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RFC 2229 - A Dictionary Server Protocol


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Network Working Group                                           R. Faith
Request for Comments: 2229                U. North Carolina, Chapel Hill
Category: Informational                                        B. Martin
                                                     Miranda Productions
                                                            October 1997

                      A Dictionary Server Protocol

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

Abstract

   The Dictionary Server Protocol (DICT) is a TCP transaction based
   query/response protocol that allows a client to access dictionary
   definitions from a set of natural language dictionary databases.

Table of Contents

   1.      Introduction .........................................    2
   1.1.    Requirements .........................................    3
   2.      Protocol Overview ....................................    3
   2.1.    Link Level ...........................................    3
   2.2.    Lexical Tokens .......................................    3
   2.3.    Commands .............................................    4
   2.4.    Responses ............................................    5
   2.4.1.  Status Responses .....................................    5
   2.4.2.  General Status Responses .............................    6
   2.4.3.  Text Responses .......................................    6
   3.      Command and Response Details .........................    7
   3.1.    Initial Connection ...................................    7
   3.2.    The DEFINE Command ...................................    9
   3.3.    The MATCH Command ....................................   10
   3.4.    A Note on Virtual Databases ..........................   12
   3.5.    The SHOW Command .....................................   13
   3.5.1.  SHOW DB ..............................................   13
   3.5.2.  SHOW STRAT ...........................................   13
   3.5.3.  SHOW INFO ............................................   14
   3.5.4.  SHOW SERVER ..........................................   14
   3.6.    The CLIENT Command ...................................   15

   3.7.    The STATUS Command ...................................   15
   3.8.    The HELP Command .....................................   15
   3.9.    The QUIT Command .....................................   16
   3.10.   The OPTION Command ...................................   16
   3.10.1. OPTION MIME ..........................................   16
   3.11.   The AUTH Command .....................................   18
   3.12.   The SASLAUTH Command .................................   18
   4.      Command Pipelining ...................................   20
   5.      URL Specification ....................................   20
   6.      Extensions ...........................................   22
   6.1.    Experimental Command Syntax ..........................   22
   6.2.    Experimental Commands and Pipelining .................   22
   7.      Summary of Response Codes ............................   23
   8.      Sample Conversations .................................   23
   8.1.    Sample 1 - HELP, DEFINE, and QUIT commands ...........   24
   8.2.    Sample 2 - SHOW commands, MATCH command ..............   25
   8.3.    Sample 3 - Server downtime ...........................   26
   8.4.    Sample 4 - Authentication ............................   26
   9.      Security Considerations ..............................   26
   10.     References ...........................................   27
   11.     Acknowledgements .....................................   29
   12.     Authors' Addresses ...................................   29
   13.     Full Copyright Statement .............................   30

1.  Introduction

   For many years, the Internet community has relied on the "webster"
   protocol for access to natural language definitions.  The webster
   protocol supports access to a single dictionary and (optionally) to a
   single thesaurus.  In recent years, the number of publicly available
   webster servers on the Internet has dramatically decreased.

   Fortunately, several freely-distributable dictionaries and lexicons
   have recently become available on the Internet.  However, these
   freely-distributable databases are not accessible via a uniform
   interface, and are not accessible from a single site.  They are often
   small and incomplete individually, but would collectively provide an
   interesting and useful database of English words.  Examples include
   the Jargon file [JARGON], the WordNet database [WORDNET], MICRA's
   version of the 1913 Webster's Revised Unabridged Dictionary
   [WEB1913], and the Free Online Dictionary of Computing [FOLDOC].
   Translating and non-English dictionaries are also becoming available
   (for example, the FOLDOC dictionary is being translated into
   Spanish).

   The webster protocol is not suitable for providing access to a large
   number of separate dictionary databases, and extensions to the
   current webster protocol were not felt to be a clean solution to the
   dictionary database problem.

   The DICT protocol is designed to provide access to multiple
   databases.  Word definitions can be requested, the word index can be
   searched (using an easily extended set of algorithms), information
   about the server can be provided (e.g., which index search strategies
   are supported, or which databases are available), and information
   about a database can be provided (e.g., copyright, citation, or
   distribution information).  Further, the DICT protocol has hooks that
   can be used to restrict access to some or all of the databases.

1.1.  Requirements

   In this document, we adopt the convention discussed in Section 1.3.2
   of [RFC1122] of using the capitalized words MUST, REQUIRED, SHOULD,
   RECOMMENDED, MAY, and OPTIONAL to define the significance of each
   particular requirement specified in this document.

   In brief: "MUST" (or "REQUIRED") means that the item is an absolute
   requirement of the specification; "SHOULD" (or "RECOMMENDED") means
   there may exist valid reasons for ignoring this item, but the full
   implications should be understood before doing so; and "MAY" (or
   "OPTIONAL") means that his item is optional, and may be omitted
   without careful consideration.

2.  Protocol Overview

2.1.  Link Level

   The DICT protocol assumes a reliable data stream such as provided by
   TCP.  When TCP is used, a DICT server listens on port 2628.

   This server is only an interface between programs and the dictionary
   databases.  It does not perform any user interaction or
   presentation-level functions.

