    Network Working Group                                         C. Feather
    Request for Comments: 3977                                      THUS plc
    Obsoletes: 977                                              October 2006
    Updates: 2980
    Category: Standards Track

                     Network News Transfer Protocol (NNTP)

    Status of This Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2006).

    Abstract

       The Network News Transfer Protocol (NNTP) has been in use in the
       Internet for a decade, and remains one of the most popular protocols
       (by volume) in use today.  This document 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.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  3
         1.1.  Author's Note . . . . . . . . . . . . . . . . . . . . . .  4
       2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       3.  Basic Concepts  . . . . . . . . . . . . . . . . . . . . . . .  6
         3.1.  Commands and Responses  . . . . . . . . . . . . . . . . .  6
           3.1.1.  Multi-line Data Blocks . . . . . . . . . . . . . . . . 8
         3.2.  Response Codes  . . . . . . . . . . . . . . . . . . . . .  9
           3.2.1.  Generic Response Codes  . . . . . . . . . . . . . . . 10
             3.2.1.1.  Examples  . . . . . . . . . . . . . . . . . . . . 12
         3.3.  Capabilities and Extensions . . . . . . . . . . . . . . . 14
           3.3.1.  Capability Descriptions . . . . . . . . . . . . . . . 14
           3.3.2.  Standard Capabilities . . . . . . . . . . . . . . . . 15
           3.3.3.  Extensions  . . . . . . . . . . . . . . . . . . . . . 16
           3.3.4.  Initial IANA Register . . . . . . . . . . . . . . . . 18
         3.4.  Mandatory and Optional Commands . . . . . . . . . . . . . 20

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           3.4.1.  Reading and Transit Servers . . . . . . . . . . . . . 21
           3.4.2.  Mode Switching  . . . . . . . . . . . . . . . . . . . 21
         3.5.  Pipelining  . . . . . . . . . . . . . . . . . . . . . . . 22
           3.5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . 23
         3.6.  Articles  . . . . . . . . . . . . . . . . . . . . . . . . 24
       4.  The WILDMAT Format  . . . . . . . . . . . . . . . . . . . . . 25
         4.1.  Wildmat Syntax  . . . . . . . . . . . . . . . . . . . . . 26
         4.2.  Wildmat Semantics . . . . . . . . . . . . . . . . . . . . 26
         4.3.  Extensions  . . . . . . . . . . . . . . . . . . . . . . . 27
         4.4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . 27
       5.  Session Administration Commands . . . . . . . . . . . . . . . 28
         5.1.  Initial Connection  . . . . . . . . . . . . . . . . . . . 28
         5.2.  CAPABILITIES  . . . . . . . . . . . . . . . . . . . . . . 29
         5.3.  MODE READER . . . . . . . . . . . . . . . . . . . . . . . 32
         5.4.  QUIT  . . . . . . . . . . . . . . . . . . . . . . . . . . 34
       6.  Article Posting and Retrieval . . . . . . . . . . . . . . . . 35
         6.1.  Group and Article Selection . . . . . . . . . . . . . . . 36
           6.1.1.  GROUP . . . . . . . . . . . . . . . . . . . . . . . . 36
           6.1.2.  LISTGROUP . . . . . . . . . . . . . . . . . . . . . . 39
           6.1.3.  LAST  . . . . . . . . . . . . . . . . . . . . . . . . 42
           6.1.4.  NEXT  . . . . . . . . . . . . . . . . . . . . . . . . 44
         6.2.  Retrieval of Articles and Article Sections  . . . . . . . 45
           6.2.1.  ARTICLE . . . . . . . . . . . . . . . . . . . . . . . 46
           6.2.2.  HEAD  . . . . . . . . . . . . . . . . . . . . . . . . 49
           6.2.3.  BODY  . . . . . . . . . . . . . . . . . . . . . . . . 51
           6.2.4.  STAT  . . . . . . . . . . . . . . . . . . . . . . . . 53
         6.3.  Article Posting . . . . . . . . . . . . . . . . . . . . . 56
           6.3.1.  POST  . . . . . . . . . . . . . . . . . . . . . . . . 56
           6.3.2.  IHAVE . . . . . . . . . . . . . . . . . . . . . . . . 58
       7.  Information Commands  . . . . . . . . . . . . . . . . . . . . 61
         7.1.  DATE  . . . . . . . . . . . . . . . . . . . . . . . . . . 61
         7.2.  HELP  . . . . . . . . . . . . . . . . . . . . . . . . . . 62
         7.3.  NEWGROUPS . . . . . . . . . . . . . . . . . . . . . . . . 63
         7.4.  NEWNEWS . . . . . . . . . . . . . . . . . . . . . . . . . 64
         7.5.  Time  . . . . . . . . . . . . . . . . . . . . . . . . . . 65
           7.5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . 66
         7.6.  The LIST Commands . . . . . . . . . . . . . . . . . . . . 66
           7.6.1.  LIST  . . . . . . . . . . . . . . . . . . . . . . . . 67
           7.6.2.  Standard LIST Keywords  . . . . . . . . . . . . . . . 69
           7.6.3.  LIST ACTIVE . . . . . . . . . . . . . . . . . . . . . 70
           7.6.4.  LIST ACTIVE.TIMES . . . . . . . . . . . . . . . . . . 71
           7.6.5.  LIST DISTRIB.PATS . . . . . . . . . . . . . . . . . . 72
           7.6.6.  LIST NEWSGROUPS . . . . . . . . . . . . . . . . . . . 73
       8.  Article Field Access Commands . . . . . . . . . . . . . . . . 73
         8.1.  Article Metadata  . . . . . . . . . . . . . . . . . . . . 74
           8.1.1.  The :bytes Metadata Item  . . . . . . . . . . . . . . 74
           8.1.2.  The :lines Metadata Item  . . . . . . . . . . . . . . 75
         8.2.  Database Consistency  . . . . . . . . . . . . . . . . . . 75

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         8.3.  OVER  . . . . . . . . . . . . . . . . . . . . . . . . . . 76
         8.4.  LIST OVERVIEW.FMT . . . . . . . . . . . . . . . . . . . . 81
         8.5.  HDR . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
         8.6.  LIST HEADERS  . . . . . . . . . . . . . . . . . . . . . . 87
       9.  Augmented BNF Syntax for NNTP . . . . . . . . . . . . . . . . 90
         9.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . 90
         9.2.  Commands  . . . . . . . . . . . . . . . . . . . . . . . . 92
         9.3.  Command Continuation  . . . . . . . . . . . . . . . . . . 93
         9.4.  Responses . . . . . . . . . . . . . . . . . . . . . . . . 93
           9.4.1.  Generic Responses . . . . . . . . . . . . . . . . . . 93
           9.4.2.  Initial Response Line Contents  . . . . . . . . . . . 94
           9.4.3.  Multi-line Response Contents  . . . . . . . . . . . . 94
         9.5.  Capability Lines  . . . . . . . . . . . . . . . . . . . . 95
         9.6.  LIST Variants . . . . . . . . . . . . . . . . . . . . . . 96
         9.7.  Articles  . . . . . . . . . . . . . . . . . . . . . . . . 97
         9.8.  General Non-terminals . . . . . . . . . . . . . . . . . . 97
         9.9.  Extensions and Validation . . . . . . . . . . . . . . . . 99
       10. Internationalisation Considerations . . . . . . . . . . . . .100
         10.1. Introduction and Historical Situation . . . . . . . . . .100
         10.2. This Specification  . . . . . . . . . . . . . . . . . . .101
         10.3. Outstanding Issues  . . . . . . . . . . . . . . . . . . .102
       11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .103
       12. Security Considerations . . . . . . . . . . . . . . . . . . .103
         12.1. Personal and Proprietary Information  . . . . . . . . . .104
         12.2. Abuse of Server Log Information . . . . . . . . . . . . .104
         12.3. Weak Authentication and Access Control  . . . . . . . . .104
         12.4. DNS Spoofing  . . . . . . . . . . . . . . . . . . . . . .104
         12.5. UTF-8 Issues  . . . . . . . . . . . . . . . . . . . . . .105
         12.6. Caching of Capability Lists . . . . . . . . . . . . . . .106
       13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .107
       14. References  . . . . . . . . . . . . . . . . . . . . . . . . .110
         14.1. Normative References  . . . . . . . . . . . . . . . . . .110
         14.2. Informative References  . . . . . . . . . . . . . . . . .110
       A.  Interaction with Other Specifications . . . . . . . . . . . .112
         A.1.  Header Folding  . . . . . . . . . . . . . . . . . . . . .112
         A.2.  Message-IDs . . . . . . . . . . . . . . . . . . . . . . .112
         A.3.  Article Posting . . . . . . . . . . . . . . . . . . . . .114
       B.  Summary of Commands . . . . . . . . . . . . . . . . . . . . .115
       C.  Summary of Response Codes . . . . . . . . . . . . . . . . . .117
       D.  Changes from RFC 977  . . . . . . . . . . . . . . . . . . . .121

    1.  Introduction

       This document specifies the Network News Transfer Protocol (NNTP),
       which is used for the distribution, inquiry, retrieval, and posting
       of Netnews articles using a reliable stream-based mechanism.  For
       news-reading clients, NNTP enables retrieval of news articles that

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       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.  NNTP is designed for efficient
       transmission of Netnews articles over a reliable full duplex
       communication channel.

       Although the protocol specification in this document is largely
       compatible with the version specified in RFC 977 [RFC977], a number
       of changes are summarised in Appendix D.  In particular:

       o  the default character set is changed from US-ASCII [ANSI1986] to
          UTF-8 [RFC3629] (note that US-ASCII is a subset of UTF-8);

       o  a number of commands that were optional in RFC 977 or that have
          been taken from RFC 2980 [RFC2980] are now mandatory; and

       o  a CAPABILITIES command has been added to allow clients to
          determine what functionality is available from a server.

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in RFC 2119 [RFC2119].

       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 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 document, the terms "client" and "client
       host" refer to a host making use of the NNTP service, while the terms
       "server" and "server host" refer to a host that offers the NNTP
       service.

    1.1.  Author's Note

       This document is written in XML using an NNTP-specific DTD.  Custom
       software is used to convert this to RFC 2629 [RFC2629] format, and
       then the public "xml2rfc" package to further reduce this to text,
       nroff source, and HTML.

       No perl was used in producing this document.

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    2.  Notation

       The following notational conventions are used in this document.

         UPPERCASE     indicates literal text to be included in the
                       command.

         lowercase     indicates a token described elsewhere.

         [brackets]    indicate that the enclosed material is optional.

         elliptical    indicates that the argument may be repeated any
         ... marks     number of times (it must occur at least once).

         vertical|bar  indicates a choice of two mutually exclusive
                       arguments (exactly one must be provided).

       The name "message-id" for a command or response argument indicates
       that it is the message-id of an article as described in Section 3.6,
       including the angle brackets.

       The name "wildmat" for an argument indicates that it is a wildmat as
       defined in Section 4.  If the argument does not meet the requirements
       of that section (for example, if it does not fit the grammar of
       Section 4.1), the NNTP server MAY place some interpretation on it
       (not specified by this document) or otherwise MUST treat it as a
       syntax error.

       Responses for each command will be described in tables listing the
       required format of a response followed by the meaning that should be
       ascribed to that response.

       The terms "NUL", "TAB", "LF", "CR, and "space" refer to the octets
       %x00, %x09, %x0A, %x0D, and %x20, respectively (that is, the octets
       with those codes in US-ASCII [ANSI1986] and thus in UTF-8 [RFC3629]).
       The term "CRLF" or "CRLF pair" means the sequence CR immediately
       followed by LF (that is, %x0D.0A).  A "printable US-ASCII character"
       is an octet in the range %x21-7E.  Quoted characters refer to the
       octets with those codes in US-ASCII (so "." and "<" refer to %x2E and
       %x3C) and will always be printable US-ASCII characters; similarly,
       "digit" refers to the octets %x30-39.

       A "keyword" MUST consist only of US-ASCII letters, digits, and the
       characters dot (".") and dash ("-") and MUST begin with a letter.
       Keywords MUST be at least three characters in length.

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       Examples in this document are not normative but serve to illustrate
       usages, arguments, and responses.  In the examples, a "[C]" will be
       used to represent the client host and an "[S]" will be used to
       represent the server host.  Most of the examples do not rely on a
       particular server state.  In some cases, however, they do assume that
       the currently selected newsgroup (see the GROUP command,
       Section 6.1.1) is invalid; when so, this is indicated at the start of
       the example.  Examples may use commands or other keywords not defined
       in this specification (such as an XENCRYPT command).  These will be
       used to illustrate some point and do not imply that any such command
       is defined elsewhere or needs to exist in any particular
       implementation.

       Terms that might be read as specifying details of a client or server
       implementation, such as "database", are used simply to ease
       description.  Provided that implementations conform to the protocol
       and format specifications in this document, no specific technique is
       mandated.

    3.  Basic Concepts

    3.1.  Commands and Responses

       NNTP operates over any reliable bi-directional 8-bit-wide data stream
       channel.  When the connection is established, the NNTP server host
       MUST send a greeting.  The client host and server host then exchange
       commands and responses (respectively) until the connection is closed
       or aborted.  If the connection used is TCP, then 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.

       The character set for all NNTP commands is UTF-8 [RFC3629].  Commands
       in NNTP MUST consist of a keyword, which MAY be followed by one or
       more arguments.  A CRLF pair MUST terminate all commands.  Multiple
       commands MUST NOT be on the same line.  Unless otherwise noted
       elsewhere in this document, arguments SHOULD consist of printable US-
       ASCII characters.  Keywords and arguments MUST each be separated by
       one or more space or TAB characters.  Command lines MUST NOT exceed
       512 octets, which includes the terminating CRLF pair.  The arguments
       MUST NOT exceed 497 octets.  A server MAY relax these limits for
       commands defined in an extension.

       Where this specification permits UTF-8 characters outside the range
       of U+0000 to U+007F, implementations MUST NOT use the Byte Order Mark
       (U+FEFF, encoding %xEF.BB.BF) and MUST use the Word Joiner (U+2060,
       encoding %xE2.91.A0) for the meaning Zero Width No-Break Space in

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       command lines and the initial lines of responses.  Implementations
       SHOULD apply these same principles throughout.

       The term "character" means a single Unicode code point.
       Implementations are not required to carry out Unicode normalisation.
       Thus, U+0084 (A-dieresis) is one character, while U+0041 U+0308 (A
       composed with dieresis) is two; the two need not be treated as
       equivalent.

       Commands may have variants; if so, they use a second keyword
       immediately after the first to indicate which variant is required.
       The only such commands in this specification are LIST and MODE.  Note
       that such variants are sometimes referred to as if they were commands
       in their own right: "the LIST ACTIVE" command should be read as
       shorthand for "the ACTIVE variant of the LIST command".

       Keywords are case insensitive; the case of keywords for commands MUST
       be ignored by the server.  Command and response arguments are case or
       language specific only when stated, either in this document or in
       other relevant specifications.

       In some cases, a command involves more data than just a single line.
       The further data may be sent either immediately after the command
       line (there are no instances of this in this specification, but there
       are in extensions such as [NNTP-STREAM]) or following a request from
       the server (indicated by a 3xx response).

       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.  The initial line of the response MUST NOT exceed
       512 octets, which includes the response code and the terminating CRLF
       pair; an extension MAY specify a greater maximum for commands that it
       defines, but not for any other command.  Single-line responses
       consist of an initial line only.  Multi-line responses consist of an
       initial line followed by a multi-line data block.

       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 a client that is sending a multi-line
       data block (such as during a POST or IHAVE command) SHOULD suffice to
       reset the autologout timer.  When the timer expires, the server
       SHOULD close the connection without sending any response to the
       client.

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    3.1.1.  Multi-line Data Blocks

       A multi-line data block is used in certain commands and responses.
       It MUST adhere to the following rules:

       1.  The block consists of a sequence of zero or more "lines", each
           being a stream of octets ending with a CRLF pair.  Apart from
           those line endings, the stream MUST NOT include the octets NUL,
           LF, or CR.

       2.  In a multi-line response, the block immediately follows the CRLF
           at the end of the initial line of the response.  When used in any
           other context, the specific command will define when the block is
           sent.

       3.  If any line of the data block begins with the "termination octet"
           ("." or %x2E), that line MUST be "dot-stuffed" by prepending an
           additional termination octet to that line of the block.

       4.  The lines of the block MUST be followed by a terminating line
           consisting of a single termination octet followed by a CRLF pair
           in the normal way.  Thus, unless it is empty, a multi-line block
           is always terminated with the five octets CRLF "." CRLF
           (%x0D.0A.2E.0D.0A).

       5.  When a multi-line block is interpreted, the "dot-stuffing" MUST
           be undone; i.e., the recipient MUST ensure that, in any line
           beginning with the termination octet followed by octets other
           than a CRLF pair, that initial termination octet is disregarded.

       6.  Likewise, the terminating line ("." CRLF or %x2E.0D.0A) MUST NOT
           be considered part of the multi-line block; i.e., the recipient
           MUST ensure that any line beginning with the termination octet
           followed immediately by a CRLF pair is disregarded.  (The first
           CRLF pair of the terminating CRLF "." CRLF of a non-empty block
           is, of course, part of the last line of the block.)

       Note that texts using an encoding (such as UTF-16 or UTF-32) that may
       contain the octets NUL, LF, or CR other than a CRLF pair cannot be
       reliably conveyed in the above format (that is, they violate the MUST
       requirement above).  However, except when stated otherwise, this
       specification does not require the content to be UTF-8, and therefore
       (subject to that same requirement) it MAY include octets above and
       below 128 mixed arbitrarily.

       This document does not place any limit on the length of a line in a
       multi-line block.  However, the standards that define the format of
       articles may do so.

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    3.2.  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 completed OK
          3xx - Command OK so far; send the rest of it
          4xx - Command was syntactically correct but failed for some reason
          5xx - Command unknown, unsupported, unavailable, or syntax error

       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 privacy extensions
          x9x - Reserved for private use (non-standard extensions)

       Certain responses contain arguments such as numbers and names in
       addition to the status indicator.  In those cases, to simplify
       interpretation by the client, the number and type of such arguments
       is fixed for each response code, as is whether the code is
       single-line or multi-line.  Any extension MUST follow this principle
       as well.  Note that, for historical reasons, the 211 response code is
       an exception to this in that the response may be single-line or
       multi-line depending on the command (GROUP or LISTGROUP) that
       generated it.  In all other cases, the client MUST only use the
       status indicator itself to determine the nature of the response.  The
       exact response codes that can be returned by any given command are
       detailed in the description of that command.

       Arguments MUST be separated from the numeric status indicator and
       from each other by a single space.  All numeric arguments MUST be in
       base 10 (decimal) format and MAY have leading zeros.  String
       arguments MUST contain at least one character and MUST NOT contain
       TAB, LF, CR, or space.  The server MAY add any text after the
       response code or last argument, 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 argument by at least
       one space.

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       The server MUST respond to any command with the appropriate generic
       response (given in Section 3.2.1) 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 specification, including new
       commands, new variants or 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 specification 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 document 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 document nor any registered extension (see
       Section 3.3.3) will specify any response codes of the x9x pattern.
       (Implementers of extensions are accordingly cautioned not to use such
       responses for extensions that may subsequently be submitted for
       registration.)

    3.2.1.  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 if it is an optional command
       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,
       including the case where more arguments are provided than the command
       specifies or the command line is longer than the server accepts, the
       response code 501 MUST be returned.  The line MUST NOT be truncated
       or split and then interpreted.  Note that where a command has
       variants depending on a second keyword (e.g., LIST ACTIVE and LIST
       NEWSGROUPS), 501 MUST be used when the base command is implemented
       but the requested variant is not, and 500 MUST be used only when the
       base command itself is not implemented.

       If an argument is required to be a base64-encoded string [RFC4648]
       (there are no such arguments in this specification, but there may be

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       in extensions) and is not validly encoded, the response code 504 MUST
       be returned.

       If the server experiences an internal fault or problem that means it
       is unable to carry out the command (for example, a necessary file is
       missing or a necessary service could not be contacted), the response
       code 403 MUST be returned.  If the server recognizes the command but
       does not provide an optional feature (for example, because it does
       not store the required information), or if it only handles a subset
       of legitimate cases (see the HDR command, Section 8.5, for an
       example), the response code 503 MUST be returned.

       If the client is not authorized to use the specified facility when
       the server is in its current state, then the appropriate one of the
       following response codes MUST be used.

       502: It is necessary to terminate the connection and to start a new
          one with the appropriate authority before the command can be used.
          Historically, some mode-switching servers (see Section 3.4.1) used
          this response to indicate that this command will become available
          after the MODE READER command (Section 5.3) is used, but this
          usage does not conform to this specification and MUST NOT be used.
          Note that the server MUST NOT close the connection immediately
          after a 502 response except at the initial connection
          (Section 5.1) and with the MODE READER command.

