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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
                   Network News Transport Protocol
                   draft-ietf-nntpext-base-16.txt
  
  1  Status of this memo
  
     This memo is an Internet-Draft and is in full conformance
     with Section 10 of RFC 2026. Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its
     areas, and its working groups.  Note that other groups may
     also distribute working documents as Internet-Drafts.
  
     Internet-Drafts are draft documents valid for a maximum of
     six months and may be updated, replaced, or made obsolete by
     other documents at any time.  It is inappropriate to use
     Internet-Drafts as reference material or to cite them other
     than as "work in progress."
  
     The list of current Internet-Drafts can be accesses at
     http://www.ietf.org/ietf/1id-abstracts.txt.
  
     The list of Internet-Draft shadow directories can be accessed
     at http://www.ietf.org/shadow.html.
  
     This section will be updated with the appropriate verbiage
     from RFC 2223 should this memo have been found ready for
     publication as an RFC. This update will include its standards
     track status.
  
     This memo is a product of the NNTP Working Group, chaired by
     Russ Allbery.
  
  2  Abstract
  
     The Network News Transport Protocol has been in use in the
     Internet for a decade and remains one of the most popular
     protocols (by volume) in use today. This memo is a
     replacement for RFC 977 and officially updates the protocol
     specification. It clarifies some vagueness in RFC 977,
     includes some new base functionality and provides a specific
     mechanism to add standardized extensions to NNTP.
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
  3   Introduction
  
     This memo specifies the Network News Transport Protocol
     (NNTP), which is used for the distribution, inquiry,
     retrieval, and posting of net news articles using a reliable
     stream-based mechanism. For news reading clients, NNTP
     enables retrieval of news articles that are stored in a
     central database, giving subscribers the ability to select
     only those articles they wish to read.
  
     The netnews model provides for indexing, cross-referencing,
     and expiration of aged messages. For server-to-server
     interaction, NNTP is designed for efficient transmission of
     net news articles over a reliable full duplex communication
     channel.
  
     Every attempt is made to ensure that the protocol
     specification in this memo is compatible with the version
     specified in RFC 977[1]. However, this version does not
     support the ill-defined SLAVE command and permits four digit
     years to be specified in the NEWNEWS and NEWGROUPS commands.
     It changes the default character set to UTF-8[2] instead of
     US-ASCII[3]. It also extends the newsgroup name matching
     capabilities already documented in RFC 977.
  
     Generally, new functionality is available using new keywords.
     Part of that new functionality involves a mechanism to
     discover what new functionality is available to clients from
     a server.
  
     This mechanism can also be used to add more functionality as
     needs merit such additions.
  
     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
     NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
     "OPTIONAL" in this memo are to be interpreted as described in
     RFC 2119[4].
  
     An implementation is not compliant if it fails to satisfy one
     or more of the MUST requirements for this protocol.  An
     implementation that satisfies all the MUST and all the SHOULD
     requirements for its protocols is said to be "unconditionally
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     compliant"; one that satisfies all the MUST requirements but
     not all the SHOULD requirements for NNTP is said to be
     "conditionally compliant".
  
     For the remainder of this memo, the term "client host" refers
     to a host making use of the NNTP service, while the term
     "server host" refers to a host that offers the NNTP service.
     In addition, where examples of interactions between a client
     host and a server host are provided a "[C]" will be used to
     represent the client host and a "[S]" will be used to
     represent the server host.
  
     For the remainder of this memo, responses will be described
     in tables listing the required format of a response followed
     by the meaning that should be ascribed to that response.
  
  4  Basic Operation.
  
     Every NNTP session MUST involve the following in this order:
        CONNECTION
        GREETING
        DISCONNECTION
  
     Other steps may occur between the GREETING and DISCONNECTION
     step. They are:
        CAPABILITIES DISCOVERY
        NEWS EXCHANGE
        CONCLUSION
  
     NNTP operates over any reliable data stream 8-bit-wide
     channel. When running over TCP/IP, the official port for the
     NNTP service is 119. Initially, the server host starts the
     NNTP service by listening on a TCP port.  When a client host
     wishes to make use of the service, it MUST establish a TCP
     connection with the server host by connecting to that host on
     the same port on which the server is listening. This is the
     CONNECTION step.  When the connection is established, the
     NNTP server host MUST send a greeting. This is the GREETING
     step. The client host and server host SHOULD then exchange
     commands and responses (respectively) until the connection is
     closed or aborted. This final step is called the
     DISCONNECTION step.
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     If there is a CONCLUSION step, it MUST immediately precede
     the DISCONNECTION step. There MUST be only one CONNECTION,
     CONCLUSION and DISCONNECTION step for each NNTP session. All
     other steps MAY be repeated as needed. For example, the
     GREETING step may be repeated if the client makes use of the
     MODE READER command (See Section 7.2 for more on the MODE
     READER command).
  
     The character set for all NNTP commands is UTF-8. Commands in
     the NNTP MUST consist of an US-ASCII case-insensitive
     keyword, which MAY be followed by one or more arguments.  An
     US-ASCII CRLF pair MUST terminate all commands. Multiple
     commands MUST NOT be on the same line. Keywords MUST consist
     of printable US-ASCII characters.  Unless otherwise noted
     elsewhere in this memo, arguments SHOULD consist of printable
     US-ASCII characters. Keywords and arguments MUST be each
     separated by one or more US-ASCII SPACE or US-ASCII TAB
     characters. Keywords MUST be at least three US-ASCII
     characters and MUST NOT exceed 12 US-ASCII characters.
     Command lines MUST NOT exceed 512 octets, which includes the
     terminating US-ASCII CRLF pair. Arguments MUST NOT exceed 497
     octets.
  
     Each response MUST start with a three-digit response code
     that is sufficient to distinguish all responses. Certain
     valid responses are defined to be multi-line; for all others,
     the response is contained in a single line. All multi-line
     responses MUST adhere to the following format:
  
     The response consists of a sequence of one or more "lines",
     each being a stream of octets ending with 0x0D 0x0A (US-ASCII
     CRLF). Apart from those line endings, the stream MUST NOT
     include the octets 0x00, 0x0A, or 0x0D (US-ASCII NUL, LF, and
     CR).
  
     The first such line contains the response code as with a
     single line response.
  
     If any subsequent line begins with the "termination octet"
     (0x2E or US_ASCII "."), that line MUST be "byte-stuffed" by
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     pre-pending an additional termination octet (0x2E) to that
     line of the response.
  
     The lines of the response MUST be followed by a terminating
     line consisting of a single termination octet (0x2E or
     US_ASCII ".")followed by CRLF in the normal way. Thus a
     multi-line response is always terminated with the five octets
     "CRLF.CRLF" (in US-ASCII).
  
     There is NO limit on the length of a line.
  
     When interpreting a multi-line response, the "byte stuffing"
     MUST be undone; i.e. the client MUST ensure that, in any line
     beginning with the termination octet followed by octets other
     than US-ASCII CRLF, that initial termination octet is
     disregarded.
  
     Likewise, the terminating line ".CRLF" (in US-ASCII) MUST NOT
     be considered part of the multi-line response; i.e. the
     client MUST ensure that any line beginning with the
     termination octet followed immediately by US-ASCII CRLF is
     disregarded; (the first CRLF of the terminating "CRLF.CRLF"
     is, of course, part of the last line of the response).
  
     NOTE: Texts using an encoding (such as UTF-16 or UTF-32) that
     may contain the NUL octet or the CR or LF octets in contexts
     other than the CRLF line ending cannot be reliably conveyed
     in the above format.
  
     Note also that, although this memo does not limit the length
     of a line in any way, the standards that define the format of
     articles may do so.
  
     An NNTP server MAY have an inactivity autologout timer.  Such
     a timer SHOULD be of at least three minutes duration, with
     the exception that there MAY be a shorter limit on how long
     the server is willing to wait for the first command from the
     client.  The receipt of any command from the client during
     the timer interval SHOULD suffice to reset the autologout
     timer.  Similarly, the receipt of any significant amount of
     data from the client while in the midst of sending a multi-
     line message to the server (such as during a POST or IHAVE
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     command) SHOULD suffice to reset the autologout timer.  When
     the timer expires, the server SHOULD close the TCP connection
     without sending any response to the client, including when
     the client is in the middle of sending a multi-line message
     to the server.
  
  4.1  Response Codes
  
     Each response MUST begin with a three-digit status indicator.
     These are status reports from the server and indicate the
     response to the last command received from the client.
  
     The first digit of the response broadly indicates the
     success, failure, or progress of the previous command.
  
        1xx - Informative message
        2xx - Command ok
        3xx - Command ok so far, send the rest of it.
        4xx - Command was correct, but couldn't be performed for
              some reason.
        5xx - Command unimplemented, or incorrect, or a serious
              program error occurred.
  
     The next digit in the code indicates the function response
     category.
  
        x0x - Connection, setup, and miscellaneous messages
        x1x - Newsgroup selection
        x2x - Article selection
        x3x - Distribution functions
        x4x - Posting
        x8x - Reserved for authentication and authorization
              extensions
        x9x - Reserved for private use (non-standard extensions)
  
     Certain responses contain parameters such as numbers and
     names in addition to the status indicator. In those cases,
     the number and type of such parameters is fixed for each
     response code to simplify interpretation by the client (any
     extension MUST follow this principle as well). In all other
     cases, the client MUST only use the status indicator itself
     to determine the nature of the response. The exact response
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     codes that can be returned in response to a given command are
     detailed in the description of the keyword that is the first
     part of the command.
  
     Parameters MUST be separated from the numeric status
     indicator and from each other by a single US-ASCII space. All
     numeric parameters MUST be in base 10 (decimal) format, and
     MAY have leading zeros. String parameters MUST contain at
     least one character and MUST NOT contain US-ASCII spaces, CR,
     LF, or tab). The server MAY add any text after the response
     code or last parameter as appropriate, and the client MUST
     NOT make decisions based on this text. Such text MUST be
     separated from the numeric status indicator or the last
     parameter by at least one US-ASCII space.
  
     The server MUST respond to any command with the appropriate
     generic response (given in section 4.1.1) if it represents
     the situation. Otherwise, each recognized command MUST return
     one of the response codes specifically listed in its
     description or in an extension. A server MAY provide
     extensions to this memo, including new commands, new features
     of existing commands, and other ways of changing the internal
     state of the server. However, the server MUST NOT produce any
     other responses to a client that does not invoke any of the
     additional features. (Therefore a client that restricts
     itself to this memo will only receive the responses that are
     listed).
  
     If a client receives an unexpected response, it SHOULD use
     the first digit of the response to determine the result. For
     example, an unexpected 2xx should be taken as success and an
     unexpected 4xx or 5xx as failure.
  
     Response codes not specified in this memo MAY be used for any
     installation-specific additional commands also not specified.
     These SHOULD be chosen to fit the pattern of x9x specified
     above.
  
     Neither this memo nor any extension registered with IANA (see
     section 12) will specify any response codes of the x9x
     pattern. (Implementers of extensions are accordingly
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     cautioned not to use such responses for extensions that may
     subsequently be submitted for registration.)
  
  4.1.1  Generic Response Codes
  
     The server MUST respond to any command with the appropriate
     one of the following generic responses if it represents the
     situation.
  
     If the command is not recognized, or it is an optional
     command or extension that is not implemented by the server,
     the response code 500 MUST be returned.
  
     If there is a syntax error in the arguments of a recognized
     command, the response code 501 MUST be returned. Note that
     where a command has variants depending on a keyword (e.g.
     LIST ACTIVE and LIST NEWSGROUPS), then 501 MUST be used when
     the requested variant is not implemented but the base command
     is.
  
     If the client is not authorized to use the specified facility
     when the server is in its current state, the response code
     502 MUST be returned. A different command might change the
     server state and permit the command if it is retried.
  
     If the server does not provide an optional feature, then the
     response code 403 MUST be returned if the omission is
     temporary (e.g. because a necessary facility is unavailable)
     and the code 503 if it is permanent  (e.g. because the server
     does not store the required information).
  