2.2.  Lexical Tokens

   Commands and replies are composed of characters from the UCS
   character set [ISO10646] using the UTF-8 [RFC2044] encoding.  More
   specifically, using the grammar conventions from [RFC822]:

                                               ; (  Octal, Decimal.)
   CHAR        =  <any UTF-8 character (1 to 6 octets)>
   CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)
                   character and DEL>          ; (    177,     127.)
   CR          =  <ASCII CR, carriage return>  ; (     15,      13.)
   LF          =  <ASCII LF, linefeed>         ; (     12,      10.)
   SPACE       =  <ASCII SP, space>            ; (     40,      32.)
   HTAB        =  <ASCII HT, horizontal-tab>   ; (     11,       9.)
   <">         =  <ASCII quote mark>           ; (     42,      34.)
   <'>         =  <ASCII single quote mark>    ; (     47,      39.)
   CRLF        =  CR LF
   WS          =  1*(SPACE / HTAB)

   dqstring    =  <"> *(dqtext/quoted-pair) <">
   dqtext      =  <any CHAR except <">, "\", and CTLs>
   sqstring    =  <'> *(dqtext/quoted-pair) <'>
   sqtext      =  <any CHAR except <'>, "\", and CTLs>
   quoted-pair =  "\" CHAR

   atom        =  1*<any CHAR except SPACE, CTLs, <'>, <">, and "\">
   string      =  *<dqstring / sqstring / quoted-pair>
   word        =  *<atom / string>
   description =  *<word / WS>
   text        =  *<word / WS>

2.3.  Commands

   Commands consist of a command word followed by zero or more
   parameters.  Commands with parameters must separate the parameters
   from each other and from the command by one or more space or tab
   characters.  Command lines must be complete with all required
   parameters, and may not contain more than one command.

   Each command line must be terminated by a CRLF.

   The grammar for commands is:

             command     = cmd-word *<WS cmd-param>
             cmd-word    = atom
             cmd-param   = database / strategy / word
             database    = atom
             strategy    = atom

   Commands are not case sensitive.

   Command lines MUST NOT exceed 1024 characters in length, counting all
   characters including spaces, separators, punctuation, and the
   trailing CRLF.  There is no provision for the continuation of command
   lines.  Since UTF-8 may encode a character using up to 6 octets, the
   command line buffer MUST be able to accept up to 6144 octets.

2.4.  Responses

   Responses are of two kinds, status and textual.

2.4.1.  Status Responses

   Status responses indicate the server's response to the last command
   received from the client.

   Status response lines begin with a 3 digit numeric code which is
   sufficient to distinguish all responses.  Some of these may herald
   the subsequent transmission of text.

   The first digit of the response broadly indicates the success,
   failure, or progress of the previous command (based generally on
   [RFC640,RFC821]):

       1yz - Positive Preliminary reply
       2yz - Positive Completion reply
       3yz - Positive Intermediate reply
       4yz - Transient Negative Completion reply
       5yz - Permanent Negative Completion reply

   The next digit in the code indicates the response category:

       x0z - Syntax
       x1z - Information (e.g., help)
       x2z - Connections
       x3z - Authentication
       x4z - Unspecified as yet
       x5z - DICT System (These replies indicate the status of the
             receiver DICT system vis-a-vis the requested transfer
             or other DICT system action.)
       x8z - Nonstandard (private implementation) extensions

   The exact response codes that should be expected from each command
   are detailed in the description of that command.

   Certain status responses contain parameters such as numbers and
   strings.  The number and type of such parameters is fixed for each
   response code to simplify interpretation of the response.  Other
   status responses do not require specific text identifiers.  Parameter

   requirements are detailed in the description of relevant commands.
   Except for specifically detailed parameters, the text following
   response codes is server-dependent.

   Parameters are separated from the numeric response code and from each
   other by a single space.  All numeric parameters are decimal, and may
   have leading zeros.  All string parameters MUST conform to the "atom"
   or "dqstring" grammar productions.

   If no parameters are present, and the server implementation provides
   no implementation-specific text, then there MAY or MAY NOT be a space
   after the response code.

   Response codes not specified in this standard may be used for any
   installation-specific additional commands also not specified.

   These should be chosen to fit the pattern of x8z specified above.
   The use of unspecified response codes for standard commands is
   prohibited.

2.4.2.  General Status Responses

   In response to every command, the following general status responses
   are possible:

             500 Syntax error, command not recognized
             501 Syntax error, illegal parameters
             502 Command not implemented
             503 Command parameter not implemented
             420 Server temporarily unavailable
             421 Server shutting down at operator request

2.4.3.  Text Responses

   Before text is sent a numeric status response line, using a 1yz code,
   will be sent indicating text will follow. Text is sent as a series of
   successive lines of textual matter, each terminated with a CRLF.  A
   single line containing only a period (decimal code 46, ".") is sent
   to indicate the end of the text (i.e., the server will send a CRLF at
   the end of the last line of text, a period, and another CRLF).

   If a line of original text contained a period as the first character
   of the line, that first period is doubled by the DICT server.
   Therefore, the client must examine the first character of each line
   received.  Those that begin with two periods must have those two
   periods collapsed into one period.  Those that contain only a single
   period followed by a CRLF indicate the end of the text response.

   If the OPTION MIME command has been given, all textual responses will
   be prefaced by a MIME header [RFC2045] followed by a single blank
   line (CRLF).  See section 3.10.1 for more details on OPTION MIME.

   Following a text response, a 2yz response code will be sent.

   Text lines MUST NOT exceed 1024 characters in length, counting all
   characters including spaces, separators, punctuation, the extra
   initial period (if needed), and the trailing CRLF.  Since UTF-8 may
   encode a character using up to 6 octets, the text line input buffer
   MUST be able to accept up to 6144 octets.

   By default, the text of the definitions MUST be composed of
   characters from the UCS character set [ISO10644] using the UTF-8
   [RFC2044] encoding.  The UTF-8 encoding has the advantage of
   preserving the full range of 7-bit US ASCII [USASCII] values.
   Clients and servers MUST support UTF-8, even if only in some minimal
   fashion.