       480: The client must authenticate itself to the server (that is, it
          must provide information as to the identity of the client) before
          the facility can be used on this connection.  This will involve
          the use of an authentication extension such as [NNTP-AUTH].

       483: The client must negotiate appropriate privacy protection on the
          connection.  This will involve the use of a privacy extension such
          as [NNTP-TLS].

       401: The client must change the state of the connection in some other
          manner.  The first argument of the response MUST be the capability
          label (see Section 5.2) of the facility that provides the
          necessary mechanism (usually an extension, which may be a private
          extension).  The server MUST NOT use this response code except as
          specified by the definition of the capability in question.

       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 connection.  Following a 400 response, clients
       SHOULD NOT simply reconnect immediately and retry the same actions.
       Rather, a client SHOULD either use an exponentially increasing delay
       between retries (e.g., double the waiting time after each 400

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       response) or present any associated text to the user for them to
       decide whether and when to retry.

       The client MUST be prepared to receive any of these responses for any
       command (except, of course, that the server MUST NOT generate a 500
       response code for mandatory commands).

    3.2.1.1.  Examples

       Example of an unknown command:

          [C] MAIL
          [S] 500 Unknown command

       Example of an unsupported command:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] NEWNEWS
          [S] LIST ACTIVE NEWSGROUPS
          [S] .
          [C] OVER
          [S] 500 Unknown command

       Example of an unsupported variant:

          [C] MODE POSTER
          [S] 501 Unknown MODE option

       Example of a syntax error:

          [C] ARTICLE a.message.id@no.angle.brackets
          [S] 501 Syntax error

       Example of an overlong command line:

          [C] HEAD 53 54 55
          [S] 501 Too many arguments

       Example of a bad wildmat:

          [C] LIST ACTIVE u[ks].*
          [S] 501 Syntax error

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       Example of a base64-encoding error (the second argument is meant to
       be base64 encoded):

          [C] XENCRYPT RSA abcd=efg
          [S] 504 Base64 encoding error

       Example of an attempt to access a facility not available to this
       connection:

          [C] MODE READER
          [S] 200 Reader mode, posting permitted
          [C] IHAVE <i.am.an.article.you.will.want@example.com>
          [S] 500 Permission denied

       Example of an attempt to access a facility requiring authentication:

          [C] GROUP secret.group
          [S] 480 Permission denied

       Example of a successful attempt following such authentication:

          [C] XSECRET fred flintstone
          [S] 290 Password for fred accepted
          [C] GROUP secret.group
          [S] 211 5 1 20 secret.group selected

       Example of an attempt to access a facility requiring privacy:

          [C] GROUP secret.group
          [S] 483 Secure connection required
          [C] XENCRYPT
          [Client and server negotiate encryption on the link]
          [S] 283 Encrypted link established
          [C] GROUP secret.group
          [S] 211 5 1 20 secret.group selected

       Example of a need to change mode before a facility is used:

          [C] GROUP binary.group
          [S] 401 XHOST Not on this virtual host
          [C] XHOST binary.news.example.org
          [S] 290 binary.news.example.org virtual host selected
          [C] GROUP binary.group
          [S] 211 5 1 77 binary.group selected

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       Example of a temporary failure:

          [C] GROUP archive.local
          [S] 403 Archive server temporarily offline

       Example of the server needing to close down immediately:

          [C] ARTICLE 123
          [S] 400 Power supply failed, running on UPS
          [Server closes connection.]

    3.3.  Capabilities and Extensions

       Not all NNTP servers provide exactly the same facilities, both
       because this specification allows variation and because servers may
       provide extensions.  A set of facilities that are related are called
       a "capability".  This specification provides a way to determine what
       capabilities are available, includes a list of standard capabilities,
       and includes a mechanism (the extension mechanism) for defining new
       capabilities.

    3.3.1.  Capability Descriptions

       A client can determine the available capabilities of the server by
       using the CAPABILITIES command (Section 5.2).  This returns a
       capability list, which is a list of capability lines.  Each line
       describes one available capability.

       Each capability line consists of one or more tokens, which MUST be
       separated by one or more space or TAB characters.  A token is a
       string of 1 or more printable UTF-8 characters (that is, either
       printable US-ASCII characters or any UTF-8 sequence outside the US-
       ASCII range, but not space or TAB).  Unless stated otherwise, tokens
       are case insensitive.  Each capability line consists of the
       following:

       o  The capability label, which is a keyword indicating the
          capability.  A capability label may be defined by this
          specification or a successor, or by an extension.

       o  The label is then followed by zero or more tokens, which are
          arguments of the capability.  The form and meaning of these tokens
          is specific to each capability.

       The server MUST ensure that the capability list accurately reflects
       the capabilities (including extensions) currently available.  If a
       capability is only available with the server in a certain state (for
       example, only after authentication), the list MUST only include the

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       capability label when the server is in that state.  Similarly, if
       only some of the commands in an extension will be available, or if
       the behaviour of the extension will change in some other manner,
       according to the state of the server, this MUST be indicated by
       different arguments in the capability line.

       Note that a capability line can only begin with a letter.  Lines
       beginning with other characters are reserved for future versions of
       this specification.  In order to interoperate with such versions,
       clients MUST be prepared to receive lines beginning with other
       characters and MUST ignore any they do not understand.

    3.3.2.  Standard Capabilities

       The following capabilities are defined by this specification.

       VERSION
          This capability MUST be advertised by all servers and MUST be the
          first capability in the capability list; it indicates the
          version(s) of NNTP that the server supports.  There must be at
          least one argument; each argument is a decimal number and MUST NOT
          have a leading zero.  Version numbers are assigned only in RFCs
          that update or replace this specification; servers MUST NOT create
          their own version numbers.

          The version number of this specification is 2.

       READER
          This capability indicates that the server implements the various
          commands useful for reading clients.

       IHAVE
          This capability indicates that the server implements the IHAVE
          command.

       POST
          This capability indicates that the server implements the POST
          command.

       NEWNEWS
          This capability indicates that the server implements the NEWNEWS
          command.

       HDR
          This capability indicates that the server implements the header
          access commands (HDR and LIST HEADERS).

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       OVER
          This capability indicates that the server implements the overview
          access commands (OVER and LIST OVERVIEW.FMT).  If and only if the
          server supports the message-id form of the OVER command, there
          must be a single argument MSGID.

       LIST
          This capability indicates that the server implements at least one
          variant of the LIST command.  There MUST be one argument for each
          variant of the LIST command supported by the server, giving the
          keyword for that variant.

       IMPLEMENTATION
          This capability MAY be provided by a server.  If so, the arguments
          SHOULD be used to provide information such as the server software
          name and version number.  The client MUST NOT use this line to
          determine capabilities of the server.  (While servers often
          provide this information in the initial greeting, clients need to
          guess whether this is the case; this capability makes it clear
          what the information is.)

       MODE-READER
          This capability indicates that the server is mode-switching
          (Section 3.4.2) and that the MODE READER command needs to be used
          to enable the READER capability.

    3.3.3.  Extensions

       Although NNTP is widely and robustly deployed, some parts of the
       Internet community might wish to extend the NNTP service.  It must be
       emphasized that any extension to NNTP 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.

       An extension is a package of associated facilities, often but not
       always including one or more new commands.  Each extension MUST
       define at least one new capability label (this will often, but need
       not, be the name of one of these new commands).  While any additional
       capability information can normally be specified using arguments to

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       that label, an extension MAY define more than one capability label.
       However, this SHOULD be limited to exceptional circumstances.

       An extension is either a private extension, or its capabilities are
       included in the IANA registry of capabilities (see Section 3.3.4) and
       it is defined in an RFC (in which case it is a "registered
       extension").  Such RFCs either must be on the standards track or must
       define an IESG-approved experimental protocol.

       The definition of an extension must include the following:

       o  a descriptive name for the extension.

       o  the capability label or labels defined by the extension (the
          capability label of a registered extension MUST NOT begin with
          "X").

       o  The syntax, values, and meanings of any arguments for each
          capability label defined by the extension.

       o  Any new NNTP commands associated with the extension (the names of
          commands associated with registered extensions MUST NOT begin with
          "X").

       o  The syntax and possible values of arguments associated with the
          new NNTP commands.

       o  The response codes and possible values of arguments for the
          responses of the new NNTP commands.

       o  Any new arguments the extension associates with any other
          pre-existing NNTP commands.

       o  Any increase in the maximum length of commands and initial
          response lines over the value specified in this document.

       o  A specific statement about the effect on pipelining that this
          extension may have (if any).

       o  A specific statement about the circumstances when use of this
          extension can alter the contents of the capabilities list (other
          than the new capability labels it defines).

       o  A specific statement about the circumstances under which the
          extension can cause any pre-existing command to produce a 401,
          480, or 483 response.

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       o  A description of how the use of MODE READER on a mode-switching
          server interacts with the extension.

       o  A description of how support for the extension affects the
          behaviour of a server and NNTP client in any other manner not
          outlined above.

       o  Formal syntax as described in Section 9.9.

       A private extension MAY or MAY NOT be included in the capabilities
       list.  If it is, the capability label MUST begin with "X".  A server
       MAY provide additional keywords (for new commands and also for new
       variants of existing commands) as part of a private extension.  To
       avoid the risk of a clash with a future registered extension, these
       keywords SHOULD begin with "X".

       If the server advertises a capability defined by a registered
       extension, it MUST implement the extension so as to fully conform
       with the specification (for example, it MUST implement all the
       commands that the extension describes as mandatory).  If it does not
       implement the extension as specified, it MUST NOT list the extension
       in the capabilities list under its registered name.  In that case, it
       MAY, but SHOULD NOT, provide a private extension (not listed, or
       listed with a different name) that implements part of the extension
       or implements the commands of the extension with a different meaning.

       A server MUST NOT send different response codes to basic NNTP
       commands documented here or to commands documented in registered
       extensions in response to the availability or use of a private
       extension.

    3.3.4.  Initial IANA Register

       IANA will maintain a registry of NNTP capability labels.  All
       capability labels in the registry MUST be keywords and MUST NOT begin
       with X.

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       The initial content of the registry consists of these entries:

       +-------------------+--------------------------+--------------------+
       | Label             | Meaning                  | Definition         |
       +-------------------+--------------------------+--------------------+
       | AUTHINFO          | Authentication           | [NNTP-AUTH]        |
       |                   |                          |                    |
       | HDR               | Batched header retrieval | Section 3.3.2,     |
       |                   |                          | Section 8.5, and   |
       |                   |                          | Section 8.6        |
       |                   |                          |                    |
       | IHAVE             | IHAVE command available  | Section 3.3.2 and  |
       |                   |                          | Section 6.3.2      |
       |                   |                          |                    |
       | IMPLEMENTATION    | Server                   | Section 3.3.2      |
       |                   | implementation-specific  |                    |
       |                   | information              |                    |
       |                   |                          |                    |
       | LIST              | LIST command variants    | Section 3.3.2 and  |
       |                   |                          | Section 7.6.1      |
       |                   |                          |                    |
       | MODE-READER       | Mode-switching server    | Section 3.4.2      |
       |                   | and MODE READER command  |                    |
       |                   | available                |                    |
       |                   |                          |                    |
       | NEWNEWS           | NEWNEWS command          | Section 3.3.2 and  |
       |                   | available                | Section 7.4        |
       |                   |                          |                    |
       | OVER              | Overview support         | Section 3.3.2,     |
       |                   |                          | Section 8.3, and   |
       |                   |                          | Section 8.4        |
       |                   |                          |                    |
       | POST              | POST command available   | Section 3.3.2 and  |
       |                   |                          | Section 6.3.1      |
       |                   |                          |                    |
       | READER            | Reader commands          | Section 3.3.2      |
       |                   | available                |                    |
       |                   |                          |                    |
       | SASL              | Supported SASL           | [NNTP-AUTH]        |
       |                   | mechanisms               |                    |
       |                   |                          |                    |
       | STARTTLS          | Transport layer security | [NNTP-TLS]         |
       |                   |                          |                    |
       | STREAMING         | Streaming feeds          | [NNTP-STREAM]      |
       |                   |                          |                    |
       | VERSION           | Supported NNTP versions  | Section 3.3.2      |
       +-------------------+--------------------------+--------------------+

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    3.4.  Mandatory and Optional Commands

       For a number of reasons, not all the commands in this specification
       are mandatory.  However, it is equally undesirable for every command
       to be optional, since this means that a client will have no idea what
       facilities are available.  Therefore, as a compromise, some of the
       commands in this specification are mandatory (they must be supported
       by all servers) while the remainder are not.  The latter are then
       subdivided into bundles, each indicated by a single capability label.

       o  If the label is included in the capability list returned by the
          server, the server MUST support all commands in that bundle.

       o  If the label is not included, the server MAY support none or some
          of the commands but SHOULD NOT support all of them.  In general,
          there will be no way for a client to determine which commands are
          supported without trying them.

       The bundles have been chosen to provide useful functionality, and
       therefore server authors are discouraged from implementing only part
       of a bundle.

       The description of each command will either indicate that it is
       mandatory, or will give, using the term "indicating capability", the
       capability label indicating whether the bundle including this command
       is available.

       Where a server does not implement a command, it MUST always generate
       a 500 generic response code (or a 501 generic response code in the
       case of a variant of a command depending on a second keyword where
       the base command is recognised).  Otherwise, the command MUST be
       fully implemented as specified; a server MUST NOT only partially
       implement any of the commands in this specification.  (Client authors
       should note that some servers not conforming to this specification
       will return a 502 generic response code to some commands that are not
       implemented.)

       Note: some commands have cases that require other commands to be used
       first.  If the former command is implemented but the latter is not,
       the former MUST still generate the relevant specific response code.
       For example, if ARTICLE (Section 6.2.1) is implemented but GROUP
       (Section 6.1.1) is not, the correct response to "ARTICLE 1234"
       remains 412.

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    3.4.1.  Reading and Transit Servers

       NNTP is traditionally used in two different ways.  The first use is
       "reading", where the client fetches articles from a large store
       maintained by the server for immediate or later presentation to a
       user and sends articles created by that user back to the server (an
       action called "posting") to be stored and distributed to other stores
       and users.  The second use is for the bulk transfer of articles from
       one store to another.  Since the hosts making this transfer tend to
       be peers in a network that transmit articles among one another, and
       not end-user systems, this process is called "peering" or "transit".
       (Even so, one host is still the client and the other is the server).

       In practice, these two uses are so different that some server
       implementations are optimised for reading or for transit and, as a
       result, do not offer the other facility or only offer limited
       features.  Other implementations are more general and offer both.
       This specification allows for this by bundling the relevant commands
       accordingly: the IHAVE command is designed for transit, while the
       commands indicated by the READER capability are designed for reading
       clients.

       Except as an effect of the MODE READER command (Section 5.3) on a
       mode-switching server, once a server advertises either or both of the
       IHAVE or READER capabilities, it MUST continue to advertise them for
       the entire session.

       A server MAY provide different modes of behaviour (transit, reader,
       or a combination) to different client connections and MAY use
       external information, such as the IP address of the client, to
       determine which mode to provide to any given connection.

       The official TCP port for the NNTP service is 119.  However, if a
       host wishes to offer separate servers for transit and reading
       clients, port 433 SHOULD be used for the transit server and 119 for
       the reading server.

    3.4.2.  Mode Switching

       An implementation MAY, but SHOULD NOT, provide both transit and
       reader facilities on the same server but require the client to select
       which it wishes to use.  Such an arrangement is called a
       "mode-switching" server.

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       A mode-switching server has two modes:

       o  Transit mode, which applies after the initial connection.

          *  It MUST advertise the MODE-READER capability.

          *  It MUST NOT advertise the READER capability.

          However, the server MAY cease to advertise the MODE-READER
          capability after the client uses any command except CAPABILITIES.

       o  Reading mode, after a successful MODE READER command (see Section
          5.3).

          *  It MUST NOT advertise the MODE-READER capability.

          *  It MUST advertise the READER capability.

          *  It MAY NOT advertise the IHAVE capability, even if it was
             advertising it in transit mode.

       A client SHOULD only issue a MODE READER command to a server if it is
       advertising the MODE-READER capability.  If the server does not
       support CAPABILITIES (and therefore does not conform to this
       specification), the client MAY use the following heuristic:

       o  If the client wishes to use any "reader" commands, it SHOULD use
          the MODE READER command immediately after the initial connection.

       o  Otherwise, it SHOULD NOT use the MODE READER command.

       In each case, it should be prepared for some commands to be
       unavailable that would have been available if it had made the other
       choice.

    3.5.  Pipelining

       NNTP is designed to operate over a reliable bi-directional
       connection, such as TCP.  Therefore, if a command does not 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 "pipelining".
       However, certain server implementations throw away all text received
       from the client following certain commands before sending their
       response.  If this happens, pipelining 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 pipelining, but also circumstances
       where it is reasonable or common for servers to behave in the above

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       manner, this document puts certain requirements on both clients and
       servers.

       Except where stated otherwise, a client MAY use pipelining.  That is,
       it may send a command before receiving the response for the previous
       command.  The server MUST allow pipelining and MUST NOT throw away
       any text received after a command.  Irrespective of whether
       pipelining is used, the server MUST process commands in the order
       they are sent.

       If the specific description of a command says it "MUST NOT be
       pipelined", that command MUST end any pipeline of commands.  That is,
       the client MUST NOT send any following command until it receives 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 pipeline; that is, the
       client MUST NOT send any command until it receives 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 pipelining does not cause a
       deadlock between transmission and reception.  The amount of text
       involved will depend on window sizes in the transmission layer;
       typically, it is 4k octets for TCP.  (Since the server only sends
       data in response to commands from the client, the converse problem
       does not occur.)

    3.5.1.  Examples

       Example of correct use of pipelining:

          [C] GROUP misc.test
          [C] STAT
          [C] NEXT
          [S] 211 1234 3000234 3002322 misc.test
          [S] 223 3000234 <45223423@example.com> retrieved
          [S] 223 3000237 <668929@example.org> retrieved

       Example of incorrect use of pipelining (the MODE READER command may
       not be pipelined):

          [C] MODE READER
          [C] DATE
          [C] NEXT
          [S] 200 Server ready, posting allowed
          [S] 223 3000237 <668929@example.org> retrieved

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       The DATE command has been thrown away by the server, so there is no
       111 response to match it.

    3.6.  Articles

       NNTP is intended to transfer articles between clients and servers.
       For the purposes of this specification, articles are required to
       conform to the rules in this section, and clients and servers MUST
       correctly process any article received from the other that does so.
       Note that this requirement applies only to the contents of
       communications over NNTP; it does not prevent the client or server
       from subsequently rejecting an article for reasons of local policy.
       Also see Appendix A for further restrictions on the format of
       articles in some uses of NNTP.

       An article consists of two parts: the headers and the body.  They are
       separated by a single empty line, or in other words by two
       consecutive CRLF pairs (if there is more than one empty line, the
       second and subsequent ones are part of the body).  In order to meet
       the general requirements of NNTP, an article MUST NOT include the
       octet NUL, MUST NOT contain the octets LF and CR other than as part
       of a CRLF pair, and MUST end with a CRLF pair.  This specification
       puts no further restrictions on the body; in particular, it MAY be
       empty.

       The headers of an article consist of one or more header lines.  Each
       header line consists of a header name, a colon, a space, the header
       content, and a CRLF, in that order.  The name consists of one or more
       printable US-ASCII characters other than colon and, for the purposes
       of this specification, is not case sensitive.  There MAY be more than
       one header line with the same name.  The content MUST NOT contain
       CRLF; it MAY be empty.  A header may be "folded"; that is, a CRLF
       pair may be placed before any TAB or space in the line.  There MUST
       still be some other octet between any two CRLF pairs in a header
       line.  (Note that folding means that the header line occupies more
       than one line when displayed or transmitted; nevertheless, it is
       still referred to as "a" header line.)  The presence or absence of
       folding does not affect the meaning of the header line; that is, the
       CRLF pairs introduced by folding are not considered part of the
       header content.  Header lines SHOULD NOT be folded before the space
       after the colon that follows the header name and SHOULD include at
       least one octet other than %x09 or %x20 between CRLF pairs.  However,
       if an article that fails to satisfy this requirement has been
       received from elsewhere, clients and servers MAY transfer it to each
       other without re-folding it.

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       The content of a header SHOULD be in UTF-8.  However, if an
       implementation receives an article from elsewhere that uses octets in
       the range 128 to 255 in some other manner, it MAY pass it to a client
       or server without modification.  Therefore, implementations MUST be
       prepared to receive such headers, and data derived from them (e.g.,
       in the responses from the OVER command, Section 8.3), and MUST NOT
       assume that they are always UTF-8.  Any external processing of those
       headers, including identifying the encoding used, is outside the
       scope of this document.

       Each article MUST have a unique message-id; two articles offered by
       an NNTP server MUST NOT have the same message-id.  For the purposes
       of this specification, message-ids are opaque strings that MUST meet
       the following requirements:

       o  A message-id MUST begin with "<", end with ">", and MUST NOT
          contain the latter except at the end.

       o  A message-id MUST be between 3 and 250 octets in length.

       o  A message-id MUST NOT contain octets other than printable US-ASCII
          characters.

       Two message-ids are the same if and only if they consist of the same
       sequence of octets.

       This specification does not describe how the message-id of an article
       is determined.  If the server does not have any way to determine a
       message-id from the article itself, it MUST synthesize one (this
       specification does not require that the article be changed as a
       result).  See also Appendix A.2.

    4.  The WILDMAT Format

       The WILDMAT format described here is based on the version first
       developed by Rich Salz [SALZ1992], which was in turn derived from the
       format used in the UNIX "find" command to articulate file names.  It
       was developed to provide a uniform mechanism for matching patterns in
       the same manner that the UNIX shell matches filenames.

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    4.1.  Wildmat Syntax

       A wildmat is described by the following ABNF [RFC4234] syntax, which
       is an extract of that in Section 9.8.

         wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
         wildmat-pattern = 1*wildmat-item
         wildmat-item = wildmat-exact / wildmat-wild
         wildmat-exact = %x22-29 / %x2B / %x2D-3E / %x40-5A / %x5E-7E /
              UTF8-non-ascii ; exclude ! * , ? [ \ ]
         wildmat-wild = "*" / "?"

       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 suppress the special meaning of characters, whereas brackets
       are used to introduce sets.  However, these usages are not universal,
       and interpretation of these characters in the context of UTF-8
       strings is potentially complex and differs from existing practice, so
       they were omitted from this specification.  A future extension to
       this specification may provide semantics for these characters.

    4.2.  Wildmat Semantics

       A wildmat is tested against a string and either matches or does not
       match.  To do this, each constituent <wildmat-pattern> 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*":

       o  The string "aaa" matches because the rightmost match is with "a*".

       o  The string "abb" does not match because the rightmost match is
          with "*b".

       o  The string "ccb" matches because the rightmost match is with
          "*c*".

       o  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

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       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.

    4.3.  Extensions

       An NNTP server or extension MAY extend the syntax or semantics of
       wildmats provided that all wildmats that meet the requirements of
       Section 4.1 have the meaning ascribed to them by Section 4.2.  Future
       editions of this document may also extend wildmats.

    4.4.  Examples

       In these examples, $ and @ are used to represent the two octets %xC2
       and %xA3, 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"
         a*,!*b,c*  Any string that begins with "a" and does not end with
                    "b", and any string that begins with "c" no matter
                    what it ends with
         a*,c*,!*b  Any string that begins with "a" or "c" and does not
                    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

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    5.  Session Administration Commands

    5.1.  Initial Connection

    5.1.1.  Usage

       This command MUST NOT be pipelined.

       Responses [1]
         200    Service available, posting allowed
         201    Service available, posting prohibited
         400    Service temporarily unavailable [2]
         502    Service permanently unavailable [2]

       [1] These are the only valid response codes for the initial greeting;
           the server MUST not return any other generic response code.

       [2] Following a 400 or 502 response, the server MUST immediately
           close the connection.

    5.1.2.  Description

       There is no command 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
       whether service is available and whether the client is permitted to
       post.

       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 the client 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. 400 SHOULD be used if the
       issue is only temporary (for example, because of load) and the client
       can expect to be able to connect successfully at some point in the
       future without making any changes. 502 MUST be used if the client is
       not permitted under any circumstances to interact with the server,
       and MAY be used if the server has insufficient information to
       determine whether the issue is temporary or permanent.

       Note: the distinction between the 200 and 201 response codes has
       turned out in practice to be insufficient; for example, some servers
       do not allow posting until the client has authenticated, while other
       clients assume that a 201 response means that posting will never be
       possible even after authentication.  Therefore, clients SHOULD use

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       the CAPABILITIES command (Section 5.2) rather than rely on this
       response.

    5.1.3.  Examples

       Example of a normal connection from an authorized client that then
       terminates the session (see Section 5.4):

          [Initial connection set-up completed.]
          [S] 200 NNTP Service Ready, posting permitted
          [C] QUIT
          [S] 205 NNTP Service exits normally
          [Server closes connection.]

       Example of a normal connection from an authorized client that is not
       permitted to post, which also immediately terminates the session:

          [Initial connection set-up completed.]
          [S] 201 NNTP Service Ready, posting prohibited
          [C] QUIT
          [S] 205 NNTP Service exits normally
          [Server closes connection.]

       Example of a normal connection from an unauthorized client:

          [Initial connection set-up completed.]
          [S] 502 NNTP Service permanently unavailable
          [Server closes connection.]

       Example of a connection from a client if the server is unable to
       provide service:

          [Initial connection set-up completed.]
          [S] 400 NNTP Service temporarily unavailable
          [Server closes connection.]

    5.2.  CAPABILITIES

    5.2.1.  Usage

       This command is mandatory.

       Syntax
         CAPABILITIES [keyword]

       Responses
         101    Capability list follows (multi-line)

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       Parameters
         keyword    additional feature, see description

    5.2.2.  Description

       The CAPABILITIES command allows a client to determine the
       capabilities of the server at any given time.

       This command MAY be issued at any time; the server MUST NOT require
       it to be issued in order to make use of any capability.  The response
       generated by this command MAY change during a session because of
       other state information (which, in turn, may be changed by the
       effects of other commands or by external events).  An NNTP client is
       only able to get the current and correct information concerning
       available capabilities at any point during a session by issuing a
       CAPABILITIES command at that point of that session and processing the
       response.

       The capability list is returned as a multi-line data block following
       the 101 response code.  Each capability is described by a separate
       capability line.  The server MUST NOT list the same capability twice
       in the response, even with different arguments.  Except that the
       VERSION capability MUST be the first line, the order in which the
       capability lines appears is not significant; the server need not even
       consistently return the same order.

       While some capabilities are likely to be always available or never
       available, others (notably extensions) will appear and disappear
       depending on server state changes within the session or on external
       events between sessions.  An NNTP client MAY cache the results of
       this command, but MUST NOT rely on the correctness of any cached
       results, whether from earlier in this session or from a previous
       session, MUST cope gracefully with the cached status being out of
       date, and SHOULD (if caching results) provide a way to force the
       cached information to be refreshed.  Furthermore, a client MUST NOT
       use cached results in relation to security, privacy, and
       authentication extensions.  See Section 12.6 for further discussion
       of this topic.

       The keyword argument is not used by this specification.  It is
       provided so that extensions or revisions to this specification can
       include extra features for this command without requiring the
       CAPABILITIES command to be used twice (once to determine if the extra
       features are available, and a second time to make use of them).  If
       the server does not recognise the argument (and it is a keyword), it
       MUST respond with the 101 response code as if the argument had been
       omitted.  If an argument is provided that the server does recognise,
       it MAY use the 101 response code or MAY use some other response code

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       (which will be defined in the specification of that feature).  If the
       argument is not a keyword, the 501 generic response code MUST be
       returned.  The server MUST NOT generate any other response code to
       the CAPABILITIES command.

    5.2.3.  Examples

       Example of a minimal response (a read-only server):

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] LIST ACTIVE NEWSGROUPS
          [S] .

       Example of a response from a server that has a range of facilities
       and that also describes itself:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] IHAVE
          [S] POST
          [S] NEWNEWS
          [S] LIST ACTIVE NEWSGROUPS ACTIVE.TIMES OVERVIEW.FMT
          [S] IMPLEMENTATION INN 4.2 2004-12-25
          [S] OVER MSGID
          [S] STREAMING
          [S] XSECRET
          [S] .

       Example of a server that supports more than one version of NNTP:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2 3
          [S] READER
          [S] LIST ACTIVE NEWSGROUPS
          [S] .

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       Example of a client attempting to use a feature of the CAPABILITIES
       command that the server does not support:

          [C] CAPABILITIES AUTOUPDATE
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] IHAVE
          [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT HEADERS
          [S] OVER MSGID
          [S] HDR
          [S] NEWNEWS
          [S] .

    5.3.  MODE READER

    5.3.1.  Usage

       Indicating capability: MODE-READER

       This command MUST NOT be pipelined.

       Syntax
         MODE READER

       Responses
         200    Posting allowed
         201    Posting prohibited
         502    Reading service permanently unavailable [1]

       [1] Following a 502 response the server MUST immediately close the
           connection.

    5.3.2.  Description

       The MODE READER command instructs a mode-switching server to switch
       modes, as described in Section 3.4.2.

       If the server is mode-switching, it switches from its transit mode to
       its reader mode, indicating this by changing the capability list
       accordingly.  It MUST then return a 200 or 201 response with the same
       meaning as for the initial greeting (as described in Section 5.1.1).
       Note that the response need not be the same as that presented during
       the initial greeting.  The client MUST NOT issue MODE READER more
       than once in a session or after any security or privacy commands are
       issued.  When the MODE READER command is issued, the server MAY reset
       its state to that immediately after the initial connection before
       switching mode.

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       If the server is not mode-switching, then the following apply:

       o  If it advertises the READER capability, it MUST return a 200 or
          201 response with the same meaning as for the initial greeting; in
          this case, the command MUST NOT affect the server state in any
          way.

       o  If it does not advertise the READER capability, it MUST return a
          502 response and then immediately close the connection.

    5.3.3.  Examples

       Example of use of the MODE READER command on a transit-only server
       (which therefore does not providing reading facilities):

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] IHAVE
          [S] .
          [C] MODE READER
          [S] 502 Transit service only
          [Server closes connection.]

       Example of use of the MODE READER command on a server that provides
       reading facilities:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] LIST ACTIVE NEWSGROUPS
          [S] .
          [C] MODE READER
          [S] 200 Reader mode, posting permitted
          [C] IHAVE <i.am.an.article.you.have@example.com>
          [S] 500 Permission denied
          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test

       Note that in both of these situations, the client SHOULD NOT use MODE
       READER.

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       Example of use of the MODE READER command on a mode-switching server:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] IHAVE
          [S] MODE-READER
          [S] .
          [C] MODE READER
          [S] 200 Reader mode, posting permitted
          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] NEWNEWS
          [S] LIST ACTIVE NEWSGROUPS
          [S] STARTTLS
          [S] .

       In this case, the server offers (but does not require) TLS privacy in
       its reading mode but not in its transit mode.

       Example of use of the MODE READER command where the client is not
       permitted to post:

          [C] MODE READER
          [S] 201 NNTP Service Ready, posting prohibited

    5.4.  QUIT

    5.4.1.  Usage

       This command is mandatory.

       Syntax
         QUIT

       Responses
         205    Connection closing

    5.4.2.  Description

       The client uses the QUIT command to terminate the session.  The
       server 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 of its transactions with the
       NNTP server.

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       If a client simply disconnects (or if 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.

       The server MUST NOT generate any response code to the QUIT command
       other than 205 or, if any arguments are provided, 501.

    5.4.3.  Examples

          [C] QUIT
          [S] 205 closing connection
          [Server closes connection.]

    6.  Article Posting and 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.  The first type of key is the message-id
       of an article and is globally unique.  The second type of key is
       composed of a newsgroup name and an article number within that
       newsgroup.  On a particular server, there MUST only be one article
       with a given number within any newsgroup, and an article MUST NOT
       have two different numbers in the same newsgroup.  An article can be
       cross-posted to multiple newsgroups, so there may be multiple keys
       that point to the same article on the same server; these MAY have
       different numbers in each newsgroup.  However, this type of key is
       not required to be globally unique, so the same key MAY refer to
       different articles on different servers.  (Note that the terms
       "group" and "newsgroup" are equivalent.)

       The final type of 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 2,147,483,647, inclusive.  The
       client and server MAY use leading zeroes in specifying article
       numbers but MUST NOT use more than 16 digits.  In some situations,
       the value zero replaces an article number to show some special
       situation.

       Note that it is likely that the article number limit of 2,147,483,647
       will be increased by a future revision or extension to this
       specification.  While servers MUST NOT send article numbers greater
       than this current limit, client and server developers are advised to

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       use internal structures and datatypes capable of handling larger
       values in anticipation of such a change.

    6.1.  Group and Article Selection

       The following commands are used to set the "currently selected
       newsgroup" and the "current article number", which are used by
       various commands.  At the start of an NNTP session, both of these
       values are set to the special value "invalid".

    6.1.1.  GROUP

    6.1.1.1.  Usage

       Indicating capability: READER

       Syntax
         GROUP group

       Responses
         211 number low high group     Group successfully selected
         411                           No such newsgroup

       Parameters
         group     Name of newsgroup
         number    Estimated number of articles in the group
         low       Reported low water mark
         high      Reported high water mark

    6.1.1.2.  Description

       The GROUP command selects a newsgroup as the currently selected
       newsgroup and returns summary information about it.

       The required argument is the name of the newsgroup to be selected
       (e.g., "news.software.nntp").  A list of valid newsgroups may be
       obtained by using the LIST ACTIVE command (see Section 7.6.3).

       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 in the group currently available.

       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

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       currently stored.  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.

       o  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.

       o  All three numbers will be zero.

       o  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
          it is 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:

       o  Articles may be removed from the group.

       o  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).

       o  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 and the high water mark has
          been adjusted accordingly, 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 in this session, and it SHOULD be no less
       than that in any previous response for that newsgroup ever sent to
       any client.  Any failure to meet the latter condition SHOULD be
       transient only.  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

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       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
       currently selected newsgroup MUST be set to that group, and the
       current article number MUST be set to the first article in the group
       (this applies even if the group is already the currently selected
       newsgroup).  If an empty newsgroup is selected, the current article
       number is made invalid.  If an invalid group is specified, the
       currently selected newsgroup and current article number MUST NOT be
       changed.

       The GROUP or LISTGROUP command (see Section 6.1.2) MUST be used by a
       client, and a successful response received, before any other command
       is used that depends on the value of the currently selected newsgroup
       or current article number.

       If the group specified is not available on the server, a 411 response
       MUST be returned.

    6.1.1.3.  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

       Example of an empty group using the preferred response:

          [C] GROUP example.currently.empty.newsgroup
          [S] 211 0 4000 3999 example.currently.empty.newsgroup

       Example of an empty group using an alternative response:

          [C] GROUP example.currently.empty.newsgroup
          [S] 211 0 0 0 example.currently.empty.newsgroup

       Example of an empty group using a different alternative response:

          [C] GROUP example.currently.empty.newsgroup
          [S] 211 0 4000 4321 example.currently.empty.newsgroup

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       Example reselecting the currently selected newsgroup:

          [C] GROUP misc.test
          [S] 211 1234 234 567 misc.test
          [C] STAT 444
          [S] 223 444 <123456@example.net> retrieved
          [C] GROUP misc.test
          [S] 211 1234 234 567 misc.test
          [C] STAT
          [S] 223 234 <different@example.net> retrieved

    6.1.2.  LISTGROUP

    6.1.2.1.  Usage

       Indicating capability: READER

       Syntax
         LISTGROUP [group [range]]

       Responses
         211 number low high group     Article numbers follow (multi-line)
         411                           No such newsgroup
         412                           No newsgroup selected [1]

       Parameters
         group     Name of newsgroup
         range     Range of articles to report
         number    Estimated number of articles in the group
         low       Reported low water mark
         high      Reported high water mark

       [1] The 412 response can only occur if no group has been specified.

    6.1.2.2.  Description

       The LISTGROUP command selects a newsgroup in the same manner as the
       GROUP command (see Section 6.1.1) but also provides a list of article
       numbers in the newsgroup.  If no group is specified, the currently
       selected newsgroup is used.

       On success, a list of article numbers is returned as a multi-line
       data block following the 211 response code (the arguments on the
       initial response line are the same as for the GROUP command).  The
       list contains one number per line and is in numerical order.  It
       lists precisely those articles that exist in the group at the moment
       of selection (therefore, an empty group produces an empty list).  If
       the optional range argument is specified, only articles within the

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       range are included in the list (therefore, the list MAY be empty even
       if the group is not).

       The range argument may be any of the following:

       o  An article number.

       o  An article number followed by a dash to indicate all following.

       o  An article number followed by a dash followed by another article
          number.

       In the last case, if the second number is less than the first number,
       then the range contains no articles.  Omitting the range is
       equivalent to the range 1- being specified.

       If the group specified is not available on the server, a 411 response
       MUST be returned.  If no group is specified and the currently
       selected newsgroup is invalid, a 412 response MUST be returned.

       Except that the group argument is optional, that a range argument can
       be specified, and that a multi-line data block follows the 211
       response code, the LISTGROUP command is identical to the GROUP
       command.  In particular, when successful, the command sets the
       current article number to the first article in the group, if any,
       even if this is not within the range specified by the second
       argument.

       Note that the range argument is a new feature in this specification
       and servers that do not support CAPABILITIES (and therefore do not
       conform to this specification) are unlikely to support it.

    6.1.2.3.  Examples

       Example of LISTGROUP being used to select a group:

          [C] LISTGROUP misc.test
          [S] 211 2000 3000234 3002322 misc.test list follows
          [S] 3000234
          [S] 3000237
          [S] 3000238
          [S] 3000239
          [S] 3002322
          [S] .

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       Example of LISTGROUP on an empty group:

          [C] LISTGROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup list follows
          [S] .

       Example of LISTGROUP on a valid, currently selected newsgroup:

          [C] GROUP misc.test
          [S] 211 2000 3000234 3002322 misc.test
          [C] LISTGROUP
          [S] 211 2000 3000234 3002322 misc.test list follows
          [S] 3000234
          [S] 3000237
          [S] 3000238
          [S] 3000239
          [S] 3002322
          [S] .

       Example of LISTGROUP with a range:

          [C] LISTGROUP misc.test 3000238-3000248
          [S] 211 2000 3000234 3002322 misc.test list follows
          [S] 3000238
          [S] 3000239
          [S] .

       Example of LISTGROUP with an empty range:

          [C] LISTGROUP misc.test 12345678-
          [S] 211 2000 3000234 3002322 misc.test list follows
          [S] .

       Example of LISTGROUP with an invalid range:

          [C] LISTGROUP misc.test 9999-111
          [S] 211 2000 3000234 3002322 misc.test list follows
          [S] .

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    6.1.3.  LAST

    6.1.3.1.  Usage

       Indicating capability: READER

       Syntax
         LAST

       Responses
         223 n message-id    Article found
         412                 No newsgroup selected
         420                 Current article number is invalid
         422                 No previous article in this group

       Parameters
         n             Article number
         message-id    Article message-id

    6.1.3.2.  Description

       If the currently selected newsgroup is valid, the current article
       number MUST be set to the previous article in that newsgroup (that
       is, the highest existing article number less than the current article
       number).  If successful, a response indicating the new current
       article number and the message-id of that article MUST be returned.
       No article text is sent in response to this command.

       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.

       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 the current article number is already the first article of the
       newsgroup, a 422 response MUST be returned.  If the current article
       number is invalid, a 420 response MUST be returned.  If the currently
       selected newsgroup is invalid, a 412 response MUST be returned.  In
       all three cases, the currently selected newsgroup and current article
       number MUST NOT be altered.

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    6.1.3.3.  Examples

       Example of a successful article retrieval using LAST:

          [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:

          [Assumes currently selected newsgroup is invalid.]
          [C] LAST
          [S] 412 no newsgroup selected

       Example of an attempt to retrieve an article using the LAST command
       when the current article number is that of the first article in the
       group:

          [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 currently selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] LAST
          [S] 420 No current article selected

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    6.1.4.  NEXT

    6.1.4.1.  Usage

       Indicating capability: READER

       Syntax
         NEXT

       Responses
         223 n message-id    Article found
         412                 No newsgroup selected
         420                 Current article number is invalid
         421                 No next article in this group

       Parameters
         n             Article number
         message-id    Article message-id

    6.1.4.2.  Description

       If the currently selected newsgroup is valid, the current article
       number MUST be set to the next article in that newsgroup (that is,
       the lowest existing article number greater than the current article
       number).  If successful, a response indicating the new current
       article number and the message-id of that article MUST be returned.
       No article text is sent in response to this command.

       If the current article number is already the last article of the
       newsgroup, a 421 response MUST be returned.  In all other aspects
       (apart, of course, from the lack of 422 response), this command is
       identical to the LAST command (Section 6.1.3).

    6.1.4.3.  Examples

       Example of a successful article retrieval using NEXT:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] NEXT
          [S] 223 3000237 <668929@example.org> retrieved

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       Example of an attempt to retrieve an article without having selected
       a group (via the GROUP command) first:

          [Assumes currently selected newsgroup is invalid.]
          [C] NEXT
          [S] 412 no newsgroup selected

       Example of an attempt to retrieve an article using the NEXT command
       when the current article number is that of the last article in the
       group:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] STAT 3002322
          [S] 223 3002322 <411@example.net> retrieved
          [C] NEXT
          [S] 421 No next article to retrieve

       Example of an attempt to retrieve an article using the NEXT command
       when the currently selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] NEXT
          [S] 420 No current article selected

    6.2.  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 three commands are described
       in terms of the differences.  As specified in Section 3.6, an article
       consists of two parts: the article headers and the article body.

       When responding to one of these commands, the server MUST present the
       entire article or appropriate part and MUST NOT attempt to alter or
       translate it in any way.

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    6.2.1.  ARTICLE

    6.2.1.1.  Usage

       Indicating capability: READER

       Syntax
         ARTICLE message-id
         ARTICLE number
         ARTICLE

       Responses

       First form (message-id specified)
         220 0|n message-id    Article follows (multi-line)
         430                   No article with that message-id

       Second form (article number specified)
         220 n message-id      Article follows (multi-line)
         412                   No newsgroup selected
         423                   No article with that number

       Third form (current article number used)
         220 n message-id      Article follows (multi-line)
         412                   No newsgroup selected
         420                   Current article number is invalid

       Parameters
         number        Requested article number
         n             Returned article number
         message-id    Article message-id

    6.2.1.2.  Description

       The ARTICLE command selects an article according to the arguments and
       presents the entire article (that is, the headers, an empty line, and
       the body, in that order) to the client.  The command has three forms.

       In the first form, a message-id is specified, and the server presents
       the article with that message-id.  In this case, the server MUST NOT
       alter the currently selected newsgroup or current article number.
       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 cross-posted to more than
       one newsgroup.

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       In the response, the article number MUST be replaced with zero,
       unless there is a currently selected newsgroup and the article is
       present in that group, in which case the server MAY use the article's
       number in that group.  (The server is not required to determine
       whether the article is in the currently selected newsgroup or, if so,
       what article number it has; the client MUST always be prepared for
       zero to be specified.)  The server MUST NOT provide an article number
       unless use of that number in a second ARTICLE command immediately
       following this one would return the same article.  Even if the server
       chooses to return article numbers in these circumstances, it need not
       do so consistently; it MAY return zero to any such command (also see
       the STAT examples, Section 6.2.4.3).

       In the second form, an article number is specified.  If there is an
       article with that number in the currently selected newsgroup, the
       server MUST set the current article number to that number.

       In the third form, the article indicated by the current article
       number in the currently selected newsgroup is used.

       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 this article
       number MUST be no less than the reported low water mark for that
       group.

       The server MUST NOT change the currently selected newsgroup as a
       result of this command.  The server MUST NOT change the current
       article number except when an article number argument was provided
       and the article exists; in particular, it MUST NOT change it
       following an unsuccessful response.

       Since the message-id 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 is returned as a multi-line data block following the 220
       response code.

       If the argument is a message-id and no such article exists, a 430
       response MUST be returned.  If the argument is a number or is omitted
       and the currently selected newsgroup is invalid, a 412 response MUST
       be returned.  If the argument is a number and that article does not
       exist in the currently selected newsgroup, a 423 response MUST be
       returned.  If the argument is omitted and the current article number
       is invalid, a 420 response MUST be returned.

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    6.2.1.3.  Examples

       Example of a successful retrieval of an article (explicitly not using
       an article number):

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] ARTICLE
          [S] 220 3000234 <45223423@example.com>
          [S] Path: pathost!demo!whitehouse!not-for-mail
          [S] From: "Demo User" <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: <45223423@example.com>
          [S]
          [S] This is just a test article.
          [S] .

       Example of a successful retrieval of an article by message-id:

          [C] ARTICLE <45223423@example.com>
          [S] 220 0 <45223423@example.com>
          [S] Path: pathost!demo!whitehouse!not-for-mail
          [S] From: "Demo User" <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: <45223423@example.com>
          [S]
          [S] This is just a test article.
          [S] .

       Example of an unsuccessful retrieval of an article by message-id:

          [C] ARTICLE <i.am.not.there@example.com>
          [S] 430 No Such Article Found

       Example of an unsuccessful retrieval of an article by number:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 news.groups
          [C] ARTICLE 300256
          [S] 423 No article with that number

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       Example of an unsuccessful retrieval of an article by number because
       no newsgroup was selected first:

          [Assumes currently selected newsgroup is invalid.]
          [C] ARTICLE 300256
          [S] 412 No newsgroup selected

       Example of an attempt to retrieve an article when the currently
       selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] ARTICLE
          [S] 420 No current article selected

    6.2.2.  HEAD

    6.2.2.1.  Usage

       This command is mandatory.

       Syntax
         HEAD message-id
         HEAD number
         HEAD

       Responses

       First form (message-id specified)
         221 0|n message-id    Headers follow (multi-line)
         430                   No article with that message-id

       Second form (article number specified)
         221 n message-id      Headers follow (multi-line)
         412                   No newsgroup selected
         423                   No article with that number

       Third form (current article number used)
         221 n message-id      Headers follow (multi-line)
         412                   No newsgroup selected
         420                   Current article number is invalid

       Parameters
         number        Requested article number
         n             Returned article number
         message-id    Article message-id

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    6.2.2.2.  Description

       The HEAD command behaves identically to the ARTICLE command except
       that, if the article exists, the response code is 221 instead of 220
       and only the headers are presented (the empty line separating the
       headers and body MUST NOT be included).

    6.2.2.3.  Examples

       Example of a successful retrieval of the headers of an article
       (explicitly not using an article number):

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HEAD
          [S] 221 3000234 <45223423@example.com>
          [S] Path: pathost!demo!whitehouse!not-for-mail
          [S] From: "Demo User" <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: <45223423@example.com>
          [S] .

       Example of a successful retrieval of the headers of an article by
       message-id:

          [C] HEAD <45223423@example.com>
          [S] 221 0 <45223423@example.com>
          [S] Path: pathost!demo!whitehouse!not-for-mail
          [S] From: "Demo User" <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: <45223423@example.com>
          [S] .

       Example of an unsuccessful retrieval of the headers of an article by
       message-id:

          [C] HEAD <i.am.not.there@example.com>
          [S] 430 No Such Article Found

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       Example of an unsuccessful retrieval of the headers of an article by
       number:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HEAD 300256
          [S] 423 No article with that number

       Example of an unsuccessful retrieval of the headers of an article by
       number because no newsgroup was selected first:

          [Assumes currently selected newsgroup is invalid.]
          [C] HEAD 300256
          [S] 412 No newsgroup selected

       Example of an attempt to retrieve the headers of an article when the
       currently selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] HEAD
          [S] 420 No current article selected

    6.2.3.  BODY

    6.2.3.1.  Usage

       Indicating capability: READER

       Syntax
         BODY message-id
         BODY number
         BODY

       Responses

       First form (message-id specified)
         222 0|n message-id    Body follows (multi-line)
         430                   No article with that message-id

       Second form (article number specified)
         222 n message-id      Body follows (multi-line)
         412                   No newsgroup selected
         423                   No article with that number

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       Third form (current article number used)
         222 n message-id      Body follows (multi-line)
         412                   No newsgroup selected
         420                   Current article number is invalid

       Parameters
         number        Requested article number
         n             Returned article number
         message-id    Article message-id

    6.2.3.2.  Description

       The BODY command behaves identically to the ARTICLE command except
       that, if the article exists, the response code is 222 instead of 220
       and only the body is presented (the empty line separating the headers
       and body MUST NOT be included).

    6.2.3.3.  Examples

       Example of a successful retrieval of the body of an article
       (explicitly not using an article number):

          [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:

          [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:

          [C] BODY <i.am.not.there@example.com>
          [S] 430 No Such Article Found

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       Example of an unsuccessful retrieval of the body of an article by
       number:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] BODY 300256
          [S] 423 No article with that number

       Example of an unsuccessful retrieval of the body of an article by
       number because no newsgroup was selected first:

          [Assumes currently selected newsgroup is invalid.]
          [C] BODY 300256
          [S] 412 No newsgroup selected

       Example of an attempt to retrieve the body of an article when the
       currently selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] BODY
          [S] 420 No current article selected

    6.2.4.  STAT

    6.2.4.1.  Usage

       This command is mandatory.

       Syntax
         STAT message-id
         STAT number
         STAT

       Responses

       First form (message-id specified)
         223 0|n message-id    Article exists
         430                   No article with that message-id

       Second form (article number specified)
         223 n message-id      Article exists
         412                   No newsgroup selected
         423                   No article with that number

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       Third form (current article number used)
         223 n message-id      Article exists
         412                   No newsgroup selected
         420                   Current article number is invalid

       Parameters
         number        Requested article number
         n             Returned article number
         message-id    Article message-id

    6.2.4.2.  Description

       The STAT command behaves identically to the ARTICLE command except
       that, if the article exists, it is NOT presented to the client and
       the response code is 223 instead of 220.  Note that the response is
       NOT multi-line.

       This command allows the client to determine whether an article exists
       and, in the second and third forms, what its message-id is, without
       having to process an arbitrary amount of text.

    6.2.4.3.  Examples

       Example of STAT on an existing article (explicitly not using an
       article number):

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] STAT
          [S] 223 3000234 <45223423@example.com>

       Example of STAT on an existing article by message-id:

          [C] STAT <45223423@example.com>
          [S] 223 0 <45223423@example.com>

       Example of STAT on an article not on the server by message-id:

          [C] STAT <i.am.not.there@example.com>
          [S] 430 No Such Article Found

       Example of STAT on an article not in the server by number:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] STAT 300256
          [S] 423 No article with that number

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       Example of STAT on an article by number when no newsgroup was
       selected first:

          [Assumes currently selected newsgroup is invalid.]
          [C] STAT 300256
          [S] 412 No newsgroup selected

       Example of STAT on an article when the currently selected newsgroup
       is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] STAT
          [S] 420 No current article selected

       Example of STAT by message-id on a server that sometimes reports the
       actual article number:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] STAT
          [S] 223 3000234 <45223423@example.com>
          [C] STAT <45223423@example.com>
          [S] 223 0 <45223423@example.com>
          [C] STAT <45223423@example.com>
          [S] 223 3000234 <45223423@example.com>
          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] STAT <45223423@example.com>
          [S] 223 0 <45223423@example.com>
          [C] GROUP alt.crossposts
          [S] 211 9999 111111 222222 alt.crossposts
          [C] STAT <45223423@example.com>
          [S] 223 123456 <45223423@example.com>
          [C] STAT
          [S] 223 111111 <23894720@example.com>

       The first STAT command establishes the identity of an article in the
       group.  The second and third show that the server may, but need not,
       give the article number when the message-id is specified.  The fourth
       STAT command shows that zero must be specified if the article isn't
       in the currently selected newsgroup.  The fifth shows that the
       number, if provided, must be that relating to the currently selected
       newsgroup.  The last one shows that the current article number is
       still not changed by the use of STAT with a message-id even if it
       returns an article number.

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    6.3.  Article Posting

       Article posting is done in one of two ways: individual article
       posting from news-reading clients using POST, and article transfer
       from other news servers using IHAVE.

    6.3.1.  POST

    6.3.1.1.  Usage

       Indicating capability: POST

       This command MUST NOT be pipelined.

       Syntax
         POST

       Responses

       Initial responses
         340    Send article to be posted
         440    Posting not permitted

       Subsequent responses
         240    Article received OK
         441    Posting failed

    6.3.1.2.  Description

       If posting is allowed, a 340 response MUST be returned to indicate
       that the article to be posted should be sent.  If posting is
       prohibited for some installation-dependent reason, a 440 response
       MUST be returned.

       If posting is permitted, the article MUST be in the format specified
       in Section 3.6 and MUST be sent by the client to the server as a
       multi-line data block (see Section 3.1.1).  Thus a single dot (".")
       on a line indicates the end of the text, and lines starting with a
       dot in the original text have that dot doubled during transmission.

       Following the presentation of the termination sequence by the client,
       the server MUST return a response indicating success or failure of
       the article transfer.  Note that response codes 340 and 440 are used
       in direct response to the POST command while 240 and 441 are returned
       after the article is sent.

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       A response of 240 SHOULD indicate that, barring unforeseen server
       errors, the posted article will be made available on the server
       and/or transferred to other servers, as appropriate, possibly
       following further processing.  In other words, articles not wanted by
       the server SHOULD be rejected with a 441 response, rather than being
       accepted and then discarded silently.  However, the client SHOULD NOT
       assume that the article has been successfully transferred unless it
       receives an affirmative response from the server and SHOULD NOT
       assume that it is being made available to other clients without
       explicitly checking (for example, using the STAT command).

       If the session is interrupted before the response is received, it is
       possible that an affirmative response was sent but has been lost.
       Therefore, in any subsequent session, the client SHOULD either check
       whether the article was successfully posted before resending or
       ensure that the server will allocate the same message-id to the new
       attempt (see Appendix A.2).  The latter approach is preferred since
       the article might not have been made available for reading yet (for
       example, it may have to go through a moderation process).

    6.3.1.3.  Examples

       Example of a successful posting:

          [C] POST
          [S] 340 Input article; end with <CR-LF>.<CR-LF>
          [C] From: "Demo User" <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:

          [C] POST
          [S] 340 Input article; end with <CR-LF>.<CR-LF>
          [C] From: "Demo User" <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] 441 Posting failed

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       Example of an attempt to post when posting is not allowed:

          [Initial connection set-up completed.]
          [S] 201 NNTP Service Ready, posting prohibited
          [C] POST
          [S] 440 Posting not permitted

    6.3.2.  IHAVE

    6.3.2.1.  Usage

       Indicating capability: IHAVE

       This command MUST NOT be pipelined.

       Syntax
         IHAVE message-id

       Responses

       Initial responses
         335    Send article to be transferred
         435    Article not wanted
         436    Transfer not possible; try again later

       Subsequent responses
         235    Article transferred OK
         436    Transfer failed; try again later
         437    Transfer rejected; do not retry

       Parameters
         message-id    Article message-id

    6.3.2.2.  Description

       The IHAVE command informs the server that the client has an article
       with the specified message-id.  If the server desires a copy of that
       article, a 335 response MUST be returned, 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), a 435
       response MUST be returned, indicating that the article is not wanted.
       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), a 436
       response MUST be returned.

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       If transmission of the article is requested, the client MUST send the
       entire article, including headers and body, to the server as a
       multi-line data block (see Section 3.1.1).  Thus, a single dot (".")
       on a line indicates the end of the text, and lines starting with a
       dot in the original text have that dot doubled during transmission.
       The server MUST return a 235 response, indicating that the article
       was successfully transferred; a 436 response, indicating that the
       transfer failed but should be tried again later; or a 437 response,
       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 use of this command indicates that the article has
       already been posted at another site and is simply being forwarded
       from another host.  However, despite this, 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 problems such as
       inappropriate newsgroups or distributions, disc space limitations,
       article lengths, garbled headers, and the like.  These are typically
       restrictions enforced by the server host's news software and not
       necessarily by 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 response.

       Because some news server software may not immediately be able to
       determine whether 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.

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    6.3.2.3.  Examples

       Example of successfully sending an article to another site:

          [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: "Demo User" <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.an.article.you.will.want@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.  Note
       that the message-id in the IHAVE command is not the same as the one
       in the article headers; while this is bad practice and SHOULD NOT be
       done, it is not forbidden.

          [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: "Demo User" <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.an.article.you.have@example.com>
          [C]
          [C] This is just a test article.
          [C] .
          [S] 437 Article rejected; don't send again

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       Example of sending an article to another site where the transfer
       fails:

          [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: "Demo User" <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.an.article.you.will.want@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:

          [C] IHAVE <i.am.an.article.you.have@example.com>
          [S] 435 Duplicate

       Example of sending an article to a site that requests that the
       article be tried again later:

          [C] IHAVE <i.am.an.article.you.defer@example.com>
          [S] 436 Retry later

    7.  Information Commands

       This section lists other commands that may be used at any time
       between the beginning of a session and its termination.  Using these
       commands does not alter any state information, but the response
       generated from their use may provide useful information to clients.

    7.1.  DATE

    7.1.1.  Usage

       Indicating capability: READER

       Syntax
         DATE

       Responses
         111 yyyymmddhhmmss    Server date and time

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       Parameters
         yyyymmddhhmmss    Current UTC date and time on server

    7.1.2.  Description

       This command exists to help clients find out the current Coordinated
       Universal Time [TF.686-1] from the server's perspective.  This
       command SHOULD NOT be used as a substitute for NTP [RFC1305] but to
       provide information that might be useful when using the NEWNEWS
       command (see Section 7.4).

       The DATE command MUST return a timestamp from the same clock as is
       used for determining article arrival and group creation times (see
       Section 6).  This clock SHOULD be monotonic, and adjustments SHOULD
       be made by running it fast or slow compared to "real" time rather
       than by making sudden jumps.  A system providing NNTP service SHOULD
       keep the system clock as accurate as possible, either with NTP or by
       some other method.

       The server MUST return a 111 response specifying the date and time on
       the server in the form yyyymmddhhmmss.  This date and time is in
       Coordinated Universal Time.

    7.1.3.  Examples

          [C] DATE
          [S] 111 19990623135624

    7.2.  HELP

    7.2.1.  Usage

       This command is mandatory.

       Syntax
         HELP

       Responses
         100    Help text follows (multi-line)

    7.2.2.  Description

       This command provides a short summary of the commands that are
       understood by this implementation of the server.  The help text will
       be presented as a multi-line data block following the 100 response
       code.

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       This text is not guaranteed to be in any particular format (but must
       be UTF-8) and MUST NOT be used by clients as a replacement for the
       CAPABILITIES command described in Section 5.2.

    7.2.3.  Examples

          [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] .

    7.3.  NEWGROUPS

    7.3.1.  Usage

       Indicating capability: READER

       Syntax
         NEWGROUPS date time [GMT]

       Responses
         231    List of new newsgroups follows (multi-line)

       Parameters
         date    Date in yymmdd or yyyymmdd format
         time    Time in hhmmss format

    7.3.2.  Description

       This command returns a list of newsgroups created on the server since
       the specified date and time.  The results are in the same format as
       the LIST ACTIVE command (see Section 7.6.3).  However, they MAY
       include groups not available on the server (and so not returned by
       LIST ACTIVE) and MAY omit groups for which the creation date is not
       available.

       The date is specified as 6 or 8 digits in the format [xx]yymmdd,
       where xx is the first two digits of the year (19-99), yy is the last
       two digits of the year (00-99), mm is the month (01-12), and dd is
       the day of the month (01-31).  Clients SHOULD specify all four digits
       of the year.  If the first two digits of the year are not specified
       (this is supported only for backward compatibility), the year is to
       be taken from the current century if yy is smaller than or equal to
       the current year, and the previous century otherwise.

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       The time is specified as 6 digits in the format hhmmss, where hh is
       the hours in the 24-hour clock (00-23), mm is the minutes (00-59),
       and ss is the seconds (00-60, to allow for leap seconds).  The token
       "GMT" specifies that the date and time are given in Coordinated
       Universal Time [TF.686-1]; if it is omitted, then the date and time
       are specified in the server's local timezone.  Note that there is no
       way of using the protocol specified in this document to establish the
       server's local timezone.

       Note that an empty list is a possible valid response and indicates
       that there are no new newsgroups since that date-time.

       Clients SHOULD make all queries using Coordinated Universal Time
       (i.e., by including the "GMT" argument) when possible.

    7.3.3.  Examples

       Example where there are new groups:

          [C] NEWGROUPS 19990624 000000 GMT
          [S] 231 list of new newsgroups follows
          [S] alt.rfc-writers.recovery 4 1 y
          [S] tx.natives.recovery 89 56 y
          [S] .

       Example where there are no new groups:

          [C] NEWGROUPS 19990624 000000 GMT
          [S] 231 list of new newsgroups follows
          [S] .

    7.4.  NEWNEWS

    7.4.1.  Usage

       Indicating capability: NEWNEWS

       Syntax
         NEWNEWS wildmat date time [GMT]

       Responses
         230    List of new articles follows (multi-line)

       Parameters
         wildmat    Newsgroups of interest
         date       Date in yymmdd or yyyymmdd format
         time       Time in hhmmss format

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    7.4.2.  Description

       This command returns a list of message-ids of articles posted or
       received on the server, in the newsgroups whose names match the
       wildmat, since the specified date and time.  One message-id is sent
       on each line; the order of the response has no specific significance
       and may vary from response to response in the same session.  A
       message-id MAY appear more than once; if it does, it has the same
       meaning as if it appeared only once.

       Date and time are in the same format as the NEWGROUPS command (see
       Section 7.3).

       Note that an empty list is a possible valid response and indicates
       that there is currently no new news in the relevant groups.

       Clients SHOULD make all queries in Coordinated Universal Time (i.e.,
       by using the "GMT" argument) when possible.

    7.4.3.  Examples

       Example where there are new articles:

          [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>
          [S] .

       Example where there are no new articles:

          [C] NEWNEWS alt.* 19990624 000000 GMT
          [S] 230 list of new articles by message-id follows
          [S] .

    7.5.  Time

       As described in Section 6, each article has an arrival timestamp.
       Each newsgroup also has a creation timestamp.  These timestamps are
       used by the NEWNEWS and NEWGROUP commands to construct their
       responses.

       Clients can ensure that they do not have gaps in lists of articles or
       groups by using the DATE command in the following manner:

       First session:
          Issue DATE command and record result.
          Issue NEWNEWS command using a previously chosen timestamp.

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       Subsequent sessions:
          Issue DATE command and hold result in temporary storage.
          Issue NEWNEWS command using timestamp saved from previous session.
          Overwrite saved timestamp with that currently in temporary
          storage.

       In order to allow for minor errors, clients MAY want to adjust the
       timestamp back by two or three minutes before using it in NEWNEWS.

    7.5.1.  Examples

       First session:

          [C] DATE
          [S] 111 20010203112233
          [C] NEWNEWS local.chat 20001231 235959 GMT
          [S] 230 list follows
          [S] <article.1@local.service>
          [S] <article.2@local.service>
          [S] <article.3@local.service>
          [S] .

       Second session (the client has subtracted 3 minutes from the
       timestamp returned previously):

          [C] DATE
          [S] 111 20010204003344
          [C] NEWNEWS local.chat 20010203 111933 GMT
          [S] 230 list follows
          [S] <article.3@local.service>
          [S] <article.4@local.service>
          [S] <article.5@local.service>
          [S] .

       Note how <article.3@local.service> arrived in the 3 minute gap and so
       is listed in both responses.

    7.6.  The LIST Commands

       The LIST family of commands all return information that is multi-line
       and that can, in general, be expected not to change during the
       session.  Often the information is related to newsgroups, in which
       case the response has one line per newsgroup and a wildmat MAY be
       provided to restrict the groups for which information is returned.

       The set of available keywords (including those provided by
       extensions) is given in the capability list with capability label
       LIST.

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    7.6.1.  LIST

    7.6.1.1.  Usage

       Indicating capability: LIST

       Syntax
         LIST [keyword [wildmat|argument]]

       Responses
         215    Information follows (multi-line)

       Parameters
         keyword     Information requested [1]
         argument    Specific to keyword
         wildmat     Groups of interest

       [1] If no keyword is provided, it defaults to ACTIVE.

    7.6.1.2.  Description

       The LIST command allows the server to provide blocks of information
       to the client.  This information may be global or may be related to
       newsgroups; in the latter case, the information may be returned
       either for all groups or only for those matching a wildmat.  Each
       block of information is represented by a different keyword.  The
       command returns the specific information identified by the keyword.

       If the information is available, it is returned as a multi-line data
       block following the 215 response code.  The format of the information
       depends on the keyword.  The information MAY be affected by the
       additional argument, but the format MUST NOT be.

       If the information is based on newsgroups and the optional wildmat
       argument is specified, the response is limited to only the groups (if
       any) whose names match the wildmat and for which the information is
       available.

       Note that an empty list is a possible valid response; for a
       newsgroup-based keyword, it indicates that there are no groups
       meeting the above criteria.

       If the keyword is not recognised, or if an argument is specified and
       the keyword does not expect one, a 501 response code MUST BE
       returned.  If the keyword is recognised but the server does not
       maintain the information, a 503 response code MUST BE returned.

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       The LIST command MUST NOT change the visible state of the server in
       any way; that is, the behaviour of subsequent commands MUST NOT be
       affected by whether the LIST command was issued.  For example, it
       MUST NOT make groups available that otherwise would not have been.

    7.6.1.3.  Examples

       Example of LIST with the ACTIVE keyword:

          [C] LIST ACTIVE
          [S] 215 list of newsgroups follows
          [S] misc.test 3002322 3000234 y
          [S] comp.risks 442001 441099 m
          [S] alt.rfc-writers.recovery 4 1 y
          [S] tx.natives.recovery 89 56 y
          [S] tx.natives.recovery.d 11 9 n
          [S] .

       Example of LIST with no keyword:

          [C] LIST
          [S] 215 list of newsgroups follows
          [S] misc.test 3002322 3000234 y
          [S] comp.risks 442001 441099 m
          [S] alt.rfc-writers.recovery 4 1 y
          [S] tx.natives.recovery 89 56 y
          [S] tx.natives.recovery.d 11 9 n
          [S] .

       The output is identical to that of the previous example.

       Example of LIST on a newsgroup-based keyword with and without
       wildmat:

          [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] .
          [C] LIST ACTIVE.TIMES tx.*
          [S] 215 information follows
          [S] tx.natives.recovery 930678923 <sob@academ.com>
          [S] .

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       Example of LIST returning an error where the keyword is recognized
       but the software does not maintain this information:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] LIST ACTIVE NEWSGROUPS ACTIVE.TIMES XTRA.DATA
          [S] .
          [C] LIST XTRA.DATA
          [S] 503 Data item not stored

       Example of LIST where the keyword is not recognised:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] LIST ACTIVE NEWSGROUPS ACTIVE.TIMES XTRA.DATA
          [S] .
          [C] LIST DISTRIB.PATS
          [S] 501 Syntax Error

    7.6.2.  Standard LIST Keywords

       This specification defines the following LIST keywords:

       +--------------+---------------+------------------------------------+
       | Keyword      | Definition    | Status                             |
       +--------------+---------------+------------------------------------+
       | ACTIVE       | Section 7.6.3 | Mandatory if the READER capability |
       |              |               | is advertised                      |
       |              |               |                                    |
       | ACTIVE.TIMES | Section 7.6.4 | Optional                           |
       |              |               |                                    |
       | DISTRIB.PATS | Section 7.6.5 | Optional                           |
       |              |               |                                    |
       | HEADERS      | Section 8.6   | Mandatory if the HDR capability is |
       |              |               | advertised                         |
       |              |               |                                    |
       | NEWSGROUPS   | Section 7.6.6 | Mandatory if the READER capability |
       |              |               | is advertised                      |
       |              |               |                                    |
       | OVERVIEW.FMT | Section 8.4   | Mandatory if the OVER capability   |
       |              |               | is advertised                      |
       +--------------+---------------+------------------------------------+

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       Where one of these LIST keywords is supported by a server, it MUST
       have the meaning given in the relevant sub-section.

    7.6.3.  LIST ACTIVE

       This keyword MUST be supported by servers advertising the READER
       capability.

       LIST ACTIVE returns a list of valid newsgroups and associated
       information.  If no wildmat is specified, the server MUST include
       every group that the client is permitted to select with the GROUP
       command (Section 6.1.1).  Each line of this list consists of four
       fields separated from each other by one or more spaces:

       o  The name of the newsgroup.
       o  The reported high water mark for the group.
       o  The reported low water mark for the group.
       o  The current status of the group on this server.

       The reported high and low water marks are as described in the GROUP
       command (see Section 6.1.1), but note that they are in the opposite
       order to the 211 response to that command.

       The status field is typically one of the following:

       "y" Posting is permitted.

       "n" Posting is not permitted.

       "m" Postings will be forwarded to the newsgroup moderator.

       The server SHOULD use these values when these meanings are required
       and MUST NOT use them with any other meaning.  Other values for the
       status may exist; the definition of these other values and the
       circumstances under which they are returned may be specified in an
       extension or may be private to the server.  A client SHOULD treat an
       unrecognized status as giving no information.

       The status of a newsgroup only indicates how posts to that newsgroup
       are normally processed and is not necessarily customised to the
       specific client.  For example, if the current client is forbidden
       from posting, then this will apply equally to groups with status "y".
       Conversely, a client with special privileges (not defined by this
       specification) might be able to post to a group with status "n".

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       For example:

          [C] LIST ACTIVE
          [S] 215 list of newsgroups follows
          [S] misc.test 3002322 3000234 y
          [S] comp.risks 442001 441099 m
          [S] alt.rfc-writers.recovery 4 1 y
          [S] tx.natives.recovery 89 56 y
          [S] tx.natives.recovery.d 11 9 n
          [S] .

       or, on an implementation that includes leading zeroes:

          [C] LIST ACTIVE
          [S] 215 list of newsgroups follows
          [S] misc.test 0003002322 0003000234 y
          [S] comp.risks 0000442001 0000441099 m
          [S] alt.rfc-writers.recovery 0000000004 0000000001 y
          [S] tx.natives.recovery 0000000089 0000000056 y
          [S] tx.natives.recovery.d 0000000011 0000000009 n
          [S] .

       The information is newsgroup based, and a wildmat MAY be specified,
       in which case the response is limited to only the groups (if any)
       whose names match the wildmat.  For example:

          [C] LIST ACTIVE *.recovery
          [S] 215 list of newsgroups follows
          [S] alt.rfc-writers.recovery 4 1 y
          [S] tx.natives.recovery 89 56 y
          [S] .

    7.6.4.  LIST ACTIVE.TIMES

       This keyword is optional.

       The active.times list is maintained by some NNTP servers to contain
       information about who created a particular newsgroup and when.  Each
       line of this list consists of three fields separated from each other
       by one or more spaces.  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 plain text intended to describe the entity that created the
       newsgroup; it is often a mailbox as defined in RFC 2822 [RFC2822].
       For example:

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          [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] .

       The list MAY omit newsgroups for which the information is unavailable
       and MAY include groups not available on the server; in particular, it
       MAY omit all groups created before the date and time of the oldest
       entry.  The client MUST NOT assume that the list is complete or that
       it matches the list returned by the LIST ACTIVE command
       (Section 7.6.3).  The NEWGROUPS command (Section 7.3) may provide a
       better way to access this information, and the results of the two
       commands SHOULD be consistent except that, if the latter is invoked
       with a date and time earlier than the oldest entry in active.times
       list, its result may include extra groups.

       The information is newsgroup based, and a wildmat MAY be specified,
       in which case the response is limited to only the groups (if any)
       whose names match the wildmat.

    7.6.5.  LIST DISTRIB.PATS

       This keyword is optional.

       The distrib.pats list is maintained by some NNTP servers to assist
       clients to choose a value for the content of the Distribution header
       of a news article being posted.  Each line of this list consists of
       three fields separated from each other by a colon (":").  The first
       field is a weight, the second field is a wildmat (which may be a
       simple newsgroup name), and the third field is a value for the
       Distribution header content.  For example:

          [C] LIST DISTRIB.PATS
          [S] 215 information follows
          [S] 10:local.*:local
          [S] 5:*:world
          [S] 20:local.here.*:thissite
          [S] .

       The client MAY use this information to construct an appropriate
       Distribution header given the name of a newsgroup.  To do so, it
       should determine the lines whose second field matches the newsgroup
       name, select from among them the line with the highest weight (with 0
       being the lowest), and use the value of the third field to construct
       the Distribution header.

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       The information is not newsgroup based, and an argument MUST NOT be
       specified.

    7.6.6.  LIST NEWSGROUPS

       This keyword MUST be supported by servers advertising the READER
       capability.

       The newsgroups list is maintained by NNTP servers to contain the name
       of each newsgroup that is available on the server and a short
       description about the purpose of the group.  Each line of this list
       consists of two fields separated from each other by one or more space
       or TAB characters (the usual practice is a single TAB).  The first
       field is the name of the newsgroup, and the second is a short
       description of the group.  For example:

          [C] LIST NEWSGROUPS
          [S] 215 information follows
          [S] misc.test General Usenet testing
          [S] alt.rfc-writers.recovery RFC Writers Recovery
          [S] tx.natives.recovery Texas Natives Recovery
          [S] .

       The list MAY omit newsgroups for which the information is unavailable
       and MAY include groups not available on the server.  The client MUST
       NOT assume that the list is complete or that it matches the list
       returned by LIST ACTIVE.

       The description SHOULD be in UTF-8.  However, servers often obtain
       the information from external sources.  These sources may have used
       different encodings (ones that use octets in the range 128 to 255 in
       some other manner) and, in that case, the server MAY pass it on
       unchanged.  Therefore, clients MUST be prepared to receive such
       descriptions.

       The information is newsgroup based, and a wildmat MAY be specified,
       in which case the response is limited to only the groups (if any)
       whose names match the wildmat.

    8.  Article Field Access Commands

       This section lists commands that may be used to access specific
       article fields; that is, headers of articles and metadata about
       articles.  These commands typically fetch data from an "overview
       database", which is a database of headers extracted from incoming
       articles plus metadata determined as the article arrives.  Only
       certain fields are included in the database.

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       This section is based on the Overview/NOV database [ROBE1995]
       developed by Geoff Collyer.

    8.1.  Article Metadata

       Article "metadata" is data about articles that does not occur within
       the article itself.  Each metadata item has a name that MUST begin
       with a colon (and that MUST NOT contain a colon elsewhere within it).
       As with header names, metadata item names are not case sensitive.

       When generating a metadata item, the server MUST compute it for
       itself and MUST NOT trust any related value provided in the article.
       (In particular, a Lines or Bytes header in the article MUST NOT be
       assumed to specify the correct number of lines or bytes in the
       article.)  If the server has access to several non-identical copies
       of an article, the value returned MUST be correct for any copy of
       that article retrieved during the same session.

       This specification defines two metadata items: ":bytes" and ":lines".
       Other metadata items may be defined by extensions.  The names of
       metadata items defined by registered extensions MUST NOT begin with
       ":x-".  To avoid the risk of a clash with a future registered
       extension, the names of metadata items defined by private extensions
       SHOULD begin with ":x-".

    8.1.1.  The :bytes Metadata Item

       The :bytes metadata item for an article is a decimal integer.  It
       SHOULD equal the number of octets in the entire article: headers,
       body, and separating empty line (counting a CRLF pair as two octets,
       and excluding both the "." CRLF terminating the response and any "."
       added for "dot-stuffing" purposes).

       Note to client implementers: some existing servers return a value
       different from that above.  The commonest reasons for this are as
       follows:

       o  Counting a CRLF pair as one octet.

       o  Including the "." character used for dot-stuffing in the number.

       o  Including the terminating "." CRLF in the number.

       o  Using one copy of an article for counting the octets but then
          returning another one that differs in some (permitted) manner.

       Implementations should be prepared for such variation and MUST NOT
       rely on the value being accurate.

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    8.1.2.  The :lines Metadata Item

       The :lines metadata item for an article is a decimal integer.  It
       MUST equal the number of lines in the article body (excluding the
       empty line separating headers and body).  Equivalently, it is two
       less than the number of CRLF pairs that the BODY command would return
       for that article (the extra two are those following the response code
       and the termination octet).

    8.2.  Database Consistency

       The information stored in the overview database may change over time.
       If the database records the content or absence of a given field (that
       is, a header or metadata item) for all articles, it is said to be
       "consistent" for that field.  If it records the content of a header
       for some articles but not for others that nevertheless included that
       header, or if it records a metadata item for some articles but not
       for others to which that item applies, it is said to be
       "inconsistent" for that field.

       The LIST OVERVIEW.FMT command SHOULD list all the fields for which
       the database is consistent at that moment.  It MAY omit such fields
       (for example, if it is not known whether the database is consistent
       or inconsistent).  It MUST NOT include fields for which the database
       is inconsistent or that are not stored in the database.  Therefore,
       if a header appears in the LIST OVERVIEW.FMT output but not in the
       OVER output for a given article, that header does not appear in the
       article (similarly for metadata items).

       These rules assume that the fields being stored in the database
       remain constant for long periods of time, and therefore the database
       will be consistent.  When the set of fields to be stored is changed,
       it will be inconsistent until either the database is rebuilt or the
       only articles remaining are those received since the change.
       Therefore, the output from LIST OVERVIEW.FMT needs to be altered
       twice.  Firstly, before any fields stop being stored they MUST be
       removed from the output; then, when the database is once more known
       to be consistent, the new fields SHOULD be added to the output.

       If the HDR command uses the overview database rather than taking
       information directly from the articles, the same issues of
       consistency and inconsistency apply, and the LIST HEADERS command
       SHOULD take the same approach as the LIST OVERVIEW.FMT command in
       resolving them.

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    8.3.  OVER

    8.3.1.  Usage

       Indicating capability: OVER

       Syntax
         OVER message-id
         OVER range
         OVER

       Responses

       First form (message-id specified)
         224    Overview information follows (multi-line)
         430    No article with that message-id

       Second form (range specified)
         224    Overview information follows (multi-line)
         412    No newsgroup selected
         423    No articles in that range

       Third form (current article number used)
         224    Overview information follows (multi-line)
         412    No newsgroup selected
         420    Current article number is invalid

       Parameters
         range         Number(s) of articles
         message-id    Message-id of article

    8.3.2.  Description

       The OVER command returns the contents of all the fields in the
       database for an article specified by message-id, or from a specified
       article or range of articles in the currently selected newsgroup.

       The message-id argument indicates a specific article.  The range
       argument may be any of the following:

       o  An article number.

       o  An article number followed by a dash to indicate all following.

       o  An article number followed by a dash followed by another article
          number.

       If neither is specified, the current article number is used.

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       Support for the first (message-id) form is optional.  If it is
       supported, the OVER capability line MUST include the argument
       "MSGID".  Otherwise, the capability line MUST NOT include this
       argument, and the OVER command MUST return the generic response code
       503 when this form is used.

       If the information is available, it is returned as a multi-line data
       block following the 224 response code and contains one line per
       article, sorted in numerical order of article number.  (Note that
       unless the argument is a range including a dash, there will be
       exactly one line in the data block.)  Each line consists of a number
       of fields separated by a TAB.  A field may be empty (in which case
       there will be two adjacent TABs), and a sequence of trailing TABs may
       be omitted.

       The first 8 fields MUST be the following, in order:

          "0" or article number (see below)
          Subject header content
          From header content
          Date header content
          Message-ID header content
          References header content
          :bytes metadata item
          :lines metadata item

       If the article is specified by message-id (the first form of the
       command), the article number MUST be replaced with zero, except that
       if there is a currently selected newsgroup and the article is present
       in that group, the server MAY use the article's number in that group.
       (See the ARTICLE command (Section 6.2.1) and STAT examples
       (Section 6.2.4.3) for more details.)  In the other two forms of the
       command, the article number MUST be returned.

       Any subsequent fields are the contents of the other headers and
       metadata held in the database.

       For the five mandatory headers, the content of each field MUST be
       based on the content of the header (that is, with the header name and
       following colon and space removed).  If the article does not contain
       that header, or if the content is empty, the field MUST be empty.
       For the two mandatory metadata items, the content of the field MUST
       be just the value, with no other text.

       For all subsequent fields that contain headers, the content MUST be
       the entire header line other than the trailing CRLF.  For all
       subsequent fields that contain metadata, the field consists of the
       metadata name, a single space, and then the value.

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       For all fields, the value is processed by first removing all CRLF
       pairs (that is, undoing any folding and removing the terminating
       CRLF) and then replacing each TAB with a single space.  If there is
       no such header in the article, no such metadata item, or no header or
       item stored in the database for that article, the corresponding field
       MUST be empty.

       Note that, after unfolding, the characters NUL, LF, and CR cannot
       occur in the header of an article offered by a conformant server.
       Nevertheless, servers SHOULD check for these characters and replace
       each one by a single space (so that, for example, CR LF LF TAB will
       become two spaces, since the CR and first LF will be removed by the
       unfolding process).  This will encourage robustness in the face of
       non-conforming data; it is also possible that future versions of this
       specification could permit these characters to appear in articles.

       The server SHOULD NOT produce output for articles that no longer
       exist.

       If the argument is a message-id and no such article exists, a 430
       response MUST be returned.  If the argument is a range or is omitted
       and the currently selected newsgroup is invalid, a 412 response MUST
       be returned.  If the argument is a range and no articles in that
       number range exist in the currently selected newsgroup, including the
       case where the second number is less than the first one, a 423
       response MUST be returned.  If the argument is omitted and the
       current article number is invalid, a 420 response MUST be returned.

    8.3.3.  Examples

       In the first four examples, TAB has been replaced by vertical bar and
       some lines have been folded for readability.

       Example of a successful retrieval of overview information for an
       article (explicitly not using an article number):

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] OVER
          [S] 224 Overview information follows
          [S] 3000234|I am just a test article|"Demo User"
              <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] .

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       Example of a successful retrieval of overview information for an
       article by message-id:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] OVER MSGID
          [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
          [S] .
          [C] OVER <45223423@example.com>
          [S] 224 Overview information follows
          [S] 0|I am just a test article|"Demo User"
              <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] .

       Note that the article number has been replaced by "0".

       Example of the same commands on a system that does not implement
       retrieval by message-id:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] OVER
          [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
          [S] .
          [C] OVER <45223423@example.com>
          [S] 503 Overview by message-id unsupported

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       Example of a successful retrieval of overview information for a range
       of articles:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] OVER 3000234-3000240
          [S] 224 Overview information follows
          [S] 3000234|I am just a test article|"Demo User"
              <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] 3000235|Another test article|nobody@nowhere.to
              (Demo User)|6 Oct 1998 04:38:45 -0500|<45223425@to.to>||
              4818|37||Distribution: fi
          [S] 3000238|Re: I am just a test article|somebody@elsewhere.to|
              7 Oct 1998 11:38:40 +1200|<kfwer3v@elsewhere.to>|
              <45223423@to.to>|9234|51
          [S] .

       Note the missing "References" and Xref headers in the second line,
       the missing trailing fields in the first and last lines, and that
       there are only results for those articles that still exist.

       Example of an unsuccessful retrieval of overview information on an
       article by number:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] OVER 300256
          [S] 423 No such article in this group

       Example of an invalid range:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] OVER 3000444-3000222
          [S] 423 Empty range

       Example of an unsuccessful retrieval of overview information by
       number because no newsgroup was selected first:

          [Assumes currently selected newsgroup is invalid.]
          [C] OVER
          [S] 412 No newsgroup selected

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       Example of an attempt to retrieve information when the currently
       selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] OVER
          [S] 420 No current article selected

    8.4.  LIST OVERVIEW.FMT

    8.4.1.  Usage

       Indicating capability: OVER

       Syntax
         LIST OVERVIEW.FMT

       Responses
         215    Information follows (multi-line)

    8.4.2.  Description

       See Section 7.6.1 for general requirements of the LIST command.

       The LIST OVERVIEW.FMT command returns a description of the fields in
       the database for which it is consistent (as described above).  The
       information is returned as a multi-line data block following the 215
       response code.  The information contains one line per field in the
       order in which they are returned by the OVER command; the first 7
       lines MUST (except for the case of letters) be exactly as follows:

           Subject:
           From:
           Date:
           Message-ID:
           References:
           :bytes
           :lines

       For compatibility with existing implementations, the last two lines
       MAY instead be:

           Bytes:
           Lines:

       even though they refer to metadata, not headers.

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       All subsequent lines MUST consist of either a header name followed by
       ":full", or the name of a piece of metadata.

       There are no leading or trailing spaces in the output.

       Note that the 7 fixed lines describe the 2nd to 8th fields of the
       OVER output.  The "full" suffix (which may use either uppercase,
       lowercase, or a mix) is a reminder that the corresponding fields
       include the header name.

       This command MAY generate different results if it is used more than
       once in a session.

       If the OVER command is not implemented, the meaning of the output
       from this command is not specified, but it must still meet the above
       syntactic requirements.

    8.4.3.  Examples

       Example of LIST OVERVIEW.FMT output corresponding to the example OVER
       output above, in the preferred format:

          [C] LIST OVERVIEW.FMT
          [S] 215 Order of fields in overview database.
          [S] Subject:
          [S] From:
          [S] Date:
          [S] Message-ID:
          [S] References:
          [S] :bytes
          [S] :lines
          [S] Xref:full
          [S] Distribution:full
          [S] .

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       Example of LIST OVERVIEW.FMT output corresponding to the example OVER
       output above, in the alternative format:

          [C] LIST OVERVIEW.FMT
          [S] 215 Order of fields in overview database.
          [S] Subject:
          [S] From:
          [S] Date:
          [S] Message-ID:
          [S] References:
          [S] Bytes:
          [S] Lines:
          [S] Xref:FULL
          [S] Distribution:FULL
          [S] .

    8.5.  HDR

    8.5.1.  Usage

       Indicating capability: HDR

       Syntax
         HDR field message-id
         HDR field range
         HDR field

       Responses

       First form (message-id specified)
         225    Headers follow (multi-line)
         430    No article with that message-id

       Second form (range specified)
         225    Headers follow (multi-line)
         412    No newsgroup selected
         423    No articles in that range

       Third form (current article number used)
         225    Headers follow (multi-line)
         412    No newsgroup selected
         420    Current article number is invalid

       Parameters
         field         Name of field
         range         Number(s) of articles
         message-id    Message-id of article

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    8.5.2.  Description

       The HDR command provides access to specific fields from an article
       specified by message-id, or from a specified article or range of
       articles in the currently selected newsgroup.  It MAY take the
       information directly from the articles or from the overview database.
       In the case of headers, an implementation MAY restrict the use of
       this command to a specific list of headers or MAY allow it to be used
       with any header; it may behave differently when it is used with a
       message-id argument and when it is used with a range or no argument.

       The required field argument is the name of a header with the colon
       omitted (e.g., "subject") or the name of a metadata item including
       the leading colon (e.g., ":bytes"), and is case insensitive.

       The message-id argument indicates a specific article.  The range
       argument may be any of the following:

       o  An article number.

       o  An article number followed by a dash to indicate all following.

       o  An article number followed by a dash followed by another article
          number.

       If neither is specified, the current article number is used.

       If the information is available, it is returned as a multi-line data
       block following the 225 response code and contains one line for each
       article in the range that exists.  (Note that unless the argument is
       a range including a dash, there will be exactly one line in the data
       block.)  The line consists of the article number, a space, and then
       the contents of the field.  In the case of a header, the header name,
       the colon, and the first space after the colon are all omitted.

       If the article is specified by message-id (the first form of the
       command), the article number MUST be replaced with zero, except that
       if there is a currently selected newsgroup and the article is present
       in that group, the server MAY use the article's number in that group.
       (See the ARTICLE command (Section 6.2.1) and STAT examples
       (Section 6.2.4.3) for more details.)  In the other two forms of the
       command, the article number MUST be returned.

       Header contents are modified as follows: all CRLF pairs are removed,
       and then each TAB is replaced with a single space.  (Note that this
       is the same transformation as is performed by the OVER command
       (Section 8.3.2), and the same comment concerning NUL, CR, and LF
       applies.)

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       Note the distinction between headers and metadata appearing to have
       the same meaning.  Headers are always taken unchanged from the
       article; metadata are always calculated.  For example, a request for
       "Lines" returns the contents of the "Lines" header of the specified
       articles, if any, no matter whether they accurately state the number
       of lines, while a request for ":lines" returns the line count
       metadata, which is always the actual number of lines irrespective of
       what any header may state.

       If the requested header is not present in the article, or if it is
       present but empty, a line for that article is included in the output,
       but the header content portion of the line is empty (the space after
       the article number MAY be retained or omitted).  If the header occurs
       in a given article more than once, only the content of the first
       occurrence is returned by HDR.  If any article number in the provided
       range does not exist in the group, no line for that article number is
       included in the output.

       If the second argument is a message-id and no such article exists, a
       430 response MUST be returned.  If the second argument is a range or
       is omitted and the currently selected newsgroup is invalid, a 412
       response MUST be returned.  If the second argument is a range and no
       articles in that number range exist in the currently selected
       newsgroup, including the case where the second number is less than
       the first one, a 423 response MUST be returned.  If the second
       argument is omitted and the current article number is invalid, a 420
       response MUST be returned.

       A server MAY only allow HDR commands for a limited set of fields; it
       may behave differently in this respect for the first (message-id)
       form from how it would for the other forms.  If so, it MUST respond
       with the generic 503 response to attempts to request other fields,
       rather than return erroneous results, such as a successful empty
       response.

       If HDR uses the overview database and it is inconsistent for the
       requested field, the server MAY return what results it can, or it MAY
       respond with the generic 503 response.  In the latter case, the field
       MUST NOT appear in the output from LIST HEADERS.

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    8.5.3.  Examples

       Example of a successful retrieval of subject lines from a range of
       articles (3000235 has no Subject header, and 3000236 is missing):

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR Subject 3000234-3000238
          [S] 225 Headers follow
          [S] 3000234 I am just a test article
          [S] 3000235
          [S] 3000237 Re: I am just a test article
          [S] 3000238 Ditto
          [S] .

       Example of a successful retrieval of line counts from a range of
       articles:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR :lines 3000234-3000238
          [S] 225 Headers follow
          [S] 3000234 42
          [S] 3000235 5
          [S] 3000237 11
          [S] 3000238 2378
          [S] .

       Example of a successful retrieval of the subject line from an article
       by message-id:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR subject <i.am.a.test.article@example.com>
          [S] 225 Header information follows
          [S] 0 I am just a test article
          [S] .

       Example of a successful retrieval of the subject line from the
       current article:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR subject
          [S] 225 Header information follows
          [S] 3000234 I am just a test article
          [S] .

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       Example of an unsuccessful retrieval of a header from an article by
       message-id:

          [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:

          [Assumes currently selected newsgroup is invalid.]
          [C] HDR subject 300256-
          [S] 412 No newsgroup selected

       Example of an unsuccessful retrieval of headers because the currently
       selected newsgroup is empty:

          [C] GROUP example.empty.newsgroup
          [S] 211 0 0 0 example.empty.newsgroup
          [C] HDR subject 1-
          [S] 423 No articles in that range

       Example of an unsuccessful retrieval of headers because the server
       does not allow HDR commands for that header:

          [C] GROUP misc.test
          [S] 211 1234 3000234 3002322 misc.test
          [C] HDR Content-Type 3000234-3000238
          [S] 503 HDR not permitted on Content-Type

    8.6.  LIST HEADERS

    8.6.1.  Usage

       Indicating capability: HDR

       Syntax
         LIST HEADERS [MSGID|RANGE]

       Responses
         215    Field list follows (multi-line)

       Parameters
         MSGID    Requests list for access by message-id
         RANGE    Requests list for access by range

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    8.6.2.  Description

       See Section 7.6.1 for general requirements of the LIST command.

       The LIST HEADERS command returns a list of fields that may be
       retrieved using the HDR command.

       The information is returned as a multi-line data block following the
       215 response code and contains one line for each field name
       (excluding the trailing colon for headers and including the leading
       colon for metadata items).  If the implementation allows any header
       to be retrieved, it MUST NOT include any header names in the list but
       MUST include the special entry ":" (a single colon on its own).  It
       MUST still explicitly list any metadata items that are available.
       The order of items in the list is not significant; the server need
       not even consistently return the same order.  The list MAY be empty
       (though in this circumstance there is little point in providing the
       HDR command).

       An implementation that also supports the OVER command SHOULD at least
       permit all the headers and metadata items listed in the output from
       the LIST OVERVIEW.FMT command.

       If the server treats the first form of the HDR command (message-id
       specified) differently from the other two forms (range specified or
       current article number used) in respect of which headers or metadata
       items are available, then the following apply:

       o  If the MSGID argument is specified, the results MUST be those
          available for the first form of the HDR command.

       o  If the RANGE argument is specified, the results MUST be those
          available for the second and third forms of the HDR command.

       o  If no argument is specified, the results MUST be those available
          in all forms of the HDR command (that is, it MUST only list those
          items listed in both the previous cases).

       If the server does not treat the various forms differently, then it
       MUST ignore any argument and always produce the same results (though
       not necessarily always in the same order).

       If the HDR command is not implemented, the meaning of the output from
       this command is not specified, but it must still meet the above
       syntactic requirements.

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    8.6.3.  Examples

       Example of an implementation providing access to only a few headers:

          [C] LIST HEADERS
          [S] 215 headers supported:
          [S] Subject
          [S] Message-ID
          [S] Xref
          [S] .

       Example of an implementation providing access to the same fields as
       the first example in Section 8.4.3:

          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] OVER
          [S] HDR
          [S] LIST ACTIVE NEWSGROUPS HEADERS OVERVIEW.FMT
          [S] .
          [C] LIST HEADERS
          [S] 215 headers and metadata items supported:
          [S] Date
          [S] Distribution
          [S] From
          [S] Message-ID
          [S] References
          [S] Subject
          [S] Xref
          [S] :bytes
          [S] :lines
          [S] .

       Example of an implementation providing access to all headers:

          [C] LIST HEADERS
          [S] 215 metadata items supported:
          [S] :
          [S] :lines
          [S] :bytes
          [S] :x-article-number
          [S] .

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       Example of an implementation distinguishing the first form of the HDR
       command from the other two forms:

          [C] LIST HEADERS RANGE
          [S] 215 metadata items supported:
          [S] :
          [S] :lines
          [S] :bytes
          [S] .
          [C] LIST HEADERS MSGID
          [S] 215 headers and metadata items supported:
          [S] Date
          [S] Distribution
          [S] From
          [S] Message-ID
          [S] References
          [S] Subject
          [S] :lines
          [S] :bytes
          [S] :x-article-number
          [S] .
          [C] LIST HEADERS
          [S] 215 headers and metadata items supported:
          [S] Date
          [S] Distribution
          [S] From
          [S] Message-ID
          [S] References
          [S] Subject
          [S] :lines
          [S] :bytes
          [S] .

       Note that :x-article-number does not appear in the last set of
       output.

    9.  Augmented BNF Syntax for NNTP

    9.1.  Introduction

       Each of the following sections describes the syntax of a major
       element of NNTP.  This syntax extends and refines the descriptions
       elsewhere in this specification and should be given precedence when
       resolving apparent conflicts.  Note that ABNF [RFC4234] strings are
       case insensitive.  Non-terminals used in several places are defined
       in a separate section at the end.

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       Between them, the non-terminals <command-line>, <command-datastream>,
       <command-continuation>, and <response> specify the text that flows
       between client and server.  A consistent naming scheme is used in
       this document for the non-terminals relating to each command, and
       SHOULD be used by the specification of registered extensions.

       For each command, the sequence is as follows:

       o  The client sends an instance of <command-line>; the syntax for the
          EXAMPLE command is <example-command>.

       o  If the client is one that immediately streams data, it sends an
          instance of <command-datastream>; the syntax for the EXAMPLE
          command is <example-datastream>.

       o  The server sends an instance of <response>.

          *  The initial response line is independent of the command that
             generated it; if the 000 response has arguments, the syntax of
             the initial line is <response-000-content>.

          *  If the response is multi-line, the initial line is followed by
             a <multi-line-data-block>.  The syntax for the contents of this
             block after "dot-stuffing" has been removed is (for the 000
             response to the EXAMPLE command) <example-000-ml-content> and
             is an instance of <multi-line-response-content>.

       o  While the latest response is one that indicates more data is
          required (in general, a 3xx response):

          *  the client sends an instance of <command-continuation>; the
             syntax for the EXAMPLE continuation following a 333 response is
             <example-333-continuation>;

          *  the server sends another instance of <response>, as above.

       (There are no commands in this specification that immediately stream
       data, but this non-terminal is defined for the convenience of
       extensions.)

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    9.2.  Commands

       This syntax defines the non-terminal <command-line>, which represents
       what is sent from the client to the server (see section 3.1 for
       limits on lengths).

         command-line = command EOL
         command = X-command
         X-command = keyword *(WS token)

         command =/ article-command /
               body-command /
               capabilities-command /
               date-command /
               group-command /
               hdr-command /
               head-command /
               help-command /
               ihave-command /
               last-command /
               list-command /
               listgroup-command /
               mode-reader-command /
               newgroups-command /
               newnews-command /
               next-command /
               over-command /
               post-command /
               quit-command /
               stat-command

         article-command = "ARTICLE" [WS article-ref]
         body-command = "BODY" [WS article-ref]
         capabilities-command = "CAPABILITIES" [WS keyword]
         date-command = "DATE"
         group-command = "GROUP" [WS newsgroup-name]
         hdr-command = "HDR" WS header-meta-name [WS range-ref]
         head-command = "HEAD" [WS article-ref]
         help-command = "HELP"
         ihave-command = "IHAVE" WS message-id
         last-command = "LAST"
         list-command = "LIST" [WS list-arguments]
         listgroup-command = "LISTGROUP" [WS newsgroup-name [WS range]]
         mode-reader-command = "MODE" WS "READER"
         newgroups-command = "NEWGROUPS" WS date-time
         newnews-command = "NEWNEWS" WS wildmat WS date-time
         next-command = "NEXT"
         over-command = "OVER" [WS range-ref]

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         post-command = "POST"
         quit-command = "QUIT"
         stat-command = "STAT" [WS article-ref]

         article-ref = article-number / message-id
         date = date2y / date4y
         date4y = 4DIGIT 2DIGIT 2DIGIT
         date2y = 2DIGIT 2DIGIT 2DIGIT
         date-time = date WS time [WS "GMT"]
         header-meta-name = header-name / metadata-name
         list-arguments = keyword [WS token]
         metadata-name = ":" 1*A-NOTCOLON
         range = article-number ["-" [article-number]]
         range-ref = range / message-id
         time = 2DIGIT 2DIGIT 2DIGIT

    9.3.  Command Continuation

       This syntax defines the further material sent by the client in the
       case of multi-stage commands and those that stream data.

         command-datastream = UNDEFINED
           ; not used, provided as a hook for extensions
         command-continuation = ihave-335-continuation /
               post-340-continuation

         ihave-335-continuation = encoded-article
         post-340-continuation = encoded-article

         encoded-article = multi-line-data-block
           ; after undoing the "dot-stuffing", this MUST match <article>

    9.4.  Responses

    9.4.1.  Generic Responses

       This syntax defines the non-terminal <response>, which represents the
       generic form of responses; that is, what is sent from the server to
       the client in response to a <command> or a <command-continuation>.

         response = simple-response / multi-line-response
         simple-response = initial-response-line
         multi-line-response = initial-response-line multi-line-data-block

         initial-response-line =
               initial-response-content [SP trailing-comment] CRLF
         initial-response-content = X-initial-response-content
         X-initial-response-content = 3DIGIT *(SP response-argument)

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         response-argument = 1*A-CHAR
         trailing-comment = *U-CHAR

    9.4.2.  Initial Response Line Contents

       This syntax defines the specific initial response lines for the
       various commands in this specification (see section 3.1 for limits on
       lengths).  Only those response codes with arguments are listed.

         initial-response-content =/ response-111-content /
               response-211-content /
               response-220-content /
               response-221-content /
               response-222-content /
               response-223-content /
               response-401-content

         response-111-content = "111" SP date4y time
         response-211-content = "211" 3(SP article-number) SP newsgroup-name
         response-220-content = "220" SP article-number SP message-id
         response-221-content = "221" SP article-number SP message-id
         response-222-content = "222" SP article-number SP message-id
         response-223-content = "223" SP article-number SP message-id
         response-401-content = "401" SP capability-label

    9.4.3.  Multi-line Response Contents

       This syntax defines the content of the various multi-line responses;
       more precisely, it defines the part of the response in the multi-line
       data block after any "dot-stuffing" has been undone.  The numeric
       portion of each non-terminal name indicates the response code that is
       followed by this data.

         multi-line-response-content = article-220-ml-content /
               body-222-ml-content /
               capabilities-101-ml-content /
               hdr-225-ml-content /
               head-221-ml-content /
               help-100-ml-content /
               list-215-ml-content /
               listgroup-211-ml-content /
               newgroups-231-ml-content /
               newnews-230-ml-content /
               over-224-ml-content

         article-220-ml-content = article
         body-222-ml-content = body
         capabilities-101-ml-content = version-line CRLF

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               *(capability-line CRLF)
         hdr-225-ml-content = *(article-number SP hdr-content CRLF)
         head-221-ml-content = 1*header
         help-100-ml-content = *(*U-CHAR CRLF)
         list-215-ml-content = list-content
         listgroup-211-ml-content = *(article-number CRLF)
         newgroups-231-ml-content = active-groups-list
         newnews-230-ml-content = *(message-id CRLF)
         over-224-ml-content = *(article-number over-content CRLF)

         active-groups-list = *(newsgroup-name SPA article-number
               SPA article-number SPA newsgroup-status CRLF)
         hdr-content = *S-NONTAB
         hdr-n-content = [(header-name ":" / metadata-name) SP hdr-content]
         list-content = body
         newsgroup-status = %x79 / %x6E / %x6D / private-status
         over-content = 1*6(TAB hdr-content) /
               7(TAB hdr-content) *(TAB hdr-n-content)
         private-status = token ; except the values in newsgroup-status

    9.5.  Capability Lines

       This syntax defines the generic form of a capability line in the
       capabilities list (see Section 3.3.1).

         capability-line = capability-entry
         capability-entry = X-capability-entry
         X-capability-entry = capability-label *(WS capability-argument)
         capability-label = keyword
         capability-argument = token

       This syntax defines the specific capability entries for the
       capabilities in this specification.

         capability-entry =/
               hdr-capability /
               ihave-capability /
               implementation-capability /
               list-capability /
               mode-reader-capability /
               newnews-capability /
               over-capability /
               post-capability /
               reader-capability

         hdr-capability = "HDR"
         ihave-capability = "IHAVE"
         implementation-capability = "IMPLEMENTATION" *(WS token)

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         list-capability = "LIST" 1*(WS keyword)
         mode-reader-capability = "MODE-READER"
         newnews-capability = "NEWNEWS"
         over-capability = "OVER" [WS "MSGID"]
         post-capability = "POST"
         reader-capability = "READER"

         version-line = "VERSION" 1*(WS version-number)
         version-number = nzDIGIT *5DIGIT

    9.6.  LIST Variants

       This section defines more specifically the keywords for the LIST
       command and the syntax of the corresponding response contents.