     If the server has to terminate the connection for some
     reason, it MUST give a 400 response code to the next command
     and then immediately close the TCP connection. It MAY give a
     401 response code to any command to indicate that termination
     is imminent (following a 401 response, it MUST NOT close the
     TCP connection immediately).
  
  4.2  Streaming
  
     NNTP is designed to operate over a reliable bi-directional
     connection such as TCP. Therefore, if a command does not
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     depend on the response to the previous one, it should not
     matter if it is sent before that response is received. Doing
     this is called "streaming". However, certain server
     implementations throw away all text received from the client
     following certain commands before sending their response. If
     this happens, streaming will be affected because one or more
     commands will have been ignored or misinterpreted, and the
     client will be matching the wrong responses to each command.
     Since there are significant benefits to streaming, but also
     circumstances where it is reasonable or common for servers to
     behave in the above manner, this memo puts certain
     requirements on both clients and servers.
  
     Except where stated otherwise, a client MAY use streaming.
     That is, it may send a command before receiving the response
     for the previous command. The server MUST allow streaming and
     MUST NOT throw away any text received after a command.
     Finally, the server MUST process commands in the order they
     are sent.
  
     If the specific description of a command describes it as "not
     streamable", that command MUST end any stream of commands.
     That is, the client MUST NOT send any following command until
     receiving the CRLF at the end of the response from the
     command. The server MAY ignore any data received after the
     command and before the CRLF at the end of the response is
     sent to the client.
  
     The initial connection must not be part of a stream; that is,
     the client MUST NOT send any command until receiving the CRLF
     at the end of the greeting.
  
     If the client uses blocking system calls to send commands, it
     MUST ensure that the amount of text sent in streaming does
     not cause a deadlock between transmission and reception. The
     amount of text involved will depend on window sizes in the
     transmission layer, and is typically 4k bytes for TCP.
  
  5   The WILDMAT format
  
     The WILDMAT format described here is based on the version
     first developed by Rich Salz [5], which in turn was derived
  
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  INTERNET DRAFT                                         C. Feather
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                                                       January 2003
  
     from the format used in the UNIX "find" command to articulate
     file names. It was developed to provide a uniform mechanism
     for matching newsgroup names in the same manner that the UNIX
     shell matches filenames.
  
  5.1  Wildmat syntax
  
     A wildmat is described by the following augmented BNF[6]
     syntax (note that this syntax contains ambiguities and
     special cases described at the end):
        wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
  
        wildmat-pattern = 1*wildmat-item
  
        wildmat-item = wildmat-exact / wildmat-wild
  
        wildmat-exact = %x21-29 / %x2B / %x2D-3E / %x40-5A / %x5E-
              7F /UTF-8-non-ascii  ; exclude * , ? [ \ ]
  
        wildmat-wild = "*" / "?"
  
     UTF-8-non-ascii is defined in section 13.
  
     This syntax must be interpreted subject to the following
     rule:
     Where a wildmat-pattern is not immediately preceded by "!",
     it shall not begin with a "!".
  
     NOTE: the characters \ , [ and ] are not allowed in wildmats,
     while * and ? are always wildcards. This should not be a
     problem since these characters cannot occur in newsgroup
     names, which is the only current use of wildmats. Backslash
     is commonly used to supress the special meaning of characters
     and brackets to introduce sets, but there is no existing
     standard practice for these in wildmats and so they were
     omitted from this memo. A future extension to this memo may
     provide semantics for these characters.
  
  5.2  Wildmat semantics
  
     A wildmat is tested against a string, and either matches or
     does not match. To do this, each constituent wildmat-pattern
  
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  INTERNET DRAFT                                         C. Feather
  Expires: July 31, 2003                                   Thus plc
                                                       January 2003
  
     is matched against the string and the rightmost pattern that
     matches is identified. If that wildmat-pattern is not
     preceded with "!", the whole wildmat matches. If it is
     preceded by "!", or if no wildmat-pattern matches, the whole
     wildmat does not match.
  
     For example, consider the wildmat "a*,!*b,*c*":
  
        the string "aaa" matches because the rightmost match is
              with "a*"
        the string "abb" does not match because the rightmost
              match is with "*b"
        the string "ccb" matches because the rightmost match is
              with "*c*"
        the string "xxx" does not match because no wildmat-pattern
              matches
  
     A wildmat-pattern matches a string if the string can be
     broken into components, each of which matches the
     corresponding wildmat-item in the pattern; the matches must
     be in the same order, and the whole string must be used in
     the match. The pattern is "anchored"; that is, the first and
     last characters in the string must match the first and last
     item respectively (unless that item is an asterisk matching
     zero characters).
  
     A wildmat-exact matches the same character (which may be more
     than one octet in UTF-8).
  
     "?" matches exactly one character (which may be more than one
     octet).
  
     "*" matches zero or more characters. It can match an empty
     string, but it cannot match a subsequence of a UTF-8 sequence
     that is not aligned to the character boundaries.
  
  5.3  Extensions
  
     An NNTP server or extension MAY extend the syntax or
     semantics of wildmats provided that all wildmats that meet
     the requirements of section 5.1 have the meaning ascribed to
  
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     them by section 5.2. Future editions of this memo may also
     extend wildmats.
  
  5.4  Examples
  
     In these examples, $ and @ are used to represent the two
     octets 0xC2 and 0xA3 respectively; $@ is thus the UTF-8
     encoding for the pound sterling symbol, shown as # in the
     descriptions.
  
     Wildmat    Description of strings that match
       abc      the one string "abc"
       abc,def  the two strings "abc" and "def"
       $@       the one character string "#"
       a*       any string that begins with "a"
       a*b      any string that begins with "a" and ends with "b"
       a*,*b    any string that begins with "a" or ends with "b"
       a*,!*b   any string that begins with "a" and does not end with
                "b"
                any string that begins with "a" and does not end with
     a*,!*b,c*  "b", and any string that begins with "c" no matter
                what it ends with
                any string that begins with "a" or "c" and does not
     a*,c*,!*b  end with "b"
       ?a*      any string with "a" as its second character
       ??a*     any string with "a" as its third character
       *a?      any string with "a" as its penultimate character
       *a??     any string with "a" as its antepenultimate character
  
  6  Format for Keyword Descriptions
  
     On the following pages are descriptions of each keyword
     recognized by the NNTP server and the responses that will be
     returned by those commands. These keywords are grouped by the
     functional step in which they are used.
  
     Each keyword is shown in upper case for clarity, although the
     NNTP server ignores case in the interpretation of commands.
     Parameters are shown as follows:
  
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  INTERNET DRAFT                                         C. Feather
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        UPPERCASE    indicates literal text to be included in the
                     command;
        lowercase    indicates a token described elsewhere;
        [brackets]   indicate that the parameter is optional;
        ellipsis...  indicates that the parameter may be repeated
                     any number of times (it must occur at least
                     once);
        vertical|bar indicates a choice of two mutually exclusive
                     parameters (exactly one must be provided).
  
     Parameters are case or language specific only when specified
     (either in this memo or in RFC 1036[7]).
  
     The name "wildmat" for a parameter indicates that it is a
     wildmat format pattern as defined in section 5. If the
     parameter does not meet the requirements of that section (for
     example, if it does not fit the grammar of 5.1) the NNTP
     server MAY place some interpretation on it (not specified by
     this memo) or otherwise MUST generate a 501 response.
  
  7   The GREETING Step
  
  7.1  Initial Connection
  
     There is no keyword presented by the client upon initial
     connection to the server. The server MUST present an
     appropriate response code as a greeting to the client.  This
     response informs the client about what steps the client
     should take to reach the news exchange step.
  
     If the server will accept further commands from the client
     including POST, the server MUST present a 200 greeting code.
     If the server will accept further commands from the client,
     but it is not authorized to post articles using the POST
     command, the server MUST present a 201 greeting code.
  
     Otherwise the server MUST present a 400 or 502 greeting code
     and then immediately close the connection. 502 MUST be used
     if the client is not permitted under any circumstances to
     interact with the server and 400 otherwise.
  
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  INTERNET DRAFT                                         C. Feather
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  7.1.1  Responses
  
         200 Service available, posting allowed
         201 Service available, posting prohibited
         400 Service temporarily unavailable
         502 Service unavailable
  
     Following a 400 or 502 response the server MUST immediately
     close the connection.
  
  7.1.2  Initial Connection Example
  
     Example of a normal connection from an authorized client
          [Initial TCP connection setup completed.]
          [C] Initial TCP connection completed
          [S] 200 NNTP Service Ready, posting permitted
     Client can send commands at this point. In this example, the
     client jumps directly to the conclusion step (See section
     10).
          [C] QUIT
          [S] 205 NNTP Service exits normally
          [Server closes connection.]
     Example of a normal connection from an unauthorized client
          [C] Initial TCP connection completed
          [S] 502 NNTP Service Unavailable
          [Server closes connection.]
     Example of a normal connection from an authorized client that
     is not permitted to post
          [Initial TCP connection setup completed.]
          [S] 201 NNTP Service Ready, posting prohibited
     Client can send commands at this point. In this example, the
     client jumps directly to the conclusion step (See section
     10).
          [C] QUIT
          [S] 205 NNTP Service exits normally
          [Server closes connection.]
     Example of a connection from any client where the server is
     unable to provide service
          [Initial TCP connection setup completed.]
          [S] 400 NNTP Service temporarily unavailable
          [Server closes connection.]
  
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  7.2  MODE READER
  
     MODE READER
     MODE READER SHOULD be sent by any client that intends to use
     any command other than IHAVE, HEAD, STAT, LIST, LIST
     EXTENSIONS, or commands advertised by the server as available
     via LIST EXTENSIONS. This command is not streamable.
  
     Servers MAY require that this command be issued before any
     other commands are sent and MAY reject any other commands
     until after a MODE READER command has been sent.
  
     The server MUST present a response using the same codes as
     the initial greeting (as described in section 7.1) to
     indicate its ability to provide reading service to the
     client.
  
     Clients SHOULD wait for a response to MODE READER after
     sending this command and SHOULD NOT send any additional
     commands until that response has been received from the
     server.
  
     Once MODE READER is sent, IHAVE (and any extensions intended
     for peer-to-peer article transfer) MAY no longer be
     permitted, even if it were permitted before the MODE READER
     command. The results of LIST EXTENSIONS MAY be different
     following a MODE READER command than prior to the issuing of
     that command.
  
     Servers are encouraged to not require this command even
     though clients SHOULD send it when appropriate. It is present
     to support some news architectures that switch between modes
     based on whether a given connection is a peer-to-peer
     connection with another server or a news reading client.
  
  7.2.1  Responses
          200 Posting permitted
          201 Posting prohibited
          400 Service temporarily unavailable
          502 Service unavailable
  
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  INTERNET DRAFT                                         C. Feather
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                                                       January 2003
  
     Following a 400 or 502 response the server MUST immediately
     close the connection.
  
     Note that the response need not be the same as the one
     presented during the initial greeting.
  
  7.2.2  MODE READER Examples
  
     Example of use of the MODE READER command by an authorized
     client
          [C] MODE READER
          [S] 200 NNTP Service Ready, posting permitted
     Client can send commands at this point. In this example, the
     client jumps directly to the conclusion step (See section
     10).
          [C] QUIT
          [S] 205 NNTP Service exits normally
          [Server closes connection.]
     Example of use of MODE READER by a client not authorized to
     receive service from the server as a news reader
          [C] MODE READER
          [S] 502 Service Unavailable
          [Server closes connection.]
     Example of a normal connection from an authorized client that
     is not permitted to post
          [C] MODE READER
          [S] 201 NNTP Service Ready, posting prohibited
     Client can send commands at this point. In this example, the
     client jumps directly to the conclusion step (See section
     10).
          [C] QUIT
          [S] 205 NNTP Service exits normally
          [Server closes connection.]
     Example of a connection from any client where the server is
     unable to provide news reader service
          [C] MODE READER
          [S] 400 NNTP Service temporarily unavailable
          [Server closes connection.]
  
  8   The CAPABILITIES DISCOVERY Step
  
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     To discover what extensions are available, an NNTP client can
     query the server with the LIST EXTENSIONS command.
     If a particular extension is unavailable, the client can
     attempt to work around it or it may wish to terminate the
     session.
  
     See section 12 for further discussion of extensions.
  
  8.1  LIST EXTENSIONS
  
     The LIST EXTENSIONS command allows a client to determine
     which extensions are supported by the server. This command
     MUST be implemented by any server that implements any
     extensions defined in this memo. This command is not
     streamable.
  
     To discover what extensions are available, an NNTP client
     SHOULD query the server early in the session for extensions
     information by issuing the LIST EXTENSIONS command. This
     command MAY be issued at anytime during a session.  It is not
     required that the client issues this command before
     attempting to make use of any extension. The response
     generated by this command MAY change during a session because
     of other state information. However, an NNTP client MUST NOT
     cache (for use in another session) any information returned
     if the LIST EXTENSIONS command succeeds. That is, an NNTP
     client is only able to get the current and correct
     information concerning available extensions during a session
     by issuing a LIST EXTENSIONS command during that session and
     processing that response.
  
     A successful response starts with a 202 code and is followed
     by a list of extensions, one per line. Each line MUST begin
     with an extension-label and optionally one or more parameters
     (separated by single spaces). The extension-label and the
     meaning of the parameters are specified as part of the
     definition of the extension. The extension-label MUST be in
     uppercase.
  
     The server MUST NOT list the same extension twice in the
     response, and MUST list all supported extensions. The order
  
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  INTERNET DRAFT                                         C. Feather
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     in which the extensions are listed is not significant. The
     server need not even consistently return the same order.
     If the server does not support any extensions, it SHOULD
     return a 402 failure response but MAY return an empty list
     instead.
  
  8.1.1  Responses
  
          202  Extension list follows (multi-line response)
          400  Service temporarily unavailable
          402  Server has no extensions
          500  Unknown Command
          501  Syntax Error
          502  Program error, function not performed
  
     Following a 502 response an extension might still be
     available, and the client MAY attempt to use it
     The LIST EXTENSIONS command is optional, and a server MAY
     issue a 500 (unknown command) or 501 (syntax error) response
     to it.
  8.1.1.1  LIST EXTENSIONS Examples
  
     Example of a successful response:
          [C] LIST EXTENSIONS
          [S] 202 Extensions supported:
          [S] OVER
          [S] HDR
          [S] LISTGROUP
          [S] .
     The particular extensions shown here are simply examples of
     what might be defined in other places, and no particular
     meaning should be attributed to them.
     Example where no extensions are available, using preferred
     format:
         [C] LIST EXTENSIONS
         [S] 402 Server has no extensions
     Example where no extensions are available, using an empty
     list:
          [C] LIST EXTENSIONS
          [S] 202 Extensions supported:
          [S] .
  
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  INTERNET DRAFT                                         C. Feather
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                                                       January 2003
  
  9   The NEWS EXCHANGE Step
  
     During this step, two basic types of transactions occur:
     *  article retrieval from the server
     *  article posting to the server
  
  9.1  Article Retrieval
  
     News reading clients have available a variety of mechanisms
     to retrieve articles via NNTP. The news articles are stored
     and indexed using three types of keys. One key is the message
     id of an article. According to RFC 1036, this identifier
     should be globally unique. Another key is composed of the
     newsgroup name and the article number within that newsgroup.
     That key MUST be unique to a particular server (there will be
     only one article with that number within a particular
     newsgroup), but is not required to be globally unique.
     Additionally, because the same article can be cross-posted to
     multiple newsgroups, there may be multiple keys that point to
     the same article on the same server. The final key is the
     arrival timestamp, giving the time that the article arrived
     at the server.
  
     The server MUST ensure that article numbers are issued in
     order of arrival timestamp; that is, articles arriving later
     MUST have higher numbers than those that arrive earlier. The
     server SHOULD allocate the next sequential unused number to
     each new article.
  
     Article numbers MUST lie between 1 and 4,294,967,295
     inclusive. The client and server SHOULD NOT use leading
     zeroes in specifying article numbers, and MUST NOT use more
     than 16 digits. In some situations, the value zero replaces
     an article number to show some special situation.
  
     Message ids are defined in RFC 2822 with the following
     modifications:
     *  A message id MUST NOT contain a US-ASCII space within any
        quoted-pair.
     *  A message id MUST NOT be longer than 250 octets.
  
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     *  RFC 2822 obsolete syntax for message ids are not supported
        by the protocol specified in this memo.
  
  9.1.1 Article Retrieval by Newsgroup Name and Article Number
  
     The following commands are used to set the current newsgroup
     name and the "current article pointer" which is used by other
     commands for article retrieval. At the start of an NNTP
     session, both of these values are undefined.
  9.1.1.1 GROUP
  
     GROUP ggg
     The required parameter ggg is the name of the newsgroup to be
     selected (e.g. "news.software.b"). A list of valid newsgroups
     may be obtained by using the LIST keyword.  See section 9.4
     for more information on the LIST keyword.
  
     The successful selection response will return the article
     numbers of the first and last articles in the group at the
     moment of selection (these numbers are referred to as the
     "reported low water mark" and the "reported high water
     mark"), and an estimate of the number of articles on file in
     the group.
  
     If the group is not empty, the estimate MUST be at least the
     actual number of articles available, and MUST be no greater
     than one more than the difference between the reported low
     and high water marks. (Some implementations will actually
     count the number of articles on file. Others will just
     subtract the low water mark from the high water mark and add
     one to get an estimate.)
  
     If the group is empty, one of the following three situations
     will occur. Clients MUST accept all three cases; servers MUST
     NOT represent an empty group in any other way.
  
     The high water mark will be one less than the low water mark,
     and the estimated article count will be zero. Servers SHOULD
     use this method to show an empty group. This is the only time
     that the high water mark can be less than the low water mark.
     All three numbers will be zero.
  
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     The high water mark is greater than or equal to the low water
     mark; the estimated article count might be zero or non-zero;
     if non-zero, the same requirements apply as for a non-empty
     group.
  
     The set of articles in a group may change after the GROUP
     command is carried out. That is:
     *  articles may be removed from the group
     *  articles may be reinstated in the group with the same
        article number, but those articles MUST have numbers no
        less than the reported low water mark (note that this is a
        reinstatement of the previous article, not a new article
        reusing the number)
     *  new articles may be added with article numbers greater
        than the reported high water mark (if an article that was
        the one with the highest number has been removed, the next
        new article will not have the number one greater than the
        reported high water mark)
  
     Except when the group is empty and all three numbers are
     zero, whenever a subsequent GROUP command for the same
     newsgroup is issued, either by the same client or a different
     client, the reported low water mark in the response MUST be
     no less than that in any previous response for that newsgroup
     sent to any client. The client may make use of the low water
     mark to remove all remembered information about articles with
     lower numbers, as these will never recur. This includes the
     situation when the high water mark is one less than the low
     water mark.
  
     No similar assumption can be made about the high water mark,
     as this can decrease if an article is removed, and then
     increase again if it is reinstated or if new articles arrive.
     When a valid group is selected by means of this command, the
     internally maintained "current article pointer" MUST be set
     to the first article in the group and the name of the current
     newsgroup MUST be set to the selected newsgroup name. If an
     invalid group is specified, the previously selected group, if
     any, and article MUST remain selected. If an empty newsgroup
     is selected, the "current article pointer" is in an
     indeterminate state and MUST NOT be used.
  
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     The GROUP keyword (or the LISTGROUP keyword, if implemented)
     MUST be used by a client and a successful response received
     before the any other command is used that depends on having
     the "current article pointer" be valid.
  
     If the group specified is not available on the server, it
     MUST return a 411 error code.
  
  9.1.1.1.1  Responses
  
            211 n l h ggg Group successfully selected (n =
                          estimated number of articles in the
                          group, l = low water mark, h = high
                          water mark, ggg = name of the group
            411           No such newsgroup
  
  9.1.1.1.2  GROUP Examples
  
     Example for a group known to the server
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
  
     Example for a group unknown to the server
          [C] GROUP example.is.sob.bradner.or.barber
          [S] 411 example.is.sob.bradner.or.barber is unknown
  9.1.1.2   LAST
  
     LAST
     If the current newsgroup is valid, the internally maintained
     "current article pointer" MUST be set to the previous article
     in the current newsgroup.  If already positioned at the first
     article of the newsgroup, an error message MUST be returned
     and the current article MUST remain selected.
  
     There MAY be no previous article in the group, although the
     current article number is not the reported low water mark.
     There MUST NOT be a previous article when the current article
     number is the reported low water mark.
  
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     Because articles can be removed and added, the results of
     multiple LAST and NEXT commands MAY not be consistent over
     the life of a particular NNTP session.
  
     If successful, a response indicating the current article
     number and a message-id string MUST be returned. No article
     text is sent in response to this command.
  9.1.1.2.1  Responses
  
            223 n a   Article found (n = number, a = message-id)
            412       No newsgroup selected
            420       Current article pointer is invalid
            422       No previous article in this group
  
  9.1.1.2.2  LAST Examples
  
      Example of a successful article retrieval using LAST
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] NEXT
          [S] 223 3000237 <668929@example.org> retrieved
          [C] LAST
          [S] 223 3000234 <45223423@example.com> retrieved
     Example of an attempt to retrieve an article without having
     selected a group (via the GROUP command) first
          [S] 200 NNTP Service ready
          [C] LAST
          [S] 412 no newsgroup selected
     Example of an attempt to retrieve an article using the LAST
     command when the current article pointer is pointing at the
     first article in the group
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] LAST
          [S] 422 No previous article to retrieve
     Example of an attempt to retrieve an article using the LAST
     command when the current group selected is empty
  
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          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] LAST
          [S] 420 No current article selected
  9.1.1.3  NEXT
  
     NEXT
     If the current newsgroup is valid, the internally maintained
     "current article pointer" MUST be advanced to the next
     article in the current newsgroup.  If no more articles remain
     in the current group, an error message MUST be returned and
     the current article MUST remain selected.
  
     If successful, a response indicating the current article
     number and the message-id string MUST be returned.  No
     article text is sent in response to this command.
  
  9.1.1.3.1  Responses
  
            223 n a   Article found (n = number, a = message-id)
            412       No newsgroup selected
            420       Current article pointer is invalid
            421       No next article in this group
  
  9.1.1.3.2  NEXT Examples
  
     Example of a successful article retrieval using NEXT
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] NEXT
          [S] 223 3000237 <668929@example.org> retrieved
     Example of an attempt to retrieve an article without having
     selected a group (via the GROUP command) first
          [S] 200 NNTP Service ready
          [C] NEXT
          [S] 412 no newsgroup selected
  
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  INTERNET DRAFT                                         C. Feather
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     Example of an attempt to retrieve an article using the NEXT
     command when the current article pointer is pointing at the
     last article in the group
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] ARTICLE 3002322
          [S] 220 3002322 <411@example.net> retrieved
          [S] Path: pathost!demo!whitehouse!not-for-mail
          [S] From: ôDemo Userö <nobody@example.net>
          [S] Newsgroups: misc.test
          [S] Subject: I am just a test article
          [S] Date: 6 Oct 1998 04:38:40 -0500
          [S] Organization: An Example Net, Uncertain, Texas
          [S] Message-ID: <411@example.net>
          [S]
          [S] This is just a test article.
          [S] .
          [C] NEXT
          [S] 421 No next article to retrieve
     Example of an attempt to retrieve an article using the NEXT
     command when the current group selected is empty
          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] NEXT
          [S] 420 No current article selected
  
  9.2  Retrieval of Articles and Article Sections
  
     The ARTICLE, BODY, HEAD, and STAT commands are very similar.
     They differ only in the parts of the article that are
     presented to the client and in the successful response code.
     The ARTICLE command is described here in full, while the
     other commands are described in terms of the differences.
     An article, as defined by RFC 1036, consists of two parts:
     the article headers and the article body. When responding to
     one of these commands, the server presents the entire article
     or appropriate part and does not attempt to alter or
     translate it in any way.
  
