uppercase UTF-8 characters
to be passed to the resolver, and DNS servers should take care when
allowing uppercase UTF-8 characters to be entered in zone data.
Downcasing in UTF-8 is locale-sensitive and the result may vary
according to the locale of the code execution.  The desired result will
always be obtained if the application and server only accept lowercase
characters.

Names encoded in UTF-8 must not exceed the size limits clarified in
[RFC2181]:  a maximum of 64 octets per label and 255 octets per name.
Character count is insufficient to determine size, since some UTF-8
characters exceed one octet in length.


. Interoperability Considerations

The UTF-8 character encoding is ideal for use with existing protocol
implementations that expect US-ASCII characters.  The representation
of a US-ASCII characters in UTF-8 is byte for byte identical to the
US-ASCII representation.  Non-UTF-8-aware DNS clients always encode
names in ASCII format and those names will always be correctly
interpreted by a UTF-8-aware DNS server.

DNS server authors may wish to provide a configuration switch on the
DNS server to allow/disallow the use of UTF-8 characters on a
per-server or per-zone basis.

A non-UTF-8-aware DNS server may accept a zone transfer of a zone
containing UTF-8 names, but it may not be able to write back those
names to a zone file or reload those names from a zone file.
Administrators should exercise caution when transferring a zone
containing UTF-8 names to a non-UTF-8-aware DNS server.


. Security Considerations

The choice of character encoding for names does not impact the
security of the DNS protocol.


. Acknowledgements

The authors of this document would like to thank the following people
for their contribution to this specification:  John McConnell,
Cliff Van Dyke and Bjorn Rettig.






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. References

[RFC1035]     P.V. Mockapetris, "Domain Names - Implementation and
              Specification," RFC 1035, ISI, Nov 1987.

[RFC2044]     F. Yergeau, "UTF-8, a transformation format of Unicode
              and ISO 10646," RFC 2044, Alis Technologies, Oct 1996.

[RFC2130]     C. Weider et. al., "The Report of the IAB Character
              Set Workshop held 29 February - 1 March 1996",
              RFC 2130, Apr 1997.

[RFC2181]     R. Elz and R. Bush, "Clarifications to the DNS
              Specification," RFC 2181, University of Melbourne and
              RGnet Inc, July 1997.

[UNICODE 2.0] The Unicode Consortium, "The Unicode Standard, Version
              2.0," Addison-Wesley, 1996. ISBN 0-201-48345-9.


. Author's Addresses

Stuart Kwan                         James Gilroy
Microsoft Corporation               Microsoft Corporation
One Microsoft Way                   One Microsoft Way
Redmond, WA  98052                  Redmond, WA  98052
USA                                 USA
<skwan@microsoft.com>               <jamesg@microsoft.com>























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