3.  Command and Response Details

   Below, each DICT command and appropriate responses are detailed.
   Each command is shown in upper case for clarity, but the DICT server
   is case-insensitive.

   Except for the AUTH and SASLAUTH commands, every command described in
   this section MUST be implemented by all DICT servers.

3.1.  Initial Connection

   When a client initially connects to a DICT server, a code 220 is sent
   if the client's IP is allowed to connect:

             220 text capabilities msg-id

   The code 220 is a banner, usually containing host name and DICT
   server version information.

   The second-to-last sequence of characters in the banner is the
   optional capabilities string, which will allow servers to declare
   support for extensions to the DICT protocol.  The capabilities string
   is defined below:

             capabilities =  ["<" msg-atom *("." msg-atom) ">"]
             msg-atom     =  1*<any CHAR except SPACE, CTLs,
                                "<", ">", ".", and "\">

   Individual capabilities are described by a single msg-atom.  For
   example, the string <html.gzip> might be used to describe a server
   that supports extensions which allow HTML or compressed output.
   Capability names beginning with "x" or "X" are reserved for
   experimental extensions, and SHOULD NOT be defined in any future DICT
   protocol specification.  Some of these capabilities may inform the
   client that certain functionality is available or can be requested.
   The following capabilities are currently defined:

       mime        The OPTION MIME command is supported
       auth        The AUTH command is supported
       kerberos_v4 The SASL Kerberos version 4 mechanism is supported
       gssapi      The SASL GSSAPI [RFC2078] mechanism is supported
       skey        The SASL S/Key [RFC1760] mechanism is supported
       external    The SASL external mechanism is supported

   The last sequence of characters in the banner is a msg-id, similar to
   the format specified in [RFC822].  The simplified description is
   given below:

       msg-id       =  "<" spec ">"            ; Unique message id
       spec         =  local-part "@" domain
       local-part   =  msg-atom *("." msg-atom)
       domain       =  msg-atom *("." msg-atom)

   Note that, in contrast to [RFC822], spaces and quoted pairs are not
   allowed in the msg-id.  This restriction makes the msg-id much easier
   for the client to locate and parse but does not significantly
   decrease any security benefits, since the msg-id may be arbitrarily
   long (as bounded by the response length limits set forth elsewhere in
   this document).

   Note also that the open and close brackets are part of the msg-id and
   should be included in the string that is used to compute the MD5
   checksum.

   This message id will be used by the client when formulating the
   authentication string used in the AUTH command.

   If the client's IP is not allowed to connect, then a code 530 is sent
   instead:

             530 Access denied

   Transient failure responses are also possible:

             420 Server temporarily unavailable
             421 Server shutting down at operator request

   For example, response code 420 should be used if the server cannot
   currently fork a server process (or cannot currently obtain other
   resources required to proceed with a usable connection), but expects
   to be able to fork or obtain these resources in the near future.

   Response code 421 should be used when the server has been shut down
   at operator request, or when conditions indicate that the ability to
   service more requests in the near future will be impossible.  This
   may be used to allow a graceful operator-mediated temporary shutdown
   of a server, or to indicate that a well known server has been
   permanently removed from service (in which case, the text message
   might provide more information).

3.2.  The DEFINE Command

   DEFINE database word

3.2.1.  Description

   This command will look up the specified word in the specified
   database.  All DICT servers MUST implement this command.

   If the database name is specified with an exclamation point (decimal
   code 33, "!"), then all of the databases will be searched until a
   match is found, and all matches in that database will be displayed.
   If the database name is specified with a star (decimal code 42, "*"),
   then all of the matches in all available databases will be displayed.
   In both of these special cases, the databases will be searched in the
   same order as that printed by the "SHOW DB" command.

   If the word was not found, then status code 552 is sent.

   If the word was found, then status code 150 is sent, indicating that
   one or more definitions follow.

   For each definition, status code 151 is sent, followed by the textual
   body of the definition.  The first three space-delimited parameters
   following status code 151 give the word retrieved, the name of the
   database (which is the same as the first column of the SHOW DB
   command), and a short description for the database (which is the same
   as the second column of the SHOW DB command).  The short name is
   suitable for printing as:

             From name:

   before the definition is printed.  This provides source information
   for the user.

   The textual body of each definition is terminated with a CRLF period
   CRLF sequence.

   After all of the definitions have been sent, status code 250 is sent.
   This command can provide optional timing information (which is server
   dependent and is not intended to be parsable by the client).  This
   additional information is useful when debugging and tuning the
   server.

3.2.2.  Responses

       550 Invalid database, use "SHOW DB" for list of databases
       552 No match
       150 n definitions retrieved - definitions follow
       151 word database name - text follows
       250 ok (optional timing information here)

   Response codes 150 and 151 require special parameters as part of
   their text.  The client can use these parameters to display
   information on the user's terminal.

   For code 150, parameters 1 indicates the number of definitions
   retrieved.

   For code 151, parameter 1 is the word retrieved, parameter 2 is the
   database name (the first name as shown by "SHOW DB") from which the
   definition has been retrieved, and parameter 3 is the the short
   database description (the second column of the "SHOW DB" command).

3.3.  The MATCH Command

   MATCH database strategy word

3.3.1.  Description

   This command searches an index for the dictionary, and reports words
   which were found using a particular strategy.  Not all strategies are
   useful for all dictionaries, and some dictionaries may support
   additional search strategies (e.g., reverse lookup).  All DICT
   servers MUST implement the MATCH command, and MUST support the
   "exact" and "prefix" strategies.  These are easy to implement and are
   generally the most useful.  Other strategies are server dependent.

   The "exact" strategy matches a word exactly, although different
   servers may treat non-alphanumeric data differently.  We have found
   that a case-insensitive comparison which ignores non-alphanumeric

   characters and which folds whitespace is useful for English-language
   dictionaries.  Other comparisons may be more appropriate for other
   languages or when using extended character sets.

   The "prefix" strategy is similar to "exact", except that it only
   compares the first part of the word.

   Different servers may implement these algorithms differently.  The
   requirement is that strategies with the names "exact" and "prefix"
   exist so that a simple client can use them.

   Other strategies that might be considered by a server implementor are
   matches based on substring, suffix, regular expressions, soundex
   [KNUTH73], and Levenshtein [PZ85] algorithms.  These last two are
   especially useful for correcting spelling errors.  Other useful
   strategies perform some sort of "reverse" lookup (i.e., by searching
   definitions to find the word that the query suggests).

   If the database name is specified with an exclamation point (decimal
   code 33, "!"), then all of the databases will be searched until a
   match is found, and all matches in that database will be displayed.
   If the database name is specified with a star (decimal code 42, "*"),
   then all of the matches in all available databases will be displayed.
   In both of these special cases, the databases will be searched in the
   same order as that printed by the "SHOW DB" command.

   If the strategy is specified using a period (decimal code 46, "."),
   then the word will be matched using a server-dependent default
   strategy, which should be the best strategy available for interactive
   spell checking.  This is usually a derivative of the Levenshtein
   algorithm [PZ85].

   If no matches are found in any of the searched databases, then status
   code 552 will be returned.

   Otherwise, status code 152 will be returned followed by a list of
   matched words, one per line, in the form:

             database word

   This makes the responses directly useful in a DEFINE command.

   The textual body of the match list is terminated with a CRLF period
   CRLF sequence.

   Following the list, status code 250 is sent, which may include
   server-specific timing and statistical information, as discussed in
   the section on the DEFINE command.

3.3.2.  Responses

        550 Invalid database, use "SHOW DB" for list of databases
        551 Invalid strategy, use "SHOW STRAT" for a list of strategies
        552 No match
        152 n matches found - text follows
        250 ok (optional timing information here)

   Response code 152 requires a special parameter as part of its text.
   Parameter 1 must be the number of matches retrieved.

3.4.  A Note on Virtual Databases

   The ability to search all of the provided databases using a single
   command is given using the special "*" and "!" databases.

   However, sometimes, a client may want to search over some but not all
   of the databases that a particular server provides.  One alternative
   is for the client to use the SHOW DB command to obtain a list of
   databases and descriptions, and then (perhaps with the help of a
   human), select a subset of these databases for an interactive search.
   Once this selection has been done once, the results can be saved, for
   example, in a client configuration file.

   Another alternative is for the server to provide "virtual" databases
   which merge several of the regular databases into one.  For example,
   a virtual database may be provided which includes all of the
   translating dictionaries, but which does not include regular
   dictionaries or thesauri.  The special "*" and "!" databases can be
   considered as names of virtual databases which provide access to all
   of the databases.  If a server implements virtual databases, then the
   special "*" and "!" databases should probably exclude other virtual
   databases (since they merely provide information duplicated in other
   databases).  If virtual databases are supported, they should be
   listed as a regular database with the SHOW DB command (although,
   since "*" and "!" are required, they need not be listed).

   Virtual databases are an implementation-specific detail which has
   absolutely no impact on the DICT protocol.  The DICT protocol views
   virtual and non-virtual databases the same way.

   We mention virtual databases here, however, because they solve a
   problem of database selection which could also have been solved by
   changes in the protocol.  For example, each dictionary could be
   assigned attributes, and the protocol could be extended to specify
   searches over databases with certain attributes.  However, this
   needlessly complicates the parsing and analysis that must be
   performed by the implementation.  Further, unless the classification

   system is extremely general, there is a risk that it would restrict
   the types of databases that can be used with the DICT protocol
   (although the protocol has been designed with human-language
   databases in mind, it is applicable to any read-only database
   application, especially those with a single semi-unique alphanumeric
   key and textual data).

3.5.  The SHOW Command

3.5.1.  SHOW DB

   SHOW DB
   SHOW DATABASES

3.5.1.1.  Description

   Displays the list of currently accessible databases, one per line, in
   the form:

             database description

   The textual body of the database list is terminated with a CRLF
   period CRLF sequence.  All DICT servers MUST implement this command.

   Note that some databases may be restricted due to client domain or
   lack of user authentication (see the AUTH and SASLAUTH commands in
   sections 3.11 and 3.12).  Information about these databases is not
   available until authentication is performed.  Until that time, the
   client will interact with the server as if the additional databases
   did not exist.

3.5.1.2.  Responses

             110 n databases present - text follows
             554 No databases present

        Response code 110 requires a special parameter.  Parameter 1
        must be the number of databases available to the user.

3.5.2.  SHOW STRAT

   SHOW STRAT
   SHOW STRATEGIES

3.5.2.1.  Description

   Displays the list of currently supported search strategies, one per
   line, in the form:

             strategy description

   The textual body of the strategy list is terminated with a CRLF
   period CRLF sequence.  All DICT servers MUST implement this command.

3.5.2.2.  Responses

             111 n strategies available - text follows
             555 No strategies available

   Response code 111 requires a special parameter.  Parameter 1 must be
   the number of strategies available.

3.5.3.  SHOW INFO

   SHOW INFO database

3.5.3.1.  Description

   Displays the source, copyright, and licensing information about the
   specified database.  The information is free-form text and is
   suitable for display to the user in the same manner as a definition.
   The textual body of the information is terminated with a CRLF period
   CRLF sequence.  All DICT servers MUST implement this command.

3.5.3.2.  Responses

             550 Invalid database, use "SHOW DB" for list of databases
             112 database information follows

   These response codes require no special parameters.

3.5.4.  SHOW SERVER

   SHOW SERVER

3.5.4.1.  Description

   Displays local server information written by the local administrator.
   This could include information about local databases or strategies,
   or administrative information such as who to contact for access to
   databases requiring authentication.  All DICT servers MUST implement
   this command.

3.5.4.2.  Responses

             114 server information follows

   This response code requires no special parameters.

3.6.  The CLIENT Command

   CLIENT text

3.6.1.  Description

   This command allows the client to provide information about itself
   for possible logging and statistical purposes.  All clients SHOULD
   send this command after connecting to the server.  All DICT servers
   MUST implement this command (note, though, that the server doesn't
   have to do anything with the information provided by the client).

3.6.2.  Responses

             250 ok (optional timing information here)

   This response code requires no special parameters.

3.7.  The STATUS Command

   STATUS

3.7.1.  Description

   Display some server-specific timing or debugging information.  This
   information may be useful in debugging or tuning a DICT server.  All
   DICT servers MUST implement this command (note, though, that the text
   part of the response is not specified and may be omitted).

3.7.2.  Responses

             210 (optional timing and statistical information here)

   This response code requires no special parameters.

3.8.  The HELP Command

   HELP

3.8.1.  Description

   Provides a short summary of commands that are understood by this
   implementation of the DICT server.  The help text will be presented
   as a textual response, terminated by a single period on a line by
   itself.  All DICT servers MUST implement this command.

3.8.2.  Responses

             113 help text follows

   This response code requires no special parameters.

3.9.  The QUIT Command

   QUIT

3.9.1.  Description

   This command is used by the client to cleanly exit the server.  All
   DICT servers MUST implement this command.

3.9.2.  Responses

             221 Closing Connection

   This response code requires no special parameters.

3.10.  The OPTION Command

3.10.1.  OPTION MIME

   OPTION MIME

3.10.1.1.  Description

   Requests that all text responses be prefaced by a MIME header
   [RFC2045] followed by a single blank line (CRLF).

   If a client requests this option, then the client MUST be able to
   parse Content-Type and Content-transfer-encoding headers, and MUST be
   able to ignore textual responses which have an unsupported content or
   encoding.  A client MUST support the UTF-8 encoding [RFC2044], which
   minimally means that the client MUST recognize UTF-8 multi-octet
   encodings and convert these into some symbol that can be printed by
   the client.

   If a client requests this option, then the server will provide a MIME
   header.  If the header is empty, the text response will start with a
   single blank line (CRLF), in which case a client MUST interpret this
   as a default header.  The default header for SASL authentication is:

             Content-type: application/octet-stream
             Content-transfer-encoding: base64

   The default header for all other textual responses is:

             Content-type: text/plain; charset=utf-8
             Content-transfer-encoding: 8bit

   If OPTION MIME is not specified by the client, then the server may
   restrict the information content provided to the client.  For
   example, a definition may be accompanied by an image and an audio
   clip, but the server cannot transmit this information unless the
   client is able to parse MIME format headers.

   Note that, because of the line length restrictions and end-of-
   response semantics, the "binary" content-transfer-encoding MUST NOT
   be used.  In the future, extensions to the protocol may be provided
   which allow a client to request binary encodings, but the default
   SHOULD always be that the client can look for a "CRLF . CRLF"
   sequence to locate the end of the current text response.  This allows
   clients to easily skip over text responses which have unsupported
   types or encodings.

   In the future, after significant experience with large databases in
   various languages has been gained, and after evaluating the need for
   specifying character sets and other encodings (e.g., compressed or
   BASE64 encoding), standard extensions to this protocol should be
   proposed which allow the client to request certain content types or
   encodings.  Care should be taken that these extensions do not require
   a handshake which defeats pipelining.  In the mean time, private
   extensions should be used to explore the parameter space to determine
   how best to implement these extensions.

   OPTION MIME is a REQUIRED server capability, all DICT servers MUST
   implement this command.

3.10.1.2.  Responses

             250 ok (optional timing information here)

   Note that some older server implementations, completed before this
   document was finalized, will return a status code 500 if this command
   is not implemented.  Clients SHOULD be able to accept this behavior,

   making default assumptions.  Clients may also examine the
   capabilities string in the status code 220 header to determine if a
   server supports this capability.

3.11.  The AUTH Command

   AUTH username authentication-string

3.11.1.  Description

   The client can authenticate itself to the server using a username and
   password.  The authentication-string will be computed as in the APOP
   protocol discussed in [RFC1939].  Briefly, the authentication-string
   is the MD5 checksum of the concatenation of the msg-id (obtained from
   the initial banner) and the "shared secret" that is stored in the
   server and client configuration files.  Since the user does not have
   to type this shared secret when accessing the server, the shared
   secret can be an arbitrarily long passphrase.  Because of the
   computational ease of computing the MD5 checksum, the shared secret
   should be significantly longer than a usual password.

   Authentication may make more dictionary databases available for the
   current session.  For example, there may be some publicly
   distributable databases available to all users, and other private
   databases available only to authenticated users.  Or, a server may
   require authentication from all users to minimize resource
   utilization on the server machine.