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  9.2.1  ARTICLE
  
     ARTICLE <message-id>
     ARTICLE [number]
     The ARTICLE command selects an article based on the arguments
     and presents the header, a blank line, and the body of that
     article. The command has two forms.
  
     In the first form, a message-id is specified (including the
     angle brackets), and the server presents the article with
     that message-id in its headers. In this case, the server MUST
     NOT alter the "current article pointer". This is both to
     facilitate the presentation of articles that may be
     referenced within another article being read, and because of
     the semantic difficulties of determining the proper sequence
     and membership of an article that may have been crossposted
     to more than one newsgroup.
  
     In the second form, an article number may be specified. If
     so, and if there is an article with that number in the
     currently selected group, the server MUST set the current
     article pointer to that number.
  
     Then, whether or not a number was specified, the article
     indicated by the current article pointer is presented to the
     client.
  
     Note that a previously valid article number MAY become
     invalid if the article has been removed. A previously invalid
     article number MAY become valid if the article has been
     reinstated, but such an article number MUST be no less than
     the reported low water mark for that group.
  
     The server MUST NOT change the currently selected group as a
     result of this command. The server MUST NOT change the
     current selected article except when an article number
     argument was provided and the article exists; in particular,
     it MUST NOT change it following an unsuccessful response.
  9.2.1.1  Responses
     First form (message-id specified):
  
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            220 0 a  Article follows (multi-line response, a =
                     message-id)
            430      No article found with that message-id
            502      Program error, function not performed
  
     Second form (optional article number specified):
  
            220 n a  Article follows (multi-line response, n =
                     article number, a = message-id)
            412      No newsgroup selected
            420      No current article selected
            423      No such article in this newsgroup
            502      Program error, function not performed
  
  
     The 420 response only occurs if no article number has been
     specified.
  
     In the 220 response, the first parameter is 0 for the first
     form and the article number (within the current group) for
     the second form. The second parameter is the message-id of
     the article (within angle brackets). This is taken from the
     message-id header line of the article (required by RFC 1036).
     If there is no such line, the message-id "<0>" MUST be used
     instead (without the double quotes).
  
     Since the message-id field is unique for each article, it may
     be used by a client to skip duplicate displays of articles
     that have been posted more than once, or to more than one
     newsgroup.
  
     The article headers and body are returned as a multi-line
     response following the initial response line.
  9.2.1.2  Examples
     Example of a successful retrieval of an article (using no
     article number)
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] ARTICLE
          [S] 220 3000234 <45223423@example.com>
  
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          [S] Path: pathost!demo!somewhere!not-for-mail
          [S] From: ôDemo Userö <nobody@example.com>
          [S] Newsgroups: misc.test
          [S] Subject: I am just a test article
          [S] Date: 6 Oct 1998 04:38:40 -0500
          [S] Organization: An Example Com, San Jose, CA
          [S] Message-ID: <45223423@example.com>
          [S]
          [S] This is just a test article.
          [S] .
     Example of a successful retrieval of an article by message-id
          [S] 200 NNTP Service Ready
          [C] ARTICLE <45223423@example.com>
          [S] 220 0 <45223423@example.com>
          [S] Path: pathost!demo!somewhere!not-for-mail
          [S] From: ôDemo Userö <nobody@example.com>
          [S] Newsgroups: misc.test
          [S] Subject: I am just a test article
          [S] Date: 6 Oct 1998 04:38:40 -0500
          [S] Organization: An Example Com, San Jose, CA
          [S] Message-ID: <45223423@example.com>
          [S]
          [S] This is just a test article.
          [S] .
     Example of an unsuccessful retrieval of an article by
     message-id
          [S] 200 NNTP Service Ready
          [C] ARTICLE <i.am.not.there@example.com>
          [S] 430 No Such Article Found
     Example of an unsuccessful retrieval of an article by number
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 news.groups
          [C] ARTICLE 300256
          [S] 423 No such article number in this group
     Example of an unsuccessful retrieval of an article by number
     because no newsgroup was selected first
          [S] 200 NNTP Service Ready
          [C] ARTICLE 300256
          [S] 412 No newsgroup selected
     Example of an attempt to retrieve an article when the current
     group selected is empty
  
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          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] ARTICLE
          [S] 420 No current article selected
     Example of a failure due to the service being unavailable
          [S] 200 NNTP Service Ready
          [C] ARTICLE <i.am.a.test.article@example.com>
          [S] 502 Service unavailable
  
  9.2.2  HEAD
  
     HEAD <message-id>
     HEAD [number]
     The HEAD command behaves identically to the ARTICLE command
     except that, if the article exists, only the headers are
     presented (the blank line separating the headers and body
     MUST NOT be included).
  9.2.2.1  Responses
  
     First form (message-id specified):
  
            221 0 a  Article follows (multi-line response, a =
                     message-id)
            430      No article found with that message-id
            502      Program error, function not performed
  
     Second form (optional article number specified):
  
            221 n a  Article follows (multi-line response, n =
                     article number, a = message-id)
            412      No newsgroup selected
            420      No current article selected
            423      No such article in this newsgroup
            502      Program error, function not performed
  
  
     Except that only the headers are included in the response,
     the 221 response behaves identically to the 220 response of
     the ARTICLE command.
  
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  9.2.2.2  Examples
  
     Example of a successful retrieval of the headers in an
     article (using no article number)
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HEAD
          [S] 221 3000234 <45223423@example.com>
          [S] Path: pathost!demo!somewhere!not-for-mail
          [S] From: ôDemo Userö <nobody@example.com>
          [S] Newsgroups: misc.test
          [S] Subject: I am just a test article
          [S] Date: 6 Oct 1998 04:38:40 -0500
          [S] Organization: An Example Com, San Jose, CA
          [S] Message-ID: <45223423@example.com>
          [S] .
     Example of a successful retrieval of the headers in an
     article by message-id
          [S] 200 NNTP Service Ready
          [C] HEAD <45223423@example.com>
          [S] 221 0 <45223423@example.com>
          [S] Path: pathost!demo!somewhere!not-for-mail
          [S] From: ôDemo Userö <nobody@example.com>
          [S] Newsgroups: misc.test
          [S] Subject: I am just a test article
          [S] Date: 6 Oct 1998 04:38:40 -0500
          [S] Organization: An Example Com, San Jose, CA
          [S] Message-ID: <45223423@example.com>
          [S] .
     Example of an unsuccessful retrieval of the header of an
     article by message-id
          [S] 200 NNTP Service Ready
          [C] HEAD <i.am.not.there@example.com>
          [S] 430 No Such Article Found
     Example of an unsuccessful retrieval of the header of an
     article by number
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HEAD 300256
          [S] 423 No such article number in this group
  
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     Example of an unsuccessful retrieval the header of an article
     by number because no newsgroup was selected first
          [S] 200 NNTP Service Ready
          [C] HEAD 300256
          [S] 412 No newsgroup selected
     Example of an attempt to retrieve the header of an article
     when the current group selected is empty
          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] HEAD
          [S] 420 No current article selected
     Example of a failure due to the service being unavailable
          [S] 200 NNTP Service Ready
          [C] HEAD <i.am.a.test.article@example.com>
          [S] 502 Service unavailable
  
  9.2.3  BODY
  
     BODY <message-id>
     BODY [number]
     The BODY command behaves identically to the ARTICLE command
     except that, if the article exists, only the body is
     presented (the blank line separating the headers and body
     MUST NOT be included).
  9.2.3.1  Responses
  
     First form (message-id specified):
  
            222 0 a  Article follows (multi-line response, a =
                     message-id)
            430      No article found with that message-id
            502      Program error, function not performed
  
     Second form (optional article number specified):
  
            222 n a  Article follows (multi-line response, n =
                     article number, a = message-id)
            412      No newsgroup selected
            420      No current article selected
            423      No such article in this newsgroup
  
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            502      Program error, function not performed
  
  
     Except that only the body is included in the response, the
     222 response behaves identically to the 220 response of the
     ARTICLE command.
  9.2.3.2  Examples
  
     Example of a successful retrieval of the body of an article
     (using no article number)
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] BODY
          [S] 222 3000234 <45223423@example.com>
          [S] This is just a test article.
          [S] .
     Example of a successful retrieval of the body of an article
     by message-id
          [S] 200 NNTP Service Ready
          [C] BODY <45223423@example.com>
          [S] 222 0 <45223423@example.com>
          [S] This is just a test article.
          [S] .
     Example of an unsuccessful retrieval of the body of an
     article by message-id
          [S] 200 NNTP Service Ready
          [C] BODY <i.am.not.there@example.com>
          [S] 430 No Such Article Found
     Example of an unsuccessful retrieval of the body of an
     article by number
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] BODY 300256
          [S] 423 No such article number in this group
     Example of an unsuccessful retrieval of the body of an
     article by number because no newsgroup was selected first
          [S] 200 NNTP Service Ready
          [C] BODY 300256
          [S] 412 No newsgroup selected
  
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     Example of an attempt to retrieve the body of an article when
     the current group selected is empty
          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] BODY
          [S] 420 No current article selected
     Example of a failure due to the service being unavailable
          [S] 200 NNTP Service Ready
          [C] BODY <i.am.a.test.article@example.com>
          [S] 502 Service unavailable
  
  9.2.4  STAT
  
     STAT <message-id>
     STAT [number]
     The STAT command behaves identically to the ARTICLE command
     except that, if the article exists, it is NOT presented to
     the client.
  
     This command allows the client to determine whether an
     article exists, and in the second form what its message-id
     is, without having to process an arbitrary amount of text.
  9.2.4.1  Responses
  
     First form (message-id specified):
  
            223 0 a  Article exists (a = message-id)
            430      No article found with that message-id
            502      Program error, function not performed
  
     Second form (optional article number specified):
  
            223 n a  Article exists (n = article number, a =
                     message-id)
            412      No newsgroup selected
            420      No current article selected
            423      No such article in this newsgroup
            502      Program error, function not performed
  
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     The parameters of the 223 response are identical to those
     that would have been given in a 220 response to the
     equivalent ARTICLE command. However, the response is NOT
     multi-line.
  9.2.4.2  Examples
  
     Example of STAT on an existing article (using no article
     number)
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] STAT
          [S] 223 3000234 <45223423@example.com>
     Example of a STAT of an existing article by message-id
          [S] 200 NNTP Service Ready
          [C] STAT <45223423@example.com>
          [S] 223 0 <45223423@example.com>
     Example of an STAT of an article not on the server by
     message-id
          [S] 200 NNTP Service Ready
          [C] STAT <i.am.not.there@example.com>
          [S] 430 No Such Article Found
     Example of STAT of an article not in the server by number
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] STAT 300256
          [S] 423 No such article number in this group
     Example of STAT of an article by number when no newsgroup was
     selected first
          [S] 200 NNTP Service Ready
          [C] STAT 300256
          [S] 412 No newsgroup selected
     Example of STAT of an article when the current group selected
     is empty
          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] STAT
          [S] 420 No current article selected
     Example of a failure due to the service being unavailable
  
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          [S] 200 NNTP Service Ready
          [C] STAT <i.am.a.test.article@example.com>
          [S] 502 Service unavailable
  
  9.3  Article Posting
  
     Article posting is done in one of two modes: individual
     article posting from news reading clients and article
     transfer from other news servers.
  
  9.3.1  POST
  
     POST
     If posting is allowed, response code 340 MUST be returned to
     indicate that the article to be posted should be sent.
     Response code 440 MUST be sent if that posting is prohibited
     for some installation-dependent reason.
  
     If posting is permitted, the article MUST be presented to the
     server by the client in the format specified by RFC 1036 (or
     by any of its successors or extensions). The text forming the
     header and body of the message to be posted MUST be sent by
     the client in the format defined above (section 0) for multi-
     line responses (except that there is no initial line
     containing a response code).  Thus a single period (".") on a
     line indicates the end of the text, and lines starting with a
     period in the original text have that period doubled during
     transmission.
  
     Following the presentation of the termination sequence by the
     client, the server MUST return a response code indicating
     success or failure of the article transfer. Note that
     response codes 340 and 440 are used in direct response to the
     POST command. Others are returned following the sending of
     the article.
  
     No attempt shall be made by the server to filter characters,
     fold or limit lines, or otherwise process incoming text. The
     intent is that the server just passes the incoming message to
     be posted to the server installation's news posting software,
     which is not defined by this memo.
  
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     The client SHOULD NOT assume that the article has been
     successfully transferred unless it receives an affirmative
     response from the server.  Since, however, the affirmative
     response may have been sent and lost, the client SHOULD use
     the same message-id in the article when resending it or check
     whether the article was successfully posted before resending
     it to ensure that the resend will not result in a duplicate
     article.
  
     This command is not streamable.
  9.3.1.1  Responses
  
            240 Article received ok
            340 Send article to be posted
            440 Posting not permitted
            441 Posting failed
  
  9.3.1.2  Examples
  
     Example of a successful posting
          [S] 200 NNTP Service Ready
          [C] POST
          [S] 340 Input article. End with <CR-LF>.<CR-LF>
          [C] From: ôDemo Userö <nobody@example.net>
          [C] Newsgroups: misc.test
          [C] Subject: I am just a test article
          [C] Organization: An Example Net
          [C]
          [C] This is just a test article.
          [C] .
          [S] 240 Article received ok
     Example of an unsuccessful posting
          [S] 200 NNTP Service Ready
          [C] POST
          [S] 340 Input article. End with <CR-LF>.<CR-LF>
          [C] From: ôDemo Userö <nobody@example.net>
          [C] Newsgroups: misc.test
          [C] Subject: I am just a test article
          [C] Organization: An Example Net
          [C]
  
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          [C] This is just a test article.
          [C] .
          [S] 441 Posting failed
     Example of an attempt to post when posting is not allowed
          [S] 201 NNTP Service Ready, read-only
          [C] POST
          [S] 440 Posting not permitted
  
  9.3.2  IHAVE
  
     IHAVE <message-id>
     The IHAVE command informs the server that the client has an
     article whose id is <message-id>.  If the server desires a
     copy of that article, it MUST return response code 335
     instructing the client to send the entire article.  If the
     server does not want the article (if, for example, the server
     already has a copy of it), response code 435 indicating that
     the article is not wanted MUST be returned.  Finally, if the
     article isn't wanted immediately but the client should retry
     later if possible (if, for example, another client is in the
     process of sending the same article to the server), response
     code 436 MUST be returned.
  
     If transmission of the article is requested, the client MUST
     send the entire article, including header and body, in the
     format defined above (section 4) for multi-line responses
     (except that there is no initial line containing a response
     code). Thus a single period (".") on a line indicates the end
     of the text, and lines starting with a period in the original
     text have that period doubled during transmission. The server
     MUST return either response code 235, indicating that the
     article was successfully transferred, response code 436,
     indicating that the transfer failed but should be tried again
     later, or response code 437, indicating that the article was
     rejected.
  
     This function differs from the POST command in that it is
     intended for use in transferring already-posted articles
     between hosts. It SHOULD NOT be used when the client is a
     personal news reading program, since this command indicates
     that the forthcoming article has already been posted at
     another site and is being forwarded from another host.
  
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     However, the server MAY elect not to post or forward the
     article if after further examination of the article it deems
     it inappropriate to do so. Reasons for such subsequent
     rejection of an article may include such problems as
     inappropriate newsgroups or distributions, disk space
     limitations, article lengths, garbled headers, and the like.
     These are typically restrictions enforced by the server
     host's news software and not necessarily the NNTP server
     itself.
  
     The client SHOULD NOT assume that the article has been
     successfully transferred unless it receives an affirmative
     response from the server.  A lack of response (such as a
     dropped network connection or a network timeout) SHOULD be
     treated the same as a 436 error response.
  
     Because some news server software may not be able immediately
     to determine whether or not an article is suitable for
     posting or forwarding, an NNTP server MAY acknowledge the
     successful transfer of the article (with a 235 response) but
     later silently discard it.
  9.3.2.1  Responses
  
            235 Article transferred ok
            335 Send article to be transferred
            435 Article not wanted, please donÆt send it.
            436 Transfer failed, try again later.
            437 Article rejected, please donÆt sent it again.
  
  9.3.2.2  Examples
  
     Example of successfully sending an article to another site
          [S] 200 NNTP Service Ready
          [C] IHAVE <i.am.an.article.you.will.want@example.com>
          [S] 335 Send it. End with <CR-LF>.<CR-LF>
          [C] Path: pathost!demo!somewhere!not-for-mail
          [C] From: ôDemo Userö <nobody@example.com>
          [C] Newsgroups: misc.test
          [C] Subject: I am just a test article
          [C] Date: 6 Oct 1998 04:38:40 -0500
  
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          [C] Organization: An Example Com, San Jose, CA
          [C] Message-ID: <i.am.a.test.article@example.com>
          [C]
          [C] This is just a test article.
          [C] .
          [S] 235 Article transferred ok
     Example of sending an article to another site that rejects it
          [S] 200 NNTP Service Ready
          [C] IHAVE <i.am.an.article.you.will.want@example.com>
          [S] 335 Send it. End with <CR-LF>.<CR-LF>
          [C] Path: pathost!demo!somewhere!not-for-mail
          [C] From: ôDemo Userö <nobody@example.com>
          [C] Newsgroups: misc.test
          [C] Subject: I am just a test article
          [C] Date: 6 Oct 1998 04:38:40 -0500
          [C] Organization: An Example Com, San Jose, CA
          [C] Message-ID: <i.am.a.test.article@example.com>
          [C]
          [C] This is just a test article.
          [C] .
          [S] 437 Article rejected. Don't send again
     Example of sending an article to another site where the
     transfer fails
          [S] 200 NNTP Service Ready
          [C] IHAVE <i.am.an.article.you.will.want@example.com>
          [S] 335 Send it. End with <CR-LF>.<CR-LF>
          [C] Path: pathost!demo!somewhere!not-for-mail
          [C] From: ôDemo Userö <nobody@example.com>
          [C] Newsgroups: misc.test
          [C] Subject: I am just a test article
          [C] Date: 6 Oct 1998 04:38:40 -0500
          [C] Organization: An Example Com, San Jose, CA
          [C] Message-ID: <i.am.a.test.article@example.com>
          [C]
          [C] This is just a test article.
          [C] .
          [S] 436 Transfer failed
     Example of sending an article to a site that already has it
          [S] 200 NNTP Service Ready
          [C] IHAVE <i.am.an.article.you.have@example.com>
          [S] 435 Duplicate
  
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     Example of sending an article to a site that requests the
     article be tried again later
          [S] 200 NNTP Service Ready
          [C] IHAVE <i.am.an.article.you.defer@example.com>
          [S] 436 Retry later
  
  9.4  The LIST Keyword
  
  9.4.1  LIST
  
     LIST [ACTIVE [wildmat]]
     The response to the LIST keyword with no parameters returns a
     list of valid newsgroups and associated information.  Each
     newsgroup is sent as a line of text in the following format:
        group first last status
     where <group> is the name of the newsgroup, <last> is the
     number of the last known article currently in that newsgroup,
     <first> is the number of the first article currently in the
     newsgroup, and <status> indicates the current status of the
     group on this server. Typically, the <status> will consist of
     the US-ASCII character 'y' where posting is permitted, 'n'
     where posting is not permitted and 'm' where postings will be
     forwarded to the newsgroup moderator by the news server.
     Other status strings may exist. The definition of these other
     values and the circumstances under which they are returned is
     covered in other specifications. Each field in the line is
     separated from its neighboring fields by one or more US-ASCII
     spaces.
  
     The <first> and <last> fields will always be numeric.  They
     may have leading zeros. The <first> field corresponds to the
     "reported low water mark" and the <last> field corresponds to
     the "reported high water mark" described in the GROUP command
     (see Section 9.1.1.1).
  
     The status of a newsgroup only indicates how posts to that
     newsgroup are processed. It does not indicate if the current
     client is permitted to post. That is indicated by the status
     code returned as part of the greeting.
     Please note that an empty list (i.e., the text body returned
     by this command consists only of the terminating period) is a
  
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     possible valid response, and indicates that there are
     currently no valid newsgroups.
  
     If the optional wildmat parameter is specified, the list is
     limited to only the groups that match the pattern.
  9.4.1.1  Responses
            215 Information Follows (multi-line response)
  
  9.4.1.2  Examples
  
     Example of LIST returning a list of newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST
          [S] 215 list of newsgroups follows
          [S] misc.test 3000234 3002322 y
          [S] alt.fc-writers.recovery 1 4 y
          [S] tx.natives.recovery 56 89 y
          [S] .
     Example of LIST returning no newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST
          [S] 215 list of newsgroups follows
          [S] .
  
  9.4.2  LIST ACTIVE.TIMES
  
     LIST ACTIVE.TIMES [wildmat]
     The active.times file is maintained by some news transport
     systems to contain information about who created a particular
     newsgroup and when. The format of this file includes three
     fields separated from each other by one or more US-ASCII
     space characters. The first field is the name of the
     newsgroup. The second is the time when this group was created
     on this news server measured in seconds since the start of
     January 1, 1970.  The third is the email address of the
     entity that created the newsgroup and must be a mailbox as
     defined in RFC 28228. When executed, the information is
     displayed following the 215 response. When display is
     completed, the server will send a period on a line by itself.
     If the information is not available, the server will return
  
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     the 503 error response. If the server does not recognize the
     command, it SHOULD return the 501 error response.
  
     If the optional wildmat parameter is specified, the list is
     limited to only the groups that match the pattern.
  9.4.2.1  Responses
  
            215 Information Follows (multi-line response)
            501 Syntax error
            503 Program error, function not performed
  9.4.2.2  Examples
  
     Example of LIST ACTIVE.TIMES returning a list of newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST ACTIVE.TIMES
          [S] 215 information follows
          [S] misc.test 930445408 <creatme@isc.org>
          [S] alt.rfc-writers.recovery 930562309 <m@example.com>
          [S] tx.natives.recovery 930678923 <sob@academ.com>
          [S]  .
     Example of LIST ACTIVE.TIMES returning an error (The server
     software is not configured to maintain this information, but
     does recognize the command as valid.)
          [S] 200 NNTP Service Ready
          [C] LIST ACTIVE.TIMES
          [S] 503 program error, function not performed
     Example of LIST ACTIVE.TIMES sent to a server that does not
     recognize this argument (e.g. The software does not maintain
     this information.)
          [S] 200 NNTP Service Ready
          [C] LIST ACTIVE.TIMES
          [S] 501 Syntax Error
  
  9.4.3  LIST DISTRIBUTIONS
  
     LIST DISTRIBUTIONS
     The distributions file is maintained by some news transport
     systems to contain information about valid values for the
     Distribution: line in a news article header and about what
     the values mean. Each line contains two fields, the value and
  
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     a short explanation on the meaning of the value. The first
     field is separated from the second field by one or more US-
     ASCII spaces. When executed, the information is displayed
     following the 215 response. When display is completed, the
     server will send a period on a line by itself. If the
     information is not available, the server will return the 503
     error response. If the server does not recognize this
     command, it SHOULD return the 501 error response.
  9.4.3.1  Responses
  
            215 Information Follows (multi-line response)
            501 Syntax error
            503 Program error, function not performed
  
  9.4.3.2  Examples
  
     Example of LIST DISTRIBUTIONS returning a list of newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST DISTRIBUTIONS
          [S] 215 information follows
          [S] usa United States of America
          [S] na North America
          [S] world All over the World
          [S] .
     Example of LIST DISTRIBUTIONS returning an error (e.g. The
     server software is not configured to maintain this
     information, but does recognize the command as valid.)
          [S] 200 NNTP Service Ready
          [C] LIST DISTRIBUTIONS
          [S] 503 program error, function not performed
     Example of LIST DISTRIBUTIONS sent to a server that does not
     recognize the command (e.g. The server does not maintain this
     information regardless of configuration.)
          [S] 200 NNTP Service Ready
          [C] LIST DISTRIBUTIONS
          [S] 501 Syntax Error
  
  9.4.4  LIST DISTRIB.PATS
  
     LIST DISTRIB.PATS
  
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     The distrib.pats file is maintained by some news transport
     systems to allow clients to choose a value for the
     Distribution: line in the header of a news article being
     posted. The information returned consists of lines, in no
     particular order, each of which contains three fields
     separated by colons. These fields are a weight, a wildmat
     (which may be a simple group name), and a Distribution:
     value, in that order.
  
     The client MAY use this information to select a Distribution:
     value based on the name of a newsgroup. To do so, it should
     determine the lines whose second field matches the newsgroup
     name, select that line with the highest weight (with 0 being
     the lowest), and use the Distribution: field from that line.
     When executed, the information is displayed following the 215
     response.  When display is completed, the server will send a
     period on a line by itself. If the information is not
     available, the server will return the 503 error response. If
     this command is not recognized, the server SHOULD return the
     501 error response.
  9.4.4.1  Responses
  
            215 Information Follows (multi-line response)
            501 Syntax error
            503 Program error, function not performed
  
  9.4.4.2  Examples
  
     Example of LIST DISTRIB.PATS returning a list of newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST DISTRIB.PATS
          [S] 215 information follows
          [S] 10:local.*:local
          [S] .
     Example of LIST DISTRIB.PATS returning an error (e.g. The
     server software is not configured to maintain this
     information, but does recognize the command as valid.)
          [S] 200 NNTP Service Ready
          [C] LIST DISTRIB.PATS
          [S] 503 program error, function not performed
  
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     Example of LIST DISTRIB.PATS sent to a server that does not
     recognize the command (e.g. The software does not maintain
     this information regardless of configuration.)
          [S] 200 NNTP Service Ready
          [C] LIST DISTRIB.PATS
          [S] 501 Syntax Error
  
  9.4.5  LIST NEWSGROUPS
  
     LIST NEWSGROUPS [wildmat]
     The newsgroups file is maintained by some news transport
     systems to contain the name of each newsgroup that is active
     on the server and a short description about the purpose of
     each newsgroup. Each line in the file contains two fields,
     the newsgroup name and a short explanation of the purpose of
     that newsgroup. The first field is separated from the second
     field by one or more US-ASCII spaces. When executed, the
     information is displayed following the 215 response. When
     display is completed, the server will send a period on a line
     by itself. If the information is not available, the server
     will return the 503 response. If the server does not
     recognize the command it should return a 501 response. If the
     optional wildmat parameter is specified, the list is limited
     to only those groups that match the wildmat (no matching is
     done on the group descriptions). If nothing is matched an
     empty list is returned, not an error.
  9.4.5.1  Responses
  
            215 Information Follows (multi-line response)
            501 Syntax error
            503 Program error, function not performed
  
  9.4.5.2  Examples
  
     Example of LIST NEWSGROUPS returning a list of newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST NEWSGROUPS
          [S] 215 information follows
          [S] misc.test General Usenet testing
          [S] alt.rfc-writers.recovery RFC Writers Recovery
  
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          [S] tx.natives.recovery Texas Natives Recovery
          [S] .
     Example of LIST NEWSGROUPS returning an error (e.g. The
     server software recognizes the command as valid, but the
     information is not available.)
          [S] 200 NNTP Service Ready
          [C] LIST NEWSGROUPS
          [S] 503 program error, function not performed
  
  9.5  Standard extensions
  
     Each of the following sections describes an extension that a
     server MAY provide. If the server provides the extension, it
     MUST include the appropriate extension label in the response
     to LIST EXTENSIONS. If it does not provide it, it MUST NOT
     include the appropriate extension label. The descriptions of
     facilities in each section are written as if the extension is
     provided. If it is not provided, the entire section should be
     ignored.
  
  9.5.1  LISTGROUP extension
  
     This extension provides one command and has the extension
     label LISTGROUP.
  9.5.1.1  The LISTGROUP Command
  
     LISTGROUP [ggg]
     The LISTGROUP command is used to get a listing of all the
     article numbers in a particular newsgroup.
  
     The optional parameter ggg is the name of the newsgroup to be
     selected (e.g. "news.software.misc").  A list of valid
     newsgroups may be obtained from the LIST command. If no group
     is specified, the current group is used as the default
     argument.
  
     The successful selection response will be a list of the
     article numbers in the group followed by a period on a line
     by itself. The list starts on the next line following the 211
     response code.
  
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     When a valid group is selected by means of this command, the
     internally maintained "current article pointer" MUST be set
     to the first article in the group and the name of the current
     newsgroup MUST be set to the selected newsgroup name. If an
     invalid group is specified, the previously selected group and
     article remain selected.  If an empty newsgroup is selected,
     the "current article pointer" may be in an indeterminate
     state and should not be used.
  
     The LISTGROUP keyword MAY be used by a client as a
     replacement for the GROUP command in establishing a valid
     "current article pointer." After a successful response is
     received, any other command may be used that depends on
     having the "current article pointer" be valid.
  
     If a group name is specified and that group is not available
     on that server, the server MUST respond with the 411 error
     code.
  
     A server that does not implement this command SHOULD return a
     500 error response.
  9.5.1.1.1  Responses
  
            211 List of article numbers follow (multi-line
                response)
            411 No such group
            412 No newsgroup currently selected
            500 Command not recognized
  
  9.5.1.1.2  Examples
  
     Example of LISTGROUP on an empty group:
          [S] 200 NNTP Service Ready
          [C] LISTGROUP example.empty.newsgroup
          [S] 211 list of article numbers follows
          [S] .
     Example of LISTGROUP on a valid current group:
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 2000 3000234 3002322 misc.test selected
  
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          [C] LISTGROUP
          [S] 211 list follows
          [S] 3000234
          [S] 3000237
          [S] 3000238
          [S] 3000239
          [S] 3002322
          [S] .
     Example of LISTGROUP failing because no group has been
     selected:
          [S] 200 NNTP Service Ready
          [C] LISTGROUP
          [S] 412 no current group
          [C] GROUP example.is.sob.bradner.or.barber
          [S] 411 no such group
          [C] LISTGROUP
          [S] 412 no current group
  
  9.5.2  The OVER Extension
  
  This extension provides two commands, OVER and LIST
  OVERVIEW.FMT. The label for this extension is OVER.
  9.5.2.1  LIST OVERVIEW.FMT
  
     LIST OVERVIEW.FMT
     The overview.fmt file is maintained by some news transport
     systems to contain the order in which header information is
     stored in the overview databases for each newsgroup.  When
     executed, news article header fields are displayed one line
     at a time in the order in which they are stored in the
     overview database[9] following the 215 response.  When
     display is completed, the server will send a period on a line
     by itself. If the information is not available, the server
     will return the 503 response.
  
     If the header has the word "full" (without quotes) after the
     colon, the header's name is prepended to its field in the
     output returned by the server.
  
     This is command is part of the optional OVER extension which
     includes the OVER command defined in section . If the OVER
  
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     extension is not implemented, then this command MUST NOT be
     implemented. If that case, the server MUST return a 501 error
     response when this command is presented by the client.
  
  9.5.2.1.1  Responses
  
            215 Information follows (multi-line response)
            501 Syntax Error
            503 Program error, function not performed
  
  9.5.2.1.2  Examples
  
     Example of LIST OVERVIEW.FMT returning a list of newsgroups
          [S] 200 NNTP Service Ready
          [C] LIST OVERVIEW.FMT
          [S] 215 Order of fields in overview database.
          [S] Subject:
          [S] From:
          [S] Date:
          [S] Message-ID:
          [S] .
     Example of LIST OVERVIEW.FMT returning an error
          [S] 200 NNTP Service Ready
          [C] LIST OVERVIEW.FMT
          [S] 503 program error, function not performed
  9.5.2.2  OVER
  
  OVER [range]
     The OVER command returns specific header information for the
     article(s) specified from the current selected group. The
     information returned in the response to this command can be
     used by clients to follow discussion threads.
     The optional range argument may be any of the following:
     *  an article number
     *  an article number followed by a dash to indicate all
        following
     *  an article number followed by a dash followed by another
        article number
  
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     If no argument is specified, then information from the
     current article is displayed. Successful responses start with
     a 224 response followed by the overview information for all
     matched messages. Once the output is complete, a period is
     sent on a line by itself. A newsgroup must have been selected
     earlier, else a 412 error response is returned. If no
     articles are in the range specified, the server returns a 420
     error response. A 502 response will be returned if the client
     only has permission to transfer articles. A 500 response
     SHOULD be returned by servers that do not implement this
     command.
  
     The output consists of one line per article, sorted in
     numerical order of article number. Each line consists of a
     number of fields separated by an US-ASCII TAB character. The
     first 8 fields MUST be the following, in order: article
     number, subject, author, date, message-ID, references, byte
     count, line count.
  
     The article number field should contain the article number
     (within the group). The line count and byte count are
     calculated by the server based on the actual size and number
     of lines in the article. The content of the rest of the
     fields is formed by taking the original header content (such
     as the raw subject line from the article), removing all US-
     ASCII CRLF pairs, and then replacing each remaining US-ASCII
     NUL, TAB, CR, or LF character with a single US-ASCII space.
     The content of any subsequent field is given by the response
     to the LIST OVERVIEW.FMT command.  A field may be empty (in
     which case there will be two adjacent US-ASCII tabs, and a
     sequence of trailing US-ASCII tabs may be omitted).
     The server SHOULD not produce output for articles that no
     longer exist.
  9.5.2.2.1  Responses
  
            224 Overview information follows (multi-line
                response)
            412 No newsgroup currently selected
            420 No article(s) selected
            500 Command not recognized
            502 Program error, functions no performed
  
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  9.5.2.2.2  Examples
  
     Example of a successful retrieval of overview information for
     an article (using no article number)
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] OVER
          [S] 224 Overview information follows
              300234|I am just a test article|öDemo Userö
               <nobody@example.com>|6 Oct 1998 04:38:40 -0500|
               <45223423@example.com>|<45454@example.net>|1234|
               17|Xref: news.example.com misc.test:3000363
          [S] .
     [Please note that the line that begins with 300234 is all one
     line that has been wrapped for readability. A vertical bar
     has been inserted to show where the US-ASCII TAB should
     actually be.]
     Example of an unsuccessful retrieval of overview information
     on an article by number
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] OVER 300256
          [S] 420 No such article in this group
     Example of an unsuccessful retrieval of overview information
     by number because no newsgroup was selected first
          [S] 200 NNTP Service Ready
          [C] OVER
          [S] 412 No newsgroup selected
     Example of an attempt to retrieve an article when the current
     group selected is empty
          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] OVER
          [S] 420 No current article selected
  
  9.5.3  The HDR Extension
  
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     This extension provides one new command, HDR. The label for
     this extension is HDR
  9.5.3.1  HDR
  
     HDR header range|<message-id>
     The HDR command is used to retrieve specific headers from
     specific articles in the currently selected group.
  
     The required header parameter is the name of a header line
     (e.g.  "subject") in a newsgroup article. See RFC-1036 for a
     list of valid header lines. The required range argument may
     be any of the following:
     *  an article number
     *  an article number followed by a dash to indicate all
        following
     *  an article number followed by a dash followed by another
        article number.
  
