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NNTP                                                          C. Feather
Internet-Draft                                                  Thus plc
Expires: March 4, 2005                                 September 3, 2004



                     Network News Transfer Protocol
                       draft-ietf-nntpext-base-24


Status of this Memo


   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."


   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.


   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


   This Internet-Draft will expire on March 4, 2005.


Copyright Notice


   Copyright (C) The Internet Society (2004).


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.


Administration




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   This document is a product of the NNTP Working Group, chaired by Russ
   Allbery and Ned Freed.


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.


Rights


   UNIX is a registered trademark of The Open Group.





































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Table of Contents


   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .    5
   2.  Notation . . . . . . . . . . . . . . . . . . . . . . . . . .    6
   3.  Basic Concepts . . . . . . . . . . . . . . . . . . . . . . .    8
     3.1   Commands and Responses . . . . . . . . . . . . . . . . .    8
     3.2   Response Codes . . . . . . . . . . . . . . . . . . . . .   10
       3.2.1   Generic Response Codes . . . . . . . . . . . . . . .   11
         3.2.1.1   Examples . . . . . . . . . . . . . . . . . . . .   13
     3.3   Pipelining . . . . . . . . . . . . . . . . . . . . . . .   14
       3.3.1   Examples . . . . . . . . . . . . . . . . . . . . . .   15
     3.4   Articles . . . . . . . . . . . . . . . . . . . . . . . .   16
   4.  The WILDMAT format . . . . . . . . . . . . . . . . . . . . .   18
     4.1   Wildmat syntax . . . . . . . . . . . . . . . . . . . . .   18
     4.2   Wildmat semantics  . . . . . . . . . . . . . . . . . . .   18
     4.3   Extensions . . . . . . . . . . . . . . . . . . . . . . .   19
     4.4   Examples . . . . . . . . . . . . . . . . . . . . . . . .   20
   5.  Session administration commands  . . . . . . . . . . . . . .   21
     5.1   Initial Connection . . . . . . . . . . . . . . . . . . .   21
     5.2   MODE READER  . . . . . . . . . . . . . . . . . . . . . .   22
     5.3   LIST EXTENSIONS  . . . . . . . . . . . . . . . . . . . .   24
     5.4   QUIT . . . . . . . . . . . . . . . . . . . . . . . . . .   26
   6.  Article posting and retrieval  . . . . . . . . . . . . . . .   28
     6.1   Group and article selection  . . . . . . . . . . . . . .   28
       6.1.1   GROUP  . . . . . . . . . . . . . