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rfc:rfc3977

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

Feather Standards Track [Page 1] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

     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

Feather Standards Track [Page 2] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

   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

Feather Standards Track [Page 3] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 4] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 5] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 6] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 7] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 9] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 10] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 11] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 12] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 14] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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).

Feather Standards Track [Page 15] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 16] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 17] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 18] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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      |
 +-------------------+--------------------------+--------------------+

Feather Standards Track [Page 19] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 20] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 21] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 22] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 23] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 24] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 25] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 73] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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] .

Feather Standards Track [Page 78] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 90] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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)

Feather Standards Track [Page 95] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

   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 /

Feather Standards Track [Page 97] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

        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.

Feather Standards Track [Page 98] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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>;

Feather Standards Track [Page 99] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 100] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 101] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

    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

Feather Standards Track [Page 102] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 103] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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

Feather Standards Track [Page 104] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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).

Feather Standards Track [Page 105] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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

Feather Standards Track [Page 106] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

    [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.

Feather Standards Track [Page 107] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 108] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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>

Feather Standards Track [Page 109] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 110] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 [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.

Feather Standards Track [Page 111] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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:

Feather Standards Track [Page 112] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

    <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):

Feather Standards Track [Page 113] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

    [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.

Feather Standards Track [Page 114] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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 |
     +-------------------+-----------------------+---------------+

Feather Standards Track [Page 116] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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.

Feather Standards Track [Page 120] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 121] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 122] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

 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.

Feather Standards Track [Page 123] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

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/

Feather Standards Track [Page 124] RFC 3977 Network News Transfer Protocol (NNTP) October 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Feather Standards Track [Page 125]

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