     The required message-id argument indicates a specific
     article. The range and message-id arguments are mutually
     exclusive.
  
     A successful response consists of a 221 code followed by the
     output from the command. The output consists of one line for
     each article where the relevant header line exists. The line
     consists of the article number, a US-ASCII space, and then
     the contents of the header (without the header name). A valid
     response includes an empty list (indicating that there were
     no matches). Once the output is complete, a period is sent on
     a line by itself. If the optional argument is a message-id
     and no such article exists, a 430 error response shall be
     returned. A 502 response shall be returned if the client only
     has permission to transfer articles. A 500 response SHOULD be
     issued by all servers that do not recognize this command.
  9.5.3.1.1  Responses
  
            221  Header follows (multi-line response)
            412  No newsgroup selected
            430  No such article
            500  Command not recognized
  
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            502  Program error, function not performed
  
  9.5.3.1.2  Examples
  
     Example of a successful retrieval of subject lines from a
     range of articles
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR Subject 3000234-300238
          [S] 221 Header Follows
          [S] 3000234 I am just a test article
          [S] 3000237 Re: I am just a test article
          [S] 3000238 Ditto
          [S] .
     Example of a successful retrieval of header from an article
     by message-id
          [S] 200 NNTP Service Ready
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR subject <i.am.a.test.article@example.com>
          [S] 221 Header information follows
          [S] 3000345 I am just a test article
          [S] .
     Example of an unsuccessful retrieval of a header from an
     article by message-id
          [S] 200 NNTP Service Ready
          [C] HDR subject <i.am.not.there@example.com>
          [S] 430 No Such Article Found
     Example of an unsuccessful retrieval of headers from articles
     by number because no newsgroup was selected first
          [S] 200 NNTP Service Ready
          [C] HDR subject 300256-
          [S] 412 No newsgroup selected
     Example of an unsuccessful retrieval of headers from articles
     by message-id because no newsgroup was selected first
          [S] 200 NNTP Service Ready
          [C] HDR subject <i.am.a.test.article@example.com>
          [S] 412 No newsgroup selected
     Example of retrieving header information when the current
     group selected is empty
  
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          [S] 200 NNTP Service Ready
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] HDR subject 0-
          [S] 221 Headers follow
              .
     Example of a failure due to restrictions configured into the
     server
          [S] 200 NNTP Service Ready
          [C] GROUP news.group
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR Subject 3000234-300238
          [S] 502 Service Unavailable
  
  10  The CONCLUSION Step
  
  10.1  QUIT
  
     QUIT
     The server process MUST acknowledge the QUIT command and then
     close the connection to the client.  This is the preferred
     method for a client to indicate that it has finished all its
     transactions with the NNTP server.
  
     If a client simply disconnects (or the connection times out
     or some other fault occurs), the server MUST gracefully cease
     its attempts to service the client, disconnecting from its
     end if necessary.
  
  10.1.1  Responses
  
            250 Connection closing
  
  10.1.2  Example
  
          [S] 200 NNTP Service Ready
          [C] QUIT
          [S] 205 closing connection
             [Server closes connection.]
  
  11  Other Keywords
  
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     There are other keywords that may be used at any time between
     the beginning of a session and its termination.  Using these
     keywords does not alter any state information, but the
     response generated from the use of these keywords may provide
     useful information to clients that use them.
  
  11.1  DATE
  
     DATE
     This command exists to help clients find out the current
     Coordinated Universal Time[10] from the server's perspective.
     This command MUST NOT be used as a substitute for NTP[11],
     but to provide information that might be useful when using
     the NEWNEWS command (see section 11.4). A system providing
     NNTP service SHOULD implement NTP for the purposes of keeping
     the system clock as accurate as possible.
  
     This command returns a one-line response code of 111 followed
     by the date and time on the server in the form
     YYYYMMDDhhmmss. This date and time is presented in
     Coordinated Universal Time.
  
  11.1.1  Response
  
            111 YYYYMMDDhhmmss Current date and time on server
                               presented in Coordinated
                               Universal Time
  
  11.1.2  Example
  
          [S] 200 NNTP Service Ready
          [C] DATE
          [S] 111 19990623135624
  
  11.2  The HELP Command
  
     HELP
     This command provides a short summary of commands that are
     understood by this implementation of the server. The help
     text will be presented as a textual response terminated by a
     single period on a line by itself.
  
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     This text is not guaranteed to be in any particular format
     and SHALL NOT be used by clients as a replacement for the
     LIST EXTENSIONS command described in section 8.1.
  
  11.2.1  Responses
  
            100 Help text follows (multi-line response)
  
  11.2.2  Example
  
          [S] 200 NNTP Service Ready
          [C] HELP
          [S] 100 Help text follows
          [S] This is some help text. There is no specific
          [S] formatting requirement for this test, though
          [S] it is customary for it to list the valid commands
          [S] and give a brief definition of what they do
          [S] .
  
  11.3  NEWGROUPS
  
     NEWGROUPS date time [GMT]
     A list of newsgroups created since <date and time> MUST be
     listed in the same format as the LIST command.
  
     The date is sent as 6 or 8 digits in the format [XX]YYMMDD,
     where XX is the first two digits of the year, YY is the last
     two digits of the year, MM is the two digits of the month
     (with leading zero, if appropriate), and DD is the day of the
     month (with leading zero, if appropriate). If the first two
     digits of the year are not specified, the year is to be taken
     from the current century if YY is smaller than or equal to
     the current year, otherwise the year is from the previous
     century.
  
     Time must also be specified.  It must be as 6 digits HHMMSS
     with HH being hours in the 24-hour clock 00-23, MM minutes
     00-59, and SS seconds 00-60, which allows for leap seconds.
     The token "GMT" specifies that the date and time are given in
     Coordinated Universal Time. If the token "GMT" is omitted
     then the date and time are specified in the server's local
     timezone. Note that there is no way using the protocol
  
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     specified in this memo to establish the server's local
     timezone.
  
     Note that an empty list (i.e., the text body returned by this
     command consists only of the terminating period) is a
     possible valid response, and indicates that there are
     currently no new newsgroups.
  
     Clients SHOULD make all queries using Coordinated Universal
     Time (i.e. by including the ôGMTö parameter) when possible.
  
  11.3.1  Responses
  
            231 List of new newsgroups follows (multi-line
                response)
  
  11.3.2  Examples
  
     Example where there are new groups
          [S] 200 NNTP Service Ready
          [C] NEWGROUPS 19990624 000000 GMT
          [S] 231 list of new newsgroups follows
          [S] alt.rfc-writers.recovery
          [S] tx.natives.recovery
          [S] .
     Example where there are no new groups
          [S] 200 NNTP Service Ready
          [C] NEWGROUPS 19990624 000000 GMT
          [S] 231 list of new newsgroups follows
          [S] .
  
  11.4  NEWNEWS
  
     NEWNEWS wildmat date time [GMT]
     A list of message-ids of articles posted or received on the
     server to the list of newsgroups that match the wildmat since
     "date" will be listed. The format of the listing will be one
     message-id per line, as though text were being sent. The
     order of the response has no specific significance and may
     vary from response to response in the same session. If a
     message-id appears more than once it has the same meaning as
  
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     if it appears only once. A single line consisting solely of
     one period followed by CR-LF will terminate the list.
  
     Date and time are in the same format as the NEWGROUPS
     command.
  
     Note that an empty list (i.e., the text body returned by this
     command consists only of the terminating period) is a
     possible valid response, and indicates that there is
     currently no new news.
  
     Clients SHOULD make all queries in Coordinated Universal Time
     (i.e. by using the ôGMTö parameter) when possible.
  
  11.4.1  Responses
  
       230 List of new articles by message-id follows (may be a
            multi-line response)
  
  11.4.2  Examples
  
     Example where there are new articles
          [S] 200 NNTP Service Ready
          [C] NEWNEWS news.*,sci.* 19990624 000000 GMT
          [S] 230 list of new articles by message-id follows
          [S] <i.am.a.new.article@example.com>
          [S] <i.am.another.new.article@example.com>
  
     Example where there are no new articles
          [S] 200 NNTP Service Ready
          [C] NEWNEWS alt.* 19990624 000000 GMT
          [S] 230 list of new articles by message-id follows
          [S] .
  
  12  Framework for NNTP Extensions
  
     Although NNTP is widely and robustly deployed, some parts of
     the Internet community might wish to extend the NNTP service.
     This memo defines a means whereby an extended NNTP client may
     query the server to determine the service extensions that it
     supports.
  
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     It must be emphasized that any extension to the NNTP service
     should not be considered lightly. NNTP's strength comes
     primarily from its simplicity.  Experience with many
     protocols has shown that:
  
     Protocols with few options tend towards ubiquity, whilst
     protocols with many options tend towards obscurity.
  
     This means that each and every extension, regardless of its
     benefits, must be carefully scrutinized with respect to its
     implementation, deployment, and interoperability costs. In
     many cases, the cost of extending the NNTP service will
     likely outweigh the benefit.
  
     Given this environment, the framework for the extensions
     described in this memo consists of:
     *  a mechanism for clients to determine a server's available
        extensions
     *  a registry of NNTP service extensions
  
     The LIST EXTENSIONS command is described in section 8.1 of
     this memo and is the mechanism for clients to use to
     determine what extensions are available for client use.
  
     The IANA shall maintain a registry of NNTP service
     extensions.
  
     An extension is identified by a unique extension-label, which
     is a string of 1 to 12 uppercase letters. The extension-label
     will often be the name of a new command that the extension
     adds. However this is not a requirement: an extension might
     not add any new commands or keywords.
  
     An extension is either a private extension or else it is
     included in the IANA registry and is defined in an RFC. Such
     RFCs either must be on the standards-track or must define an
     IESG-approved experimental protocol.
  
     The definition of an extension must include:
     *  a descriptive name for the extension
  
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     *  the extension-label (which is returned by LIST EXTENSIONS
        to indicate to the client that the server supports this
        particular extension)
     *  the syntax, values, and meanings of any parameters
        following the extension-label in the output of LIST
        EXTENSIONS
     *  any new NNTP keywords associated with the extension
     *  the syntax and possible values of parameters associated
        with the new NNTP keywords
     *  any new parameters the extension associates with any other
        pre-existing NNTP keywords
     *  how support for the extension affects the behavior of a
        server and NNTP client
     *  any increase in the maximum length of commands over the
        value specified in this memo
     *  a specific statement about the effect on streaming this
        extension may have (if any)
  
     The extension-label of private extensions MUST begin with
     "X". The extension-label of registered extensions MUST NOT
     begin with "X".
  
     Any keyword values presented in the NNTP response that do not
     begin with "X" MUST correspond to a standard, standards-
     track, or IESG-approved experimental NNTP service extension
     registered with IANA.  A conforming server MUST NOT offer non
     "X" prefixed keyword values that are not described in a
     registered extension.
  
     Except where stated otherwise, the commands in this memo are
     understood (even if not supported) by all servers and are not
     described in the list of features returned by the LIST
     EXTENSIONS command.
  
     A server MAY provide additional keywords - either new
     commands or new parameters to existing commands - as part of
     a private extension. These new keywords MUST begin with "X".
  
     A server MUST NOT send different response codes to basic NNTP
     commands documented here or commands documented in registered
     extensions in response to the availability or use of a
     private extension.
  
