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rfc:rfc2229

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

Faith & Martin Informational [Page 1] RFC 2229 A Dictionary Server Protocol October 1997

 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).

Faith & Martin Informational [Page 2] RFC 2229 A Dictionary Server Protocol October 1997

 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]:

Faith & Martin Informational [Page 3] RFC 2229 A Dictionary Server Protocol October 1997

                                             ; (  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.

Faith & Martin Informational [Page 4] RFC 2229 A Dictionary Server Protocol October 1997

 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

Faith & Martin Informational [Page 5] RFC 2229 A Dictionary Server Protocol October 1997

 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.

Faith & Martin Informational [Page 6] RFC 2229 A Dictionary Server Protocol October 1997

 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 "\">

Faith & Martin Informational [Page 7] RFC 2229 A Dictionary Server Protocol October 1997

 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

Faith & Martin Informational [Page 8] RFC 2229 A Dictionary Server Protocol October 1997

 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.

Faith & Martin Informational [Page 9] RFC 2229 A Dictionary Server Protocol October 1997

 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

Faith & Martin Informational [Page 10] RFC 2229 A Dictionary Server Protocol October 1997

 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.

Faith & Martin Informational [Page 11] RFC 2229 A Dictionary Server Protocol October 1997

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

Faith & Martin Informational [Page 12] RFC 2229 A Dictionary Server Protocol October 1997

 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

Faith & Martin Informational [Page 13] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 14] RFC 2229 A Dictionary Server Protocol October 1997

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

Faith & Martin Informational [Page 15] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 16] RFC 2229 A Dictionary Server Protocol October 1997

 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,

Faith & Martin Informational [Page 17] RFC 2229 A Dictionary Server Protocol October 1997

 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

Faith & Martin Informational [Page 18] RFC 2229 A Dictionary Server Protocol October 1997

 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

Faith & Martin Informational [Page 19] RFC 2229 A Dictionary Server Protocol October 1997

      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).

Faith & Martin Informational [Page 20] RFC 2229 A Dictionary Server Protocol October 1997

 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:::

Faith & Martin Informational [Page 21] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 22] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 23] RFC 2229 A Dictionary Server Protocol October 1997

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

Faith & Martin Informational [Page 24] RFC 2229 A Dictionary Server Protocol October 1997

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

Faith & Martin Informational [Page 25] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 26] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 27] RFC 2229 A Dictionary Server Protocol October 1997

 [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/>

Faith & Martin Informational [Page 28] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 29] RFC 2229 A Dictionary Server Protocol October 1997

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.

Faith & Martin Informational [Page 30]

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