   Authentication is an optional server capability.  The AUTH command
   MAY be implemented by a DICT server.

3.11.2.  Responses

             230 Authentication successful
             531 Access denied, use "SHOW INFO" for server information

   These response codes require no special parameters.

3.12.  The SASLAUTH Command

   SASLAUTH mechanism initial-response
   SASLRESP response

3.12.1.  Description

   The Simple Authentication and Security Layer (SASL) is currently
   being developed [RFC2222].  The DICT protocol reserves the SASLAUTH
   and SASLRESP commands for this method of authentication.  The results

   of successful authentication with SALSAUTH will be the same as the
   results of successful AUTH authentication: more dictionary databases
   may become available for the current session.

   The initial-response is an optional parameter for the SASLAUTH
   command, encoded using BASE64 encoding [RFC2045].  Some SASL
   mechanisms may allow the use of this parameter.  If SASL
   authentication is supported by a DICT server, then this parameter
   MUST also be supported.

   A typical SASL authentication will be initiated by the client using
   the SASLAUTH command.  The server will reply with status code 130,
   followed by a challenge.  The challenge will be followed by status
   code 330, indicating that the client must now send a response to the
   server.

   Depending on the details of the SASL mechanism currently in use, the
   server will either continue the exchange using status code 130, a
   challenge, and status code 330; or the server will use status code
   230 or 531 to indicate authentication was successful or has failed.

   The challenges sent by the server are defined by the mechanisms as
   binary tokens of arbitrary length, and should be sent using a
   standard DICT textual response, as described in section 2.4.3.  If
   OPTION MIME is not set, then BASE64 encoding MUST be used.  If

   OPTION MIME is set, then BASE64 is the default encoding, as specified
   in section 3.10.1.

   The client will send all responses using the SASLRESP command and a
   BASE64-encoded parameter.  The responses sent by the client are
   defined by the mechanisms as binary tokens of arbitrary length.
   Remember that DICT command lines may only be 1024 characters in
   length, so the response provided by a DICT client is limited.

   If the mechanism specified in the SASLAUTH command is not supported,
   then status code 532 will be returned.

   Authentication is an optional server capability.  The SASLAUTH
   command MAY be implemented by a DICT server.

3.12.2.  Responses

             130 challenge follows
             330 send response
             230 Authentication successful
             531 Access denied, use "SHOW INFO" for server information
             532 Access denied, unknown mechanism

        These response codes require no special parameters.

4.  Command Pipelining

   All DICT servers MUST be able to accept multiple commands in a single
   TCP send operation.  Using a single TCP send operation for multiple
   commands can improved DICT performance significantly, especially in
   the face of high latency network links.

   The possible implementation problems for a DICT server which would
   prevent command pipelining are similar to the problems that prevent
   pipelining in an SMTP server.  These problems are discussed in detail
   in [RFC1854], which should be consulted by all DICT server
   implementors.

   The main implication is that a DICT server implementation MUST NOT
   flush or otherwise lose the contents of the TCP input buffer under
   any circumstances whatsoever.

   A DICT client may pipeline several commands and must check the
   responses to each command individually.  If the server has shut down,
   it is possible that all of the commands will not be processed.  For
   example, a simple DICT client may pipeline a CLIENT, DEFINE, and QUIT
   command sequence as it is connecting to the server.  If the server is
   shut down, the initial response code sent by the server may be 420
   (temporarily unavailable) instead of 220 (banner).  In this case, the
   definition cannot be retrieved, and the client should report and
   error or retry the command.  If the server is working, it may be able
   to send back the banner, definition, and termination message in a
   single TCP send operation.

5.  URL Specification

   The DICT URL scheme is used to refer to definitions or word lists
   available using the DICT protocol:

   dict://<user>;<auth>@<host>:<port>/d:<word>:<database>:<n>
   dict://<user>;<auth>@<host>:<port>/m:<word>:<database>:<strat>:<n>

   The "/d" syntax specifies the DEFINE command (section 3.2), whereas
   the "/m" specifies the MATCH command (section 3.3).

   Some or all of "<user>;<auth>@", ":<port>", "<database>", "<strat>",
   and "<n>" may be omitted.

   "<n>" will usually be omitted, but when included, it specifies the
   nth definition or match of a word.  A method for extracting exactly
   this information from the server is not available using the DICT
   protocol.  However, a client using the URL specification could obtain
   all of the definitions or matches, and then select the one that is
   specified.

   If "<user>;<auth>@" is omitted, no authentication is done.  If
   ":<port>" is omitted, the default port (2628) SHOULD be used.  If
   "<database>" is omitted, "!" SHOULD be used (see section 3.2).  If
   "<strat>" is omitted, "." SHOULD be used (see section 3.3).

   "<user>;<auth>@" specifies the username and the type of
   authentication performed.  For "<auth>", the string "AUTH" indicates
   that APOP authentication using the AUTH command will be performed,
   whereas the string "SASLAUTH=<auth_type>" indicates that the SASLAUTH
   and SASLRESP commands will be used, with "<auth_type>" indicating the
   type of SASL authentication which will be used.  If "<auth_type>" is
   a star (decimal code 42, "*"), then the client will select some type
   of authentication.

   Whenever authentication is required, the client SHOULD request
   additional information (e.g., a passphrase) from the user.  In
   contrast to [RFC1738], clear text passwords are not permitted in the
   URL.

