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Network Working Group                                       R. RosenbaumRequest for Comments: 1464                 Digital Equipment Corporation                                                                May 1993Using the Domain Name SystemTo Store Arbitrary String AttributesStatus 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 (=).Rosenbaum                                                       [Page 1]

RFC 1464          Storing Arbitrary Attributes in DNS           May 1993   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).Rosenbaum                                                       [Page 2]

RFC 1464          Storing Arbitrary Attributes in DNS           May 1993   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.Rosenbaum                                                       [Page 3]

RFC 1464          Storing Arbitrary Attributes in DNS           May 19935.  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: rosenbaum@lkg.dec.comRosenbaum                                                       [Page 4]