Network Working Group                                       R. Rosenbaum
Request for Comments: 1464                 Digital Equipment Corporation
                                                               May 1993


                    Using the Domain Name System
                 To Store Arbitrary String Attributes

Status of this Memo

  This memo defines an Experimental Protocol for the Internet
  community.  Discussion and suggestions for improvement are requested.
  Please refer to the current edition of the "IAB Official Protocol
  Standards" for the standardization state and status of this protocol.
  Distribution of this memo is unlimited.

Abstract

  While the Domain Name System (DNS) [2,3] is generally used to store
  predefined types of information (e.g., addresses of hosts), it is
  possible to use it to store information that has not been previously
  classified.

  This paper describes a simple means to associate arbitrary string
  information (ASCII text) with attributes that have not been defined
  by the DNS.  It uses DNS TXT resource records to store the
  information.  It requires no change to current DNS implementations.

1.  Introduction

  The Domain Name System is designed to store information that has both
  a predefined type and structure.  Examples include IP addresses of
  hosts and names of mail exchangers.  It would be useful to take
  advantage of the widespread use and scaleability of the DNS to store
  information that has not been previously defined.

  This paper proposes the use of the DNS TXT resource record (defined
  in STD 13, RFC 1035) to contain new types of information.  The
  principal advantage of such an approach is that it requires no change
  to most existing DNS servers.  It is not intended to replace the
  process by which new resource records are defined and implemented.

2.  Format of TXT record

  To store new types of information, the TXT record uses a structured
  format in its TXT-DATA field.  The format consists of the attribute
  name followed by the value of the attribute.  The name and value are
  separated by an equals sign (=).



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  For example, the following TXT records contain attributes specified
  in this fashion:

       host.widgets.com   IN   TXT   "printer=lpr5"
       sam.widgets.com    IN   TXT   "favorite drink=orange juice"

  The general syntax is:

       <owner> <class> <ttl> TXT "<attribute name>=<attribute value>"

  Attribute Names

  Any printable ASCII character is permitted for the attribute name.
  If an equals sign is embedded in the attribute name, it must be
  quoted with a preceding grave accent (or backquote: "`").  A
  backquote must also be quoted with an additional "`".

  Attribute Name Matching Rules

  The attribute name is considered case-insensitive.  For example, a
  lookup of the attribute "Favorite Drink" would match a TXT record
  containing "favorite drink=Earl Grey tea".

  During lookups, TXT records that do not contain an unquoted "=" are
  ignored.  TXT records that seem to contain a null attribute name,
  that is, the TXT-DATA starts with the character "=", are also
  ignored.

  Leading and trailing whitespace (spaces and tabs) in the attribute
  name are ignored unless they are quoted (with a "`").  For example,
  "abc" matches " abc<tab>" but does not match "` abc".

  Note that most DNS server implementations require a backslash (\) or
  double quote (") in a text string to be quoted with a preceding
  backslash.  Accent grave ("`") was chosen as a quoting character in
  this syntax to avoid confusion with "\" (and remove the need for
  confusing strings that include sequences like "\\\\").

  Attribute Values

  All printable ASCII characters are permitted in the attribute value.
  No characters need to be quoted with a "`".  In other words, the
  first unquoted equals sign in the TXT record is the name/value
  delimiter.  All subsequent characters are part of the value.

  Once again, note that in most implementations the backslash character
  is an active quoting character (and must, itself, be quoted).




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  All whitespace in the attribute value is returned to the requestor
  (it is up to the application to decide if it is significant.)

  Examples

  <sp> indicates a space character.

  Attribute    Attribute       Internal Form           External Form
  Name         Value           (server to resolver)    (TXT record)

  color        blue            color=blue              "color=blue"
  equation     a=4             equation=a=4            "equation=a=4"
  a=a          true            a`=a=true               "a`=a=true"
  a\=a false           a\`=a=false             "a\\`=a=false"
  =            \=              `==\=                   "`==\\="

  string       "Cat"           string="Cat"            "string=\"Cat\""
  string2      `abc`           string2=``abc``         "string2=``abc``"
  novalue                      novalue=                "novalue="
  a b          c d             a b=c d                 "a b=c d"
  abc<sp>      123<sp>         abc` =123<sp>           "abc` =123 "

3.  Application Usage

  The attributes can be accessed by the standard resolver library, but
  it is recommended that a library routine designed specially for this
  attribute format be used.  Such a routine might provide an analogue
  to gethostbyname:

        getattributebyname(objectname,          name of object
                           attributename,       name of attribute
                           attributevalue,      pointer to buffer
                           attributevaluelen)   length of buffer

  This routine would remove all quoting characters before returning the
  information to the caller.  A more complex routine could return
  attributes with multiple values, or several different attributes.

4.  Attribute Name Registration

  To permit ease of interoperability and to reduce the chance of naming
  conflicts, a registration process for well known attribute names
  might be established.  This could be a periodically updated list of
  names and/or adherence to other name registration mechanisms such as
  published object identifiers.

  This paper does not address attribute name registration.




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5.  Restrictions

  Some DNS server implementations place limits on the size or number of
  TXT records associated with a particular owner.  Certain
  implementations may not support TXT records at all.

6.  REFERENCES and BIBLIOGRAPHY

  [1] Stahl, M., "Domain Administrators Guide", RFC 1032, Network
      Information Center, SRI International, November 1987.

  [2] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
      13, RFC 1034, USC/Information Sciences Institute, November 1987.

  [3] Mockapetris, P., "Domain Names - Implementation and
      Specification", STD 13, RFC 1035, USC/Information Sciences
      Institute, November 1987.

  [4] Mockapetris, P., "DNS Encoding of Network Names and Other Types",
      RFC 1101, USC/Information Sciences Institute, April 1989.

7.  Security Considerations

  Security issues are not discussed in this memo.

8. Author's Address

  Rich Rosenbaum
  Digital Equipment Corporation
  550 King Street, LKG2-2/Z7
  Littleton, MA  01460-1289

  Phone: 508-486-5922
  Email: [email protected]

















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