         ; active
         list-arguments =/ "ACTIVE" [WS wildmat]
         list-content =/ list-active-content
         list-active-content = active-groups-list

         ; active.times
         list-arguments =/ "ACTIVE.TIMES" [WS wildmat]
         list-content =/ list-active-times-content
         list-active-times-content =
               *(newsgroup-name SPA 1*DIGIT SPA newsgroup-creator CRLF)
         newsgroup-creator = U-TEXT

         ; distrib.pats
         list-arguments =/ "DISTRIB.PATS"
         list-content =/ list-distrib-pats-content
         list-distrib-pats-content =
               *(1*DIGIT ":" wildmat ":" distribution CRLF)
         distribution = token

         ; headers
         list-arguments =/ "HEADERS" [WS ("MSGID" / "RANGE")]
         list-content =/ list-headers-content
         list-headers-content = *(header-meta-name CRLF) /
               *((metadata-name / ":") CRLF)

         ; newsgroups
         list-arguments =/ "NEWSGROUPS" [WS wildmat]
         list-content =/ list-newsgroups-content
         list-newsgroups-content =

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               *(newsgroup-name WS newsgroup-description CRLF)
         newsgroup-description = S-TEXT

         ; overview.fmt
         list-arguments =/ "OVERVIEW.FMT"
         list-content =/ list-overview-fmt-content
         list-overview-fmt-content = "Subject:" CRLF
               "From:" CRLF
               "Date:" CRLF
               "Message-ID:" CRLF
               "References:" CRLF
               ( ":bytes" CRLF ":lines" / "Bytes:" CRLF "Lines:") CRLF
               *((header-name ":full" / metadata-name) CRLF)

    9.7.  Articles

       This syntax defines the non-terminal <article>, which represents the
       format of an article as described in Section 3.6.

         article = 1*header CRLF body
         header = header-name ":" [CRLF] SP header-content CRLF
         header-content = *(S-CHAR / [CRLF] WS)
         body = *(*B-CHAR CRLF)

    9.8.  General Non-terminals

       These non-terminals are used at various places in the syntax and are
       collected here for convenience.  A few of these non-terminals are not
       used in this specification but are provided for the consistency and
       convenience of extension authors.

         multi-line-data-block = content-lines termination
         content-lines = *([content-text] CRLF)
         content-text = (".." / B-NONDOT) *B-CHAR
         termination = "." CRLF

         article-number = 1*16DIGIT
         header-name = 1*A-NOTCOLON
         keyword = ALPHA 2*(ALPHA / DIGIT / "." / "-")
         message-id = "<" 1*248A-NOTGT ">"
         newsgroup-name = 1*wildmat-exact
         token = 1*P-CHAR

         wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
         wildmat-pattern = 1*wildmat-item
         wildmat-item = wildmat-exact / wildmat-wild
         wildmat-exact = %x22-29 / %x2B / %x2D-3E / %x40-5A / %x5E-7E /

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              UTF8-non-ascii  ; exclude ! * , ? [ \ ]
         wildmat-wild = "*" / "?"

         base64 = *(4base64-char) [base64-terminal]
         base64-char = UPPER / LOWER / DIGIT / "+" / "/"
         base64-terminal = 2base64-char "==" / 3base64-char "="

         ; Assorted special character sets
         ;   A- means based on US-ASCII, excluding controls and SP
         ;   P- means based on UTF-8, excluding controls and SP
         ;   U- means based on UTF-8, excluding NUL CR and LF
         ;   B- means based on bytes, excluding NUL CR and LF
         A-CHAR     = %x21-7E
         A-NOTCOLON = %x21-39 / %x3B-7E  ; exclude ":"
         A-NOTGT    = %x21-3D / %x3F-7E  ; exclude ">"
         P-CHAR     = A-CHAR / UTF8-non-ascii
         U-CHAR     = CTRL / TAB / SP / A-CHAR / UTF8-non-ascii
         U-NONTAB   = CTRL /       SP / A-CHAR / UTF8-non-ascii
         U-TEXT     = P-CHAR *U-CHAR
         B-CHAR     = CTRL / TAB / SP / %x21-FF
         B-NONDOT   = CTRL / TAB / SP / %x21-2D / %x2F-FF  ; exclude "."

         ALPHA = UPPER / LOWER   ; use only when case-insensitive
         CR = %x0D
         CRLF = CR LF
         CTRL = %x01-08 / %x0B-0C / %x0E-1F
         DIGIT = %x30-39
         nzDIGIT = %x31-39
         EOL = *(SP / TAB) CRLF
         LF = %x0A
         LOWER = %x61-7A
         SP = %x20
         SPA = 1*SP
         TAB = %x09
         UPPER = %x41-5A
         UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4
         UTF8-2    = %xC2-DF UTF8-tail
         UTF8-3    = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2UTF8-tail /
                     %xED %x80-9F UTF8-tail / %xEE-EF 2UTF8-tail
         UTF8-4    = %xF0 %x90-BF 2UTF8-tail / %xF1-F3 3UTF8-tail /
                     %xF4 %x80-8F 2UTF8-tail
         UTF8-tail = %x80-BF
         WS = 1*(SP / TAB)

       The following non-terminals require special consideration.  They
       represent situations where material SHOULD be restricted to UTF-8,
       but implementations MUST be able to cope with other character
       encodings.  Therefore, there are two sets of definitions for them.

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       Implementations MUST accept any content that meets this syntax:

         S-CHAR   = %x21-FF
         S-NONTAB = CTRL / SP / S-CHAR
         S-TEXT   = (CTRL / S-CHAR) *B-CHAR

       and MAY pass such content on unaltered.

       When generating new content or re-encoding existing content,
       implementations SHOULD conform to this syntax:

         S-CHAR   = P-CHAR
         S-NONTAB = U-NONTAB
         S-TEXT   = U-TEXT

    9.9.  Extensions and Validation

       The specification of a registered extension MUST include formal
       syntax that defines additional forms for the following non-terminals:

       command
          for each new command other than a variant of the LIST command -
          the syntax of each command MUST be compatible with the definition
          of <X-command>;

       command-datastream
          for each new command that immediately streams data;

       command-continuation
          for each new command that sends further material after the initial
          command line - the syntax of each continuation MUST be exactly
          what is sent to the server, including any escape mechanisms such
          as "dot-stuffing";

       initial-response-content
          for each new response code that has arguments - the syntax of each
          response MUST be compatible with the definition of <X-initial-
          response-content>;

       multi-line-response-content
          for each new response code that has a multi-line response - the
          syntax MUST show the response after the lines containing the
          response code and the terminating octet have been removed and any
          "dot-stuffing" undone;

       capability-entry
          for each new capability label - the syntax of each entry MUST be
          compatible with the definition of <X-capability-entry>;

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       list-arguments
          for each new variant of the LIST command - the syntax of each
          entry MUST be compatible with the definition of <X-command>;

       list-content
          for each new variant of the LIST command - the syntax MUST show
          the response after the lines containing the 215 response code and
          the terminating octet have been removed and any "dot-stuffing"
          undone.

       The =/ notation of ABNF [RFC4234] and the naming conventions
       described in Section 9.1 SHOULD be used for this.

       When the syntax in this specification, or syntax based on it, is
       validated, it should be noted that:

       o  the non-terminals <command-line>, <command-datastream>,
          <command-continuation>, <response>, and
          <multi-line-response-content> describe basic concepts of the
          protocol and are not referred to by any other rule;

       o  the non-terminal <base64> is provided for the convenience of
          extension authors and is not referred to by any rule in this
          specification;

       o  for the reasons given above, the non-terminals <S-CHAR>,
          <S-NONTAB>, and <S-TEXT> each have two definitions; and

       o  the non-terminal <UNDEFINED> is deliberately not defined.

    10.  Internationalisation Considerations

    10.1.  Introduction and Historical Situation

       RFC 977 [RFC977] was written at a time when internationalisation was
       not seen as a significant issue.  As such, it was written on the
       assumption that all communication would be in ASCII and use only a
       7-bit transport layer, although in practice just about all known
       implementations are 8-bit clean.

       Since then, Usenet and NNTP have spread throughout the world.  In the
       absence of standards for handling the issues of language and
       character sets, countries, newsgroup hierarchies, and individuals
       have found a variety of solutions that work for them but that are not
       necessarily appropriate elsewhere.  For example, some have adopted a
       default 8-bit character set appropriate to their needs (such as
       ISO/IEC 8859-1 in Western Europe or KOI-8 in Russia), others have
       used ASCII (either US-ASCII or national variants) in headers but

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       local 16-bit character sets in article bodies, and still others have
       gone for a combination of MIME [RFC2045] and UTF-8.  With the
       increased use of MIME in email, it is becoming more common to find
       NNTP articles containing MIME headers that identify the character set
       of the body, but this is far from universal.

       The resulting confusion does not help interoperability.

       One point that has been generally accepted is that articles can
       contain octets with the top bit set, and NNTP is only expected to
       operate on 8-bit clean transport paths.

    10.2.  This Specification

       Part of the role of this present specification is to eliminate this
       confusion and promote interoperability as far as possible.  At the
       same time, it is necessary to accept the existence of the present
       situation and not break existing implementations and arrangements
       gratuitously, even if they are less than optimal.  Therefore, the
       current practice described above has been taken into consideration in
       producing this specification.

       This specification extends NNTP from US-ASCII [ANSI1986] to UTF-8
       [RFC3629].  Except in the two areas discussed below, UTF-8 (which is
       a superset of US-ASCII) is mandatory, and implementations MUST NOT
       use any other encoding.

       Firstly, the use of MIME for article headers and bodies is strongly
       recommended.  However, given widely divergent existing practices, an
       attempt to require a particular encoding and tagging standard would
       be premature at this time.  Accordingly, this specification allows
       the use of arbitrary 8-bit data in articles subject to the following
       requirements and recommendations.

       o  The names of headers (e.g., "From" or "Subject") MUST be in
          US-ASCII.

       o  Header values SHOULD use US-ASCII or an encoding based on it, such
          as RFC 2047 [RFC2047], until such time as another approach has
          been standardised.  At present, 8-bit encodings (including UTF-8)
          SHOULD NOT be used because they are likely to cause
          interoperability problems.

       o  The character set of article bodies SHOULD be indicated in the
          article headers, and this SHOULD be done in accordance with MIME.

       o  Where an article is obtained from an external source, an
          implementation MAY pass it on and derive data from it (such as the

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          response to the HDR command), even though the article or the data
          does not meet the above requirements.  Implementations MUST
          transfer such articles and data correctly and unchanged; they MUST
          NOT attempt to convert or re-encode the article or derived data.
          (Nevertheless, a client or server MAY elect not to post or forward
          the article if, after further examination of the article, it deems
          it inappropriate to do so.)

       This requirement affects the ARTICLE (Section 6.2.1), BODY
       (Section 6.2.3), HDR (Section 8.5), HEAD (Section 6.2.2), IHAVE
       (Section 6.3.2), OVER (Section 8.3), and POST (Section 6.3.1)
       commands.

       Secondly, the following requirements are placed on the newsgroups
       list returned by the LIST NEWSGROUPS command (Section 7.6.6):

       o  Although this specification allows UTF-8 for newsgroup names, they
          SHOULD be restricted to US-ASCII until a successor to RFC 1036
          [RFC1036] standardises another approach. 8-bit encodings SHOULD
          NOT be used because they are likely to cause interoperability
          problems.

       o  The newsgroup description SHOULD be in US-ASCII or UTF-8 unless
          and until a successor to RFC 1036 standardises other encoding
          arrangements.  8-bit encodings other than UTF-8 SHOULD NOT be used
          because they are likely to cause interoperability problems.

       o  Implementations that obtain this data from an external source MUST
          handle it correctly even if it does not meet the above
          requirements.  Implementations (in particular, clients) MUST
          handle such data correctly.

    10.3.  Outstanding Issues

       While the primary use of NNTP is for transmitting articles that
       conform to RFC 1036 (Netnews articles), it is also used for other
       formats (see Appendix A).  It is therefore most appropriate that
       internationalisation issues related to article formats be addressed
       in the relevant specifications.  For Netnews articles, this is any
       successor to RFC 1036.  For email messages, it is RFC 2822 [RFC2822].

       Of course, any article transmitted via NNTP needs to conform to this
       specification as well.

       Restricting newsgroup names to UTF-8 is not a complete solution.  In
       particular, when new newsgroup names are created or a user is asked
       to enter a newsgroup name, some scheme of canonicalisation will need
       to take place.  This specification does not attempt to define that

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       canonicalization; further work is needed in this area, in conjunction
       with the article format specifications.  Until such specifications
       are published, implementations SHOULD match newsgroup names octet by
       octet.  It is anticipated that any approved scheme will be applied
       "at the edges", and therefore octet-by-octet comparison will continue
       to apply to most, if not all, uses of newsgroup names in NNTP.

       In the meantime, any implementation experimenting with UTF-8
       newsgroup names is strongly cautioned that a future specification may
       require that those names be canonicalized when used with NNTP in a
       way that is not compatible with their experiments.

       Since the primary use of NNTP is with Netnews, and since newsgroup
       descriptions are normally distributed through specially formatted
       articles, it is recommended that the internationalisation issues
       related to them be addressed in any successor to RFC 1036.

    11.  IANA Considerations

       This specification requires IANA to keep a registry of capability
       labels.  The initial contents of this registry are specified in
       Section 3.3.4.  As described in Section 3.3.3, labels beginning with
       X are reserved for private use, while all other names are expected to
       be associated with a specification in an RFC on the standards track
       or defining an IESG-approved experimental protocol.

       Different entries in the registry MUST use different capability
       labels.

       Different entries in the registry MUST NOT use the same command name.
       For this purpose, variants distinguished by a second or subsequent
       keyword (e.g., "LIST HEADERS" and "LIST OVERVIEW.FMT") count as
       different commands.  If there is a need for two extensions to use the
       same command, a single harmonised specification MUST be registered.

    12.  Security Considerations

       This section is meant to inform application developers, information
       providers, and users of the security limitations in NNTP as described
       by this document.  The discussion does not include definitive
       solutions to the problems revealed, though it does make some
       suggestions for reducing security risks.

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    12.1.  Personal and Proprietary Information

       NNTP, because it was created to distribute network news articles,
       will forward whatever information is stored in those articles.
       Specification of that information is outside this scope of this
       document, 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 that personal and/or proprietary information in
       material that is added automatically to articles (e.g., in headers)
       is not disclosed inadvertently.  Additionally, effective and easily
       understood mechanisms to manage the distribution of news articles
       SHOULD be provided to NNTP Server administrators, so that they are
       able to report with confidence the likely spread of any particular
       set of news articles.

    12.2.  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 this 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.

    12.3.  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 specification does permit the
       creation of extensions to NNTP for such purposes; one such extension
       is [NNTP-AUTH].  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.

    12.4.  DNS Spoofing

       Many existing NNTP implementations authorize incoming connections by
       checking the IP address of that connection 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

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       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 cache 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 increase.  Observing this requirement thus reduces this
       potential security vulnerability.

       This requirement also improves the load-balancing behaviour 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.

    12.5.  UTF-8 Issues

       UTF-8 [RFC3629] permits only certain sequences of octets and
       designates others as either malformed or "illegal".  The Unicode
       standard identifies a number of security issues related to illegal
       sequences and forbids their generation by conforming implementations.

       Implementations of this specification MUST NOT generate malformed or
       illegal sequences and SHOULD detect them and take some appropriate
       action.  This could include the following:

       o  Generating a 501 response code.

       o  Replacing such sequences by the sequence %xEF.BF.BD, which encodes
          the "replacement character" U+FFFD.

       o  Closing the connection.

       o  Replacing such sequences by a "guessed" valid sequence (based on
          properties of the UTF-8 encoding).

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       In the last case, the implementation MUST ensure that any replacement
       cannot be used to bypass validity or security checks.  For example,
       the illegal sequence %xC0.A0 is an over-long encoding for space
       (%x20).  If it is replaced by the correct encoding in a command line,
       this needs to happen before the command line is parsed into
       individual arguments.  If the replacement came after parsing, it
       would be possible to generate an argument with an embedded space,
       which is forbidden.  Use of the "replacement character" does not have
       this problem, since it is permitted wherever non-US-ASCII characters
       are.  Implementations SHOULD use one of the first two solutions where
       the general structure of the NNTP stream remains intact and SHOULD
       close the connection if it is no longer possible to parse it
       sensibly.

    12.6.  Caching of Capability Lists

       The CAPABILITIES command provides a capability list, which is
       information about the current capabilities of the server.  Whenever
       there is a relevant change to the server state, the results of this
       command are required to change accordingly.

       In most situations, the capabilities list in a given server state
       will not change from session to session; for example, a given
       extension will be installed permanently on a server.  Some clients
       may therefore wish to remember which extensions a server supports to
       avoid the delay of an additional command and response, particularly
       if they open multiple connections in the same session.

       However, information about extensions related to security and privacy
       MUST NOT be cached, since this could allow a variety of attacks.

       For example, consider a server that permits the use of cleartext
       passwords on links that are encrypted but not otherwise:

          [Initial connection set-up completed.]
          [S] 200 NNTP Service Ready, posting permitted
          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] NEWNEWS
          [S] POST
          [S] XENCRYPT
          [S] LIST ACTIVE NEWSGROUPS
          [S] .
          [C] XENCRYPT
          [Client and server negotiate encryption on the link]
          [S] 283 Encrypted link established

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          [C] CAPABILITIES
          [S] 101 Capability list:
          [S] VERSION 2
          [S] READER
          [S] NEWNEWS
          [S] POST
          [S] XSECRET
          [S] LIST ACTIVE NEWSGROUPS
          [S] .
          [C] XSECRET fred flintstone
          [S] 290 Password for fred accepted

       If the client caches the last capabilities list, then on the next
       session it will attempt to use XSECRET on an unencrypted link:

          [Initial connection set-up completed.]
          [S] 200 NNTP Service Ready, posting permitted
          [C] XSECRET fred flintstone
          [S] 483 Only permitted on secure links

       This exposes the password to any eavesdropper.  While the primary
       cause of this is passing a secret without first checking the security
       of the link, caching of capability lists can increase the risk.

       Any security extension should include requirements to check the
       security state of the link in a manner appropriate to that extension.

       Caching should normally only be considered for anonymous clients that
       do not use any security or privacy extensions and for which the time
       required for an additional command and response is a noticeable
       issue.

    13.  Acknowledgements

       This document is the result of much effort by the present and past
       members of the NNTP Working Group, chaired by Russ Allbery and Ned
       Freed.  It could not have been produced without them.

       The author acknowledges the original authors of NNTP as documented in
       RFC 977 [RFC977]: Brian Kantor and Phil Lapsey.

       The author gratefully acknowledges the following:

       o  The work of the NNTP committee chaired by Eliot Lear.  The
          organization of this document was influenced by the last available
          version from this working group.  A special thanks to Eliot for
          generously providing the original machine-readable sources for
          that document.

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       o  The work of the DRUMS working group, specifically RFC 1869
          [RFC1869], that drove the original thinking that led to the
          CAPABILITIES command and the extensions mechanism detailed in this
          document.

       o  The authors of RFC 2616 [RFC2616] 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.

       o  The comments and additional information provided by the following
          individuals in preparing one or more of the progenitors of this
          document:

             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, including 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.

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       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.

       Other people who contributed to this document include:

          Matthias Andree
          Greg Andruk
          Daniel Barclay
          Maurizio Codogno
          Mark Crispin
          Andrew Gierth
          Juergen Helbing
          Scott Hollenbeck
          Urs Janssen
          Charles Lindsey
          Ade Lovett
          David Magda
          Ken Murchison
          Francois Petillon
          Peter Robinson
          Rob Siemborski
          Howard Swinehart
          Ruud van Tol
          Jeffrey Vinocur
          Erik Warmelink

       The author thanks them all and apologises to anyone omitted.

       Finally, the present author gratefully acknowledges the vast amount
       of work put into previous versions by the previous author:

          Stan Barber <sob@academ.com>

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    14.  References

    14.1.  Normative References

       [ANSI1986]    American National Standards Institute, "Coded Character
                     Set - 7-bit American Standard Code for Information
                     Interchange", ANSI X3.4, 1986.

       [RFC977]      Kantor, B. and P. Lapsley, "Network News Transfer
                     Protocol", RFC 977, February 1986.

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

       [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
                     Extensions) Part Three: Message Header Extensions for
                     Non-ASCII Text", RFC 2047, November 1996.

       [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", STD 63, RFC 3629, November 2003.

       [RFC4234]     Crocker, D., Ed. and P. Overell, "Augmented BNF for
                     Syntax Specifications: ABNF", RFC 4234, October 2005.

       [RFC4648]     Josefsson, S., "The Base16, Base32, and Base64 Data
                     Encodings", RFC 4648, October 2006.

       [TF.686-1]    International Telecommunications Union - Radio,
                     "Glossary, ITU-R Recommendation TF.686-1",
                     ITU-R Recommendation TF.686-1, October 1997.

    14.2.  Informative References

       [NNTP-AUTH]   Vinocur, J., Murchison, K., and C. Newman, "Network
                     News Transfer Protocol (NNTP) Extension for
                     Authentication",
                     RFC 4643, October 2006.

       [NNTP-STREAM] Vinocur, J. and K. Murchison, "Network News Transfer
                     Protocol (NNTP) Extension for Streaming Feeds",
                     RFC 4644, October 2006.

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       [NNTP-TLS]    Murchison, K., Vinocur, J., and C. Newman, "Using
                     Transport Layer Security (TLS) with Network News
                     Transfer Protocol (NNTP)", RFC 4642, October 2006.

       [RFC1036]     Horton, M. and R. Adams, "Standard for interchange of
                     USENET messages", RFC 1036, December 1987.

       [RFC1305]     Mills, D., "Network Time Protocol (Version 3)
                     Specification, Implementation and Analysis", RFC 1305,
                     March 1992.

       [RFC1869]     Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
                     Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
                     November 1995.

       [RFC2616]     Fielding,  R., Gettys, J., Mogul, J., Frystyk, H.,
                     Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                     Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

       [RFC2629]     Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
                     June 1999.

       [RFC2822]     Resnick, P., "Internet Message Format", RFC 2822, April
                     2001.

       [RFC2980]     Barber, S., "Common NNTP Extensions", RFC 2980, October
                     2000.

       [ROBE1995]    Robertson, R., "FAQ: Overview database / NOV General
                     Information", January 1995.

                     There is no definitive copy of this document known to
                     the author.  It was previously posted as the Usenet
                     article <news:nov-faq-1-930909720@agate.Berkeley.EDU>

       [SALZ1992]    Salz, R., "Manual Page for wildmat(3) from the INN 1.4
                     distribution, Revision 1.10", April 1992.

                     There is no definitive copy of this document known to
                     the author.

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    Appendix A.  Interaction with Other Specifications

       NNTP is most often used for transferring articles that conform to
       RFC 1036 [RFC1036] (such articles are called "Netnews articles"
       here).  It is also sometimes used for transferring email messages
       that conform to RFC 2822 [RFC2822] (such articles are called "email
       articles" here).  In this situation, articles must conform both to
       this specification and to that other one; this appendix describes
       some relevant issues.

    A.1.  Header Folding

       NNTP allows a header line to be folded (by inserting a CRLF pair)
       before any space or TAB character.

       Both email and Netnews articles are required to have at least one
       octet other than space or TAB on each header line.  Thus, folding can
       only happen at one point in each sequence of consecutive spaces or
       TABs.  Netnews articles are further required to have the header name,
       colon, and following space all on the first line; folding may only
       happen beyond that space.  Finally, some non-conforming software will
       remove trailing spaces and TABs from a line.  Therefore, it might be
       inadvisable to fold a header after a space or TAB.

       For maximum safety, header lines SHOULD conform to the following
       syntax rather than to that in Section 9.7.

         header = header-name ":" SP [header-content] CRLF
         header-content = [WS] token *( [CRLF] WS token )

    A.2.  Message-IDs

       Every article handled by an NNTP server MUST have a unique
       message-id.  For the purposes of this specification, a message-id is
       an arbitrary opaque string that merely needs to meet certain
       syntactic requirements and is just a way to refer to the article.

       Because there is a significant risk that old articles will be
       reinjected into the global Usenet system, RFC 1036 [RFC1036] requires
       that message-ids are globally unique for all time.

       This specification states that message-ids are the same if and only
       if they consist of the same sequence of octets.  Other specifications
       may define two different sequences as being equal because they are
       putting an interpretation on particular characters.  RFC 2822
       [RFC2822] has a concept of "quoted" and "escaped" characters.  It
       therefore considers the three message-ids:

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          <ab.cd@example.com>
          <"ab.cd"@example.com>
          <"ab.\cd"@example.com>

       as being identical.  Therefore, an NNTP implementation handing email
       articles must ensure that only one of these three appears in the
       protocol and that the other two are converted to it as and when
       necessary, such as when a client checks the results of a NEWNEWS
       command against an internal database of message-ids.  Note that
       RFC 1036 [RFC1036] never treats two different strings as being
       identical.  Its successor (as of the time of writing) restricts the
       syntax of message-ids so that, whenever RFC 2822 would treat two
       strings as equivalent, only one of them is valid (in the above
       example, only the first string is valid).

       This specification does not describe how the message-id of an article
       is determined; it may be deduced from the contents of the article or
       derived from some external source.  If the server is also conforming
       to another specification that contains a definition of message-id
       compatible with this one, the server SHOULD use those message-ids.  A
       common approach, and one that SHOULD be used for email and Netnews
       articles, is to extract the message-id from the contents of a header
       with name "Message-ID".  This may not be as simple as copying the
       entire header contents; it may be necessary to strip off comments and
       undo quoting, or to reduce "equivalent" message-ids to a canonical
       form.

       If an article is obtained through the IHAVE command, there will be a
       message-id provided with the command.  The server MAY either use it
       or determine one from the article contents.  However, whichever it
       does, it SHOULD ensure that, if the IHAVE command is repeated with
       the same argument and article, it will be recognized as a duplicate.

       If an article does not contain a message-id that the server can
       identify, it MUST synthesize one.  This could, for example, be a
       simple sequence number or be based on the date and time when the
       article arrived.  When email or Netnews articles are handled, a
       Message-ID header SHOULD be added to ensure global consistency and
       uniqueness.

       Note that, because the message-id might not have been derived from
       the Message-ID header in the article, the following example is
       legitimate (though unusual):

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          [C] HEAD <45223423@example.com>
          [S] 221 0 <45223423@example.com>
          [S] Path: pathost!demo!whitehouse!not-for-mail
          [S] Message-ID: <1234@example.net>
          [S] From: "Demo User" <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] .

    A.3.  Article Posting

       As far as NNTP is concerned, the POST and IHAVE commands provide the
       same basic facilities in a slightly different way.  However, they
       have rather different intentions.

       The IHAVE command is intended for transmitting conforming articles
       between a system of NNTP servers, with all articles perhaps also
       conforming to another specification (e.g., all articles are Netnews
       articles).  It is expected that the client will already have done any
       necessary validation (or that it has in turn obtained the article
       from a third party that has done so); therefore, the contents SHOULD
       be left unchanged.

       In contrast, the POST command is intended for use when an end-user is
       injecting a newly created article into a such a system.  The article
       being transferred might not be a conforming email or Netnews article,
       and the server is expected to validate it and, if necessary, to
       convert it to the right form for onward distribution.  This is often
       done by a separate piece of software on the server installation; if
       so, the NNTP server SHOULD pass the incoming article to that software
       unaltered, making no attempt to filter characters, to fold or limit
       lines, or to process the incoming text otherwise.

       The POST command can fail in various ways, and clients should be
       prepared to re-send an article.  When doing so, however, it is often
       important to ensure (as far as possible) that the same message-id is
       allocated to both attempts so that the server, or other servers, can
       recognize the two articles as duplicates.  In the case of email or
       Netnews articles, therefore, the posted article SHOULD contain a
       header with the name "Message-ID", and the contents of this header
       SHOULD be identical on each attempt.  The server SHOULD ensure that
       two POSTed articles with the same contents for this header are
       recognized as identical and that the same message-id is allocated,
       whether or not those contents are suitable for use as the message-id.

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    Appendix B.  Summary of Commands

       This section contains a list of every command defined in this
       document, ordered by command name and by indicating capability.

                             Ordered by command name:

           +-------------------+-----------------------+---------------+
           | Command           | Indicating capability | Definition    |
           +-------------------+-----------------------+---------------+
           | ARTICLE           | READER                | Section 6.2.1 |
           | BODY              | READER                | Section 6.2.3 |
           | CAPABILITIES      | mandatory             | Section 5.2   |
           | DATE              | READER                | Section 7.1   |
           | GROUP             | READER                | Section 6.1.1 |
           | HDR               | HDR                   | Section 8.5   |
           | HEAD              | mandatory             | Section 6.2.2 |
           | HELP              | mandatory             | Section 7.2   |
           | IHAVE             | IHAVE                 | Section 6.3.2 |
           | LAST              | READER                | Section 6.1.3 |
           | LIST              | LIST                  | Section 7.6.1 |
           | LIST ACTIVE.TIMES | LIST                  | Section 7.6.4 |
           | LIST ACTIVE       | LIST                  | Section 7.6.3 |
           | LIST DISTRIB.PATS | LIST                  | Section 7.6.5 |
           | LIST HEADERS      | HDR                   | Section 8.6   |
           | LIST NEWSGROUPS   | LIST                  | Section 7.6.6 |
           | LIST OVERVIEW.FMT | OVER                  | Section 8.4   |
           | LISTGROUP         | READER                | Section 6.1.2 |
           | MODE READER       | MODE-READER           | Section 5.3   |
           | NEWGROUPS         | READER                | Section 7.3   |
           | NEWNEWS           | NEWNEWS               | Section 7.4   |
           | NEXT              | READER                | Section 6.1.4 |
           | OVER              | OVER                  | Section 8.3   |
           | POST              | POST                  | Section 6.3.1 |
           | QUIT              | mandatory             | Section 5.4   |
           | STAT              | mandatory             | Section 6.2.4 |
           +-------------------+-----------------------+---------------+

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                         Ordered by indicating capability:

           +-------------------+-----------------------+---------------+
           | Command           | Indicating capability | Definition    |
           +-------------------+-----------------------+---------------+
           | CAPABILITIES      | mandatory             | Section 5.2   |
           | HEAD              | mandatory             | Section 6.2.2 |
           | HELP              | mandatory             | Section 7.2   |
           | QUIT              | mandatory             | Section 5.4   |
           | STAT              | mandatory             | Section 6.2.4 |
           | HDR               | HDR                   | Section 8.5   |
           | LIST HEADERS      | HDR                   | Section 8.6   |
           | IHAVE             | IHAVE                 | Section 6.3.2 |
           | LIST              | LIST                  | Section 7.6.1 |
           | LIST ACTIVE       | LIST                  | Section 7.6.3 |
           | LIST ACTIVE.TIMES | LIST                  | Section 7.6.4 |
           | LIST DISTRIB.PATS | LIST                  | Section 7.6.5 |
           | LIST NEWSGROUPS   | LIST                  | Section 7.6.6 |
           | MODE READER       | MODE-READER           | Section 5.3   |
           | NEWNEWS           | NEWNEWS               | Section 7.4   |
           | OVER              | OVER                  | Section 8.3   |
           | LIST OVERVIEW.FMT | OVER                  | Section 8.4   |
           | POST              | POST                  | Section 6.3.1 |
           | ARTICLE           | READER                | Section 6.2.1 |
           | BODY              | READER                | Section 6.2.3 |
           | DATE              | READER                | Section 7.1   |
           | GROUP             | READER                | Section 6.1.1 |
           | LAST              | READER                | Section 6.1.3 |
           | LISTGROUP         | READER                | Section 6.1.2 |
           | NEWGROUPS         | READER                | Section 7.3   |
           | NEXT              | READER                | Section 6.1.4 |
           +-------------------+-----------------------+---------------+

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    Appendix C.  Summary of Response Codes

       This section contains a list of every response code defined in this
       document and indicates whether it is multi-line, which commands can
       generate it, what arguments it has, and what its meaning is.

       Response code 100 (multi-line)
          Generated by: HELP
          Meaning: help text follows.

       Response code 101 (multi-line)
          Generated by: CAPABILITIES
          Meaning: capabilities list follows.

       Response code 111
          Generated by: DATE
          1 argument: yyyymmddhhmmss
          Meaning: server date and time.

       Response code 200
          Generated by: initial connection, MODE READER
          Meaning: service available, posting allowed.

       Response code 201
          Generated by: initial connection, MODE READER
          Meaning: service available, posting prohibited.

       Response code 205
          Generated by: QUIT
          Meaning: connection closing (the server immediately closes the
          connection).

       Response code 211
          The 211 response code has two completely different forms,
          depending on which command generated it:

             (not multi-line)
             Generated by: GROUP
             4 arguments: number low high group
             Meaning: group selected.

             (multi-line)
             Generated by: LISTGROUP
             4 arguments: number low high group
             Meaning: article numbers follow.

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       Response code 215 (multi-line)
          Generated by: LIST
          Meaning: information follows.

       Response code 220 (multi-line)
          Generated by: ARTICLE
          2 arguments: n message-id
          Meaning: article follows.

       Response code 221 (multi-line)
          Generated by: HEAD
          2 arguments: n message-id
          Meaning: article headers follow.

       Response code 222 (multi-line)
          Generated by: BODY
          2 arguments: n message-id
          Meaning: article body follows.

       Response code 223
          Generated by: LAST, NEXT, STAT
          2 arguments: n message-id
          Meaning: article exists and selected.

       Response code 224 (multi-line)
          Generated by: OVER
          Meaning: overview information follows.

       Response code 225 (multi-line)
          Generated by: HDR
          Meaning: headers follow.

       Response code 230 (multi-line)
          Generated by: NEWNEWS
          Meaning: list of new articles follows.

       Response code 231 (multi-line)
          Generated by: NEWGROUPS
          Meaning: list of new newsgroups follows.

       Response code 235
          Generated by: IHAVE (second stage)
          Meaning: article transferred OK.

       Response code 240
          Generated by: POST (second stage)
          Meaning: article received OK.

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       Response code 335
          Generated by: IHAVE (first stage)
          Meaning: send article to be transferred.

       Response code 340
          Generated by: POST (first stage)
          Meaning: send article to be posted.

       Response code 400
          Generic response and generated by initial connection
          Meaning: service not available or no longer available (the server
          immediately closes the connection).

       Response code 401
          Generic response
          1 argument: capability-label
          Meaning: the server is in the wrong mode; the indicated capability
          should be used to change the mode.

       Response code 403
          Generic response
          Meaning: internal fault or problem preventing action being taken.

       Response code 411
          Generated by: GROUP, LISTGROUP
          Meaning: no such newsgroup.

       Response code 412
          Generated by: ARTICLE, BODY, GROUP, HDR, HEAD, LAST, LISTGROUP,
          NEXT, OVER, STAT
          Meaning: no newsgroup selected.

       Response code 420
          Generated by: ARTICLE, BODY, HDR, HEAD, LAST, NEXT, OVER, STAT
          Meaning: current article number is invalid.

       Response code 421
          Generated by: NEXT
          Meaning: no next article in this group.

       Response code 422
          Generated by: LAST
          Meaning: no previous article in this group.

       Response code 423
          Generated by: ARTICLE, BODY, HDR, HEAD, OVER, STAT
          Meaning: no article with that number or in that range.

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       Response code 430
          Generated by: ARTICLE, BODY, HDR, HEAD, OVER, STAT
          Meaning: no article with that message-id.

       Response code 435
          Generated by: IHAVE (first stage)
          Meaning: article not wanted.

       Response code 436
          Generated by: IHAVE (either stage)
          Meaning: transfer not possible (first stage) or failed (second
          stage); try again later.

       Response code 437
          Generated by: IHAVE (second stage)
          Meaning: transfer rejected; do not retry.

       Response code 440
          Generated by: POST (first stage)
          Meaning: posting not permitted.

       Response code 441
          Generated by: POST (second stage)
          Meaning: posting failed.

       Response code 480
          Generic response
          Meaning: command unavailable until the client has authenticated
          itself.

       Response code 483
          Generic response
          Meaning: command unavailable until suitable privacy has been
          arranged.

       Response code 500
          Generic response
          Meaning: unknown command.

       Response code 501
          Generic response
          Meaning: syntax error in command.

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       Response code 502
          Generic response and generated by initial connection

          Meaning for the initial connection and the MODE READER command:
          service permanently unavailable (the server immediately closes the
          connection).

          Meaning for all other commands: command not permitted (and there
          is no way for the client to change this).

       Response code 503
          Generic response
          Meaning: feature not supported.

       Response code 504
          Generic response
          Meaning: error in base64-encoding [RFC4648] of an argument.

    Appendix D.  Changes from RFC 977

       In general every attempt has been made to ensure that the protocol
       specification in this document is compatible with the version
       specified in RFC 977 [RFC977] and the various facilities adopted from
       RFC 2980 [RFC2980].  However, there have been a number of changes,
       some compatible and some not.

       This appendix lists these changes.  It is not guaranteed to be
       exhaustive or correct and MUST NOT be relied on.

       o  A formal syntax specification (Section 9) has been added.

       o  The default character set is changed from US-ASCII [ANSI1986] to
          UTF-8 [RFC3629] (note that US-ASCII is a subset of UTF-8).  This
          matter is discussed further in Section 10.

       o  All articles are required to have a message-id, eliminating the
          "<0>" placeholder used in RFC 977 in some responses.

       o  The newsgroup name matching capabilities already documented in
          RFC 977 ("wildmats", Section 4) are clarified and extended.  The
          new facilities (e.g., the use of commas and exclamation marks) are
          allowed wherever wildmats appear in the protocol.

       o  Support for pipelining of commands (Section 3.5) is made
          mandatory.

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       o  The principles behind response codes (Section 3.2) have been
          tidied up.  In particular:

          *  the x8x response code family, formerly used for private
             extensions, is now reserved for authentication and privacy
             extensions;

          *  the x9x response code family, formerly intended for debugging
             facilities, are now reserved for private extensions;

          *  the 502 and 503 generic response codes (Section 3.2.1) have
             been redefined;

          *  new 401, 403, 480, 483, and 504 generic response codes have
             been added.

       o  The rules for article numbering (Section 6) have been clarified
          (also see Section 6.1.1.2).

       o  The SLAVE command (which was ill-defined) is removed from the
          protocol.

       o  Four-digit years are permitted in the NEWNEWS (Section 7.4) and
          NEWGROUPS (Section 7.3) commands (two-digit years are still
          permitted).  The optional distribution parameter to these commands
          has been removed.

       o  The LIST command (Section 7.6.1) is greatly extended; the original
          is available as LIST ACTIVE, while new variants include
          ACTIVE.TIMES, DISTRIB.PATS, and NEWSGROUPS.  A new "m" status flag
          is added to the LIST ACTIVE response.

       o  A new CAPABILITIES command (Section 5.2) allows clients to
          determine what facilities are supported by a server.

       o  The DATE command (Section 7.1) is adopted from RFC 2980
          effectively unchanged.

       o  The LISTGROUP command (Section 6.1.2) is adopted from RFC 2980.
          An optional range argument has been added, and the 211 initial
          response line now has the same format as the 211 response from the
          GROUP command.

       o  The MODE READER command (Section 5.3) is adopted from RFC 2980 and
          its meaning and effects clarified.

       o  The XHDR command in RFC 2980 has been formalised as the new HDR
          (Section 8.5) and LIST HEADERS (Section 8.6) commands.

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       o  The XOVER command in RFC 2980 has been formalised as the new OVER
          (Section 8.3) and LIST OVERVIEW.FMT (Section 8.4) commands.  The
          former can be applied to a message-id as well as to a range.

       o  The concept of article metadata (Section 8.1) has been formalised,
          allowing the Bytes and Lines pseudo-headers to be deprecated.

       Client authors should note in particular that lack of support for the
       CAPABILITIES command is a good indication that the server does not
       support this specification.

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    Author's Address

       Clive D.W. Feather
       THUS plc
       322 Regents Park Road
       London
       N3  2QQ
       United Kingdom

       Phone: +44 20 8495 6138
       Fax:   +44 870 051 9937
       EMail: clive@demon.net
       URI:   http://www.davros.org/

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    Full Copyright Statement

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