   Trailing colons may be omitted.  For example, the following URLs
   might specify definitions or matches:

             dict://dict.org/d:shortcake:
             dict://dict.org/d:shortcake:*
             dict://dict.org/d:shortcake:wordnet:
             dict://dict.org/d:shortcake:wordnet:1
             dict://dict.org/d:abcdefgh
             dict://dict.org/d:sun
             dict://dict.org/d:sun::1

             dict://dict.org/m:sun
             dict://dict.org/m:sun::soundex
             dict://dict.org/m:sun:wordnet::1
             dict://dict.org/m:sun::soundex:1
             dict://dict.org/m:sun:::

6.  Extensions

   This protocol was designed so that flat text databases can be used
   with a server after a minimum of analysis and formatting.  Our
   experience is that merely constructing an index for a database may be
   sufficient to make it useful with a DICT server.  The ability to
   serve preformatted text is especially important since freely-
   available databases are often distributed as flat text files without
   any semantic mark-up information (and often contain "ASCII art" which
   precludes the automation of even simple formatting).

   However, given a database with sufficient mark-up information, it may
   be possible to generate output in a variety of different formats
   (e.g., simple HTML or more sophisticated SGML).  The specification of
   formatting is beyond the scope of this document.  The requirements
   for negotiation of format (including character set and other
   encodings) is complex and should be examined over time as more
   experience is gained.  We suggest that the use of different formats,
   as well as other server features, be explored as extensions to the
   protocol.

6.1.  Experimental Command Syntax

   Single-letter commands are reserved for debugging and testing, SHOULD
   NOT be defined in any future DICT protocol specification, and MUST
   NOT be used by any client software.

   Commands beginning with the letter "X" are reserved for experimental
   extensions, and SHOULD NOT be defined in any future DICT protocol
   specification.  Authors of client software should understand that
   these commands are not part of the DICT protocol and may not be
   available on all DICT servers.

6.2.  Experimental Commands and Pipelining

   Experimental commands should be designed so that a client can
   pipeline the experimental commands without knowing if a server
   supports the commands (e.g., instead of using feature negotiation).
   If the server does not support the commands, then a response code in
   the 5yz series (usually 500) will be given, notifying the client that
   the extension is not supported.  Of course, depending on the
   complexity of the extensions added, feature negotiation may be
   necessary.  To help minimize negotiation time, server-supported
   features may be announced in the banner (code 220) using the optional
   capabilities parameter.

7.  Summary of Response Codes

   Below is a summary of response codes.  A star (*) in the first column
   indicates the response has defined arguments that must be provided.

      * 110 n databases present - text follows
      * 111 n strategies available - text follows
        112 database information follows
        113 help text follows
        114 server information follows
        130 challenge follows
      * 150 n definitions retrieved - definitions follow
      * 151 word database name - text follows
      * 152 n matches found - text follows
        210 (optional timing and statistical information here)
      * 220 text msg-id
        221 Closing Connection
        230 Authentication successful
        250 ok (optional timing information here)
        330 send response
        420 Server temporarily unavailable
        421 Server shutting down at operator request
        500 Syntax error, command not recognized
        501 Syntax error, illegal parameters
        502 Command not implemented
        503 Command parameter not implemented
        530 Access denied
        531 Access denied, use "SHOW INFO" for server information
        532 Access denied, unknown mechanism
        550 Invalid database, use "SHOW DB" for list of databases
        551 Invalid strategy, use "SHOW STRAT" for a list of strategies
        552 No match
        554 No databases present
        555 No strategies available

8.  Sample Conversations

   Theses are samples of the conversations that might be expected with
   a typical DICT server.  The notation "C:" indicates commands set by
   the client, and "S:" indicates responses sent by the server.  Blank
   lines are included for clarity and do not indicate actual newlines
   in the transaction.

8.1.  Sample 1 - HELP, DEFINE, and QUIT commands

C: [ client initiates connection ]

S: 220 dict.org dictd (version 0.9) <27831.860032493@dict.org>

C: HELP

S: 113 Help text follows
S: DEFINE database word            look up word in database
S: MATCH database strategy word    match word in database using strategy
S: [ more server-dependent help text ]
S: .
S: 250 Command complete

C: DEFINE ! penguin

S: 150 1 definitions found: list follows
S: 151 "penguin" wn "WordNet 1.5" : definition text follows
S: penguin
S:   1. n: short-legged flightless birds of cold southern esp. Antarctic
S:      regions having webbed feet and wings modified as flippers
S: .
S: 250 Command complete

C: DEFINE * shortcake

S: 150 2 definitions found: list follows
S: 151 "shortcake" wn "WordNet 1.5" : text follows
S: shortcake
S:   1. n: very short biscuit spread with sweetened fruit and usu.
S:      whipped cream
S: .
S: 151 "Shortcake" web1913 "Webster's Dictionary (1913)" : text follows
S: Shortcake
S:    \Short"cake`\, n.
S:    An unsweetened breakfast cake shortened with butter or lard,
S:    rolled thin, and baked.
S: .
S: 250 Command complete

C: DEFINE abcdefgh

S: 552 No match

C: quit

S: 221 Closing connection

8.2.  Sample 2 - SHOW commands, MATCH command

C: SHOW DB

S: 110 3 databases present: list follows
S: wn "WordNet 1.5"
S: foldoc "Free On-Line Dictionary of Computing"
S: jargon "Hacker Jargon File"
S: .
S: 250 Command complete

C: SHOW STRAT

S: 111 5 strategies present: list follows
S: exact "Match words exactly"
S: prefix "Match word prefixes"
S: substring "Match substrings anywhere in word"
S: regex "Match using regular expressions"
S: reverse "Match words given definition keywords"
S: .
S: 250 Command complete

C: MATCH foldoc regex "s.si"