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  12.1  Initial IANA Registry
  
     The IANA's initial registry of NNTP service extensions
     consists of these entries:
    Service Extension   NNTP Extension Label    Added Behavior
    Overview Support           OVER         Defined in this memo
    Specific Article
                              LISTGROUP       Defined in this memo
         Numbers
     Header Pattern
                                 HDR          Defined in this memo
        Matching
  
  13  Augmented BNF Syntax for NNTP Commands
  
     This syntax defines the non-terminal "command". The non-
     terminal "parameter" is used for command parameters whose
     syntax is specified elsewhere. The syntax is in alphabetical
     order. Note that ABNF strings are case insensitive.
  
        article-command = "ARTICLE" [1*WSP (msg-id / article-
              number)] *WSP CRLF
        article-number = 1*16DIGIT
        argument = parameter ; excluding sequence ".."
        body-command = "BODY" [1*WSP (msg-id / article-number)]
              *WSP CRLF
        command = article-command /
              body-command /
              date-command /
              group-command /
              head-command /
              help-command /
              ihave-command /
              last-command /
              list-active-times-command /
              list-distrib-pats-command /
              list-distributions-command /
              list-extensions-command /
              list-newsgroups-command /
              list-overview-fmt-command /
              list-command /
              listgroup-command /
              mode-reader-command /
  
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              newgroups-command /
              newnews-command /
              next-command /
              over-command /
              hdr-command /
              post-command /
              quit-command /
              stat-command
        CR = %x0D
        CRLF = CR LF
        date-command = "DATE" *WSP CRLF
        date = 6*8DIGIT
        DIGIT = %x30-39
        group-command = "GROUP" 1*WSP wildmat *WSP CRLF
        hdr-command = "HDR" 1*WSP header 1*WSP (range / msg-id)
              *WSP CRLF
        head-command = "HEAD" [1*WSP (msg-id / article-number)]
              *WSP CRLF
        header = parameter
        help-command = "HELP" *WSP CRLF
        HT = %x09
        ihave-command = "IHAVE" 1*WSP msg-id *WSP CRLF
        last-command = "LAST" *WSP CRLF
        LF = %x0A
        list-active-times-command = "LIST" 1*WSP "ACTIVE.TIMES"
              [1*WSP wildmat] *WSP CRLF
        list-command = "LIST" [1*WSP "ACTIVE" [1*WSP wildmat]]
              *WSP CRLF
        list-distrib-pats-command = "LIST" 1*WSP "DISTRIB.PATS"
              *WSP CRLF
        list-distributions-command = "LIST" 1*WSP "DISTRIBUTIONS"
              *WSP CRLF
        list-extensions-command = "LIST" 1*WSP "EXTENSIONS" *WSP
              CRLF
        list-newsgroups-command = "LIST" 1*WSP "NEWSGROUPS" [1*WSP
              wildmat] *WSP CRLF
        list-overview-fmt-command = "LIST" 1*WSP "OVERVIEW.FMT"
              *WSP CRLF
        listgroup-command = "LISTGROUP" [1*WSP wildmat] *WSP CRLF
        mode-reader-command = "MODE" 1*WSP "READER" *WSP CRLF
        msg-id = <defined in section 9.1 of this memo>
  
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        newgroups-command = "NEWGROUPS" 1*WSP date 1*WSP time
              1*WSP "GMT"] *WSP CRLF
        newnews-command = "NEWNEWS" 1*WSP wildmat 1*WSP date
           1*WSP time [1*WSP "GMTö]*WSP CRLF
        next-command = "NEXT" *WSP CRLF
        over-command = "OVER" [1*WSP range] *WSP CRLF
        parameter = 1*(%x21-FF) ; generic command parameter
        post-command = "POST" *WSP CRLF
        quit-command = "QUIT" *WSP CRLF
        range = article-number ["-" [article-number]]
        SP = %x20
        stat-command = "STAT" [1*WSP (msg-id / article-number)]
              *WSP CRLF
        time = 6DIGIT
        UTF-8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4 / UTF8-5 /
              UTF8-6
        UTF8-1 = %x80-BF
        UTF8-2 = %xC0-DF UTF8-1
        UTF8-3 = %xE0-EF 2UTF8-1
        UTF8-4 = %xF0-F7 3UTF8-1
        UTF8-5 = %xF8-FB 4UTF8-1
        UTF8-6 = %xFC-FD 5UTF8-1
        wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
        wildmat-pattern = 1*wildmat-item
        wildmat-item = wildmat-exact / wildmat-wild
        wildmat-exact = %x21-29 / %x2B / %x2D-3E / %x40-5A / %x5E-
              7F /UTF-8-non-ascii  ; exclude * , ? [ \ ]
        wildmat-wild = "*" / "?"
        WSP = SP / HT
  
  14  Security Considerations
  
     This section is meant to inform application developers,
     information providers, and users of the security limitations
     in NNTP as described by this memo. The discussion does not
     include definitive solutions to the problems revealed, though
     it does make some suggestions for reducing security risks.
  
  14.1  Personal and Proprietary Information
  
     NNTP, because it was created to distribute network news
     articles, will forward whatever information is stored in
  
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     those articles. Specification of that information is outside
     this scope of this memo, but it is likely that some personal
     and/or proprietary information is available in some of those
     articles. It is very important that designers and
     implementers provide informative warnings to users so
     personal and/or proprietary information is not disclosed
     inadvertently. Additionally, effective and easily understood
     mechanisms to manage the distribution of news articles must
     be provided to NNTP Server administrators, so that they are
     able to report with confidence what information is and is not
     being forwarded in news articles passing though their
     servers.
  
  14.2  Abuse of Server Log Information
  
     A server is in the position to save session data about a
     user's requests that might identify their reading patterns or
     subjects of interest. This information is clearly
     confidential in nature and its handling can be constrained by
     law in certain countries. People using the NNTP protocol to
     provide data are responsible for ensuring that such material
     is not distributed without the permission of any individuals
     that are identifiable by the published results.
  
  14.3  Weak Authentication and Access Control
  
     There is no user-based or token-based authentication in the
     basic NNTP specification. Access is normally controlled by
     server configuration files. Those files specify access by
     using domain names or IP addresses. However, this memo does
     permit the creation of extensions to the NNTP protocol itself
     for such purposes. While including such mechanisms is
     optional, doing so is strongly encouraged.
  
     Other mechanisms are also available. For example, a proxy
     server could be put in place that requires authentication
     before connecting via the proxy to the NNTP server.
  
  14.4  DNS Spoofing
  
     Many existing NNTP implementations authorize incoming
     connections by checking the IP address of that connection
  
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     against the IP addresses obtained via DNS lookups of lists of
     domain names given in local configuration files.  Servers
     that use this type of authentication, and clients that find a
     server by doing a DNS lookup of the server name, rely very
     heavily on the Domain Name Service, and are thus generally
     prone to security attacks based on the deliberate
     misassociation of IP addresses and DNS names. Clients and
     servers need to be cautious in assuming the continuing
     validity of an IP number/DNS name association.
  
     In particular, NNTP clients and servers SHOULD rely on their
     name resolver for confirmation of an IP number/DNS name
     association, rather than caching the result of previous host
     name lookups. Many platforms already can cache host name
     lookups locally when appropriate, and they SHOULD be
     configured to do so. It is proper for these lookups to be
     cached, however, only when the TTL (Time To Live) information
     reported by the name server makes it likely that the cached
     information will remain useful.
  
     If NNTP clients or servers cache the results of host name
     lookups in order to achieve a performance improvement, they
     MUST observe the TTL information reported by DNS.
     If NNTP clients or servers do not observe this rule, they
     could be spoofed when a previously accessed server's IP
     address changes. As network renumbering is expected to become
     increasingly common, the possibility of this form of attack
     will grow. Observing this requirement thus reduces this
     potential security vulnerability.
  
     This requirement also improves the load-balancing behavior of
     clients for replicated servers using the same DNS name and
     reduces the likelihood of a user's experiencing failure in
     accessing sites that use that strategy.
  
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  15  References
  
     1 Kantor, B and P. Lapsley, "Network News Transfer Protocol",
        RFC-977, U.C. San Diego and U.C. Berkeley, February, 1986.
     2 Yergeau, F., "UTF-8, a transformation format of ISO 10646",
        RFC 2279, Alis Technologies, January, 1998.
     3 Coded Character Set-7-bit American Standard Code for
        Information Interchange, ANSI x3.4-1986.
     4 Bradner, Scott, "Keywords for use in RFCs to Indicate
        Requirement Levels", RFC-2119, Harvard University, March,
        1997.
     5 Salz, Rich, Manual Page for wildmat(3) from the INN 1.4
        distribution, UUNET Technologies, Revision 1.10, April,
        1992.
     6 Crocker, D. and Overell, P., "Augmented BNF for Syntax
        Specifications: ABNF", RFC-2234, Internet Mail Consortium
        and Demon Internet, Ltd., November, 1997.
     7 Horton, M. and Adams, R., "Standard for the Interchange of
        USENET messages", RFC 1036, AT&T Bell Laboratories and The
        Center for Seismic Studies, December, 1987.
     8 Resnick, P., Editor, "Internet Message Format", RFC 2822,
        QUALCOMM Incorporated, April 2001.
     9 Robertson, Rob, "FAQ: Overview database / NOV General
        Information",
        ftp://ftp.uu.net/networking/news/nntp/inn/faq-nov.Z,
        January, 1995.
     10 International Telecommunications Union-Radio, "Glossary",
        ITU-R Recommendation TF.686-1, October, 1997.
     11 Mills, David L., "Network Time Protocol (Version 3),
        Specification, Implementation and Analysis", RFC-1305,
        University of Delaware, March 1992.
  
  16  Notes
  
     UNIX is a registered trademark of the X/Open Company Ltd.
  
  17  Acknowledgments
  
     The author acknowledges the original authors of NNTP as
     documented in RFC 977: Brian Kantor and Phil Lapsey.
  
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     The author gratefully acknowledges the work of the NNTP
     committee chaired by Eliot Lear. The organization of this
     memo was influenced by the last available draft from this
     working group. A special thanks to Eliot for generously
     providing the original machine-readable sources for that
     document.
  
     The author gratefully acknowledges the work of the Marshall
     Rose & John G. Meyers in RFC 1939 and the work of the DRUMS
     working group, specifically RFC 1869, which is the basis of
     the NNTP extensions mechanism detailed in this memo.
  
     The author gratefully acknowledges the authors of RFC 2616
     for providing specific and relevant examples of security
     issues that should be considered for HTTP. Since many of the
     same considerations exist for NNTP, those examples that are
     relevant have been included here with some minor rewrites.
  
     The author gratefully acknowledges the comments and
     additional information provided by the following individuals
     in preparing one or more of the progenitors of this memo:
        Russ Allbery <rra@stanford.edu>
        Wayne Davison <davison@armory.com>
        Chris Lewis <clewis@bnr.ca>
        Tom Limoncelli <tal@mars.superlink.net>
        Eric Schnoebelen <eric@egsner.cirr.com>
        Rich Salz <rsalz@osf.org>
  
     This work was motivated by the work of various news reader
     authors and news server authors, which includes those listed
     below:
        Rick Adams-Original author of the NNTP extensions to the
              RN news reader and last maintainer of Bnews
        Stan Barber-Original author of the NNTP extensions to the
              news readers that are part of Bnews.
        Geoff Collyer-Original author of the OVERVIEW database
              proposal and one of the original authors of CNEWS
        Dan Curry-Original author of the xvnews news reader
        Wayne Davison-Author of the first threading extensions to
              the RN news reader (commonly called TRN).
        Geoff Huston-Original author of ANU NEWS
        Phil Lapsey-Original author of the UNIX reference
              implementation for NNTP
        Iain Lea-Original maintainer of the TIN news reader
        Chris Lewis-First known implementer of the AUTHINFO
              GENERIC extension
        Rich Salz-Original author of INN
        Henry Spencer-One of the original authors of CNEWS
        Kim Storm-Original author of the NN news reader
  
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     The present author gratefully acknowledges the vast amount of
     work put into previous drafts by the original author:
        Stan Barber <sob@academ.com>
  
  18  Author's Address
  
  Clive Feather
  Thus plc
  322 Regents Park Road
  London
  N3  2QQ
  United Kingdom
  
  Email: clive@demon.net
     or: clive@davros.org
  Tel: +44 20 8371 1138
  
  This memo expires July 31, 2003.
  
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