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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
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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).
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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]:
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; ( 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.
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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
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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.
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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 "\">
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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
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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.
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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
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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.
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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
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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
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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.
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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
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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.
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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,
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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
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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
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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).
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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:::
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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.
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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.
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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
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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
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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.
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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/>
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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.
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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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