S: 152 7 matches found: list follows
S: foldoc Fast SCSI
S: foldoc SCSI
S: foldoc SCSI-1
S: foldoc SCSI-2
S: foldoc SCSI-3
S: foldoc Ultra-SCSI
S: foldoc Wide SCSI
S: .
S: 250 Command complete

C: MATCH wn substring "abcdefgh"

S: 552 No match

8.3.  Sample 3 - Server downtime

C: [ client initiates connection ]

S: 420 Server temporarily unavailable

C: [ client initiates connection ]

S: 421 Server shutting down at operator request

8.4.  Sample 4 - Authentication

C: [ client initiates connection ]

S: 220 dict.org dictd (version 0.9) <27831.860032493@dict.org>

C: SHOW DB

S: 110 1 database present: list follows
S: free "Free database"
S: .
S: 250 Command complete

C: AUTH joesmith authentication-string

S: 230 Authentication successful

C: SHOW DB

S: 110 2 databases present: list follows
S: free "Free database"
S: licensed "Local licensed database"
S: .
S: 250 Command complete

9.  Security Considerations

   This RFC raises no security issues.

10.  References

   [ASCII] US-ASCII. Coded Character Set - 7-Bit American Standard
        Code for Information Interchange. Standard ANSI X3.4-1986,
        ANSI, 1986.

   [FOLDOC] Howe, Denis, ed.  The Free On-Line Dictionary of
        Computing, <URL:http://wombat.doc.ic.ac.uk/>

   [ISO10646] ISO/IEC 10646-1:1993. International Standard --
        Information technology -- Universal Multiple-Octet Coded
        Character Set (UCS) -- Part 1: Architecture and Basic
        Multilingual Plane.  UTF-8 is described in Annex R, adopted
        but not yet published.  UTF-16 is described in Annex Q,
        adopted but not yet published.

   [JARGON] The on-line hacker Jargon File, version 4.0.0, 25 JUL
        1996, <URL:http://www.ccil.org/jargon/>

   [KNUTH73] Knuth, Donald E. "The Art of Computer Programming",
        Volume 3: Sorting and Searching (Addison-Wesley Publishing
        Co., 1973, pages 391 and 392). Knuth notes that the soundex
        method was originally described by Margaret K. Odell and
        Robert C. Russell [US Patents 1261167 (1918) and 1435663
        (1922)].

   [PZ85] Pollock, Joseph J. and Zamora, Antonio, "Automatic spelling
        correction in scientific and scholarly text," CACM, 27(4):
        Apr. 1985, 358-368.

   [RFC640] Postel, J., "Revised FTP Reply Codes", RFC 640, June,
        1975.

   [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
        RFC 821, August 1982.

   [RFC822] Crocker, D., "Standard for the Format of ARPA Internet
        Text Messages", STD 11, RFC 822, August 1982.

   [RFC977] Kantor, B., and P. Lapsley, "Network News Transfer
        Protocol: A Proposed Standard for the Stream-Based
        Transmission of News", RFC 977, February 1986.

   [RFC2045] Freed, N., and N. Borenstein, "Multipurpose Internet
        Mail Extensions (MIME) Part One: Format of Internet Message
        Bodies", RFC 2045, November 1996.

   [RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
        Resource Locators (URL)", RFC 1738, December 1994.

   [RFC1760] Haller, N., "The S/KEY One-Time Password System",
        RFC 1760, February 1995.

   [RFC1985] Freed, N., and A. Cargille, "SMTP Service Extension for
        Command Pipelining", RFC 1854, October 1995.

   [RFC1939] Myers, J., and M. Rose, "Post Office Protocol - Version 3",
        STD 53, RFC 1939, May 1996.

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

   [RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
        and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
        RFC 2068, January 1997.

   [RFC2078] Linn, J., "Generic Security Service Application Program
        Interface, Version 2", RFC 2078, January 1997.

   [RFC2222] Myers, J., "Simple Authentication and Security Layer
        (SASL)", RFC 2222, October 1997.

   [WEB1913] Webster's Revised Unabridged Dictionary (G & C. Merriam
        Co., 1913, edited by Noah Porter).  Online version prepared by
        MICRA, Inc., Plainfield, N.J. and edited by Patrick Cassidy
        <cassidy@micra.com>.  For further information, see
   <URL:ftp://uiarchive.cso.uiuc.edu/pub/etext/gutenberg/etext96/pgw*>,
        and
   <URL:http://humanities.uchicago.edu/forms_unrest/webster.form.html>

   [WORDNET] Miller, G.A. (1990), ed. WordNet: An On-Line Lexical
        Database. International Journal of Lexicography. Volume 3,
        Number 4.  <URL:http://www.cogsci.princeton.edu/~wn/>

11.  Acknowledgements

   Thanks to Arnt Gulbrandsen and Nicolai Langfeldt for many helpful
   discussions.  Thanks to Bennet Yee, Doug Hoffman, Kevin Martin, and
   Jay Kominek for extensive testing and feedback on the initial
   implementations of the DICT server.  Thanks to Zhong Shao for advice
   and support.

   Thanks to Brian Kanto, Phil Lapsley, and Jon Postel for writing
   exemplary RFCs which were consulted during the preparation of this
   document.

   Thanks to Harald T. Alvestrand, whose comments helped improve this
   document.

12.  Authors' Addresses

   Rickard E. Faith
   EMail: faith@cs.unc.edu (or faith@acm.org)

   Bret Martin
   EMail: bamartin@miranda.org

   The majority of this work was completed while Bret Martin was a
   student at Yale University.

13.  Full Copyright Statement

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published
   andand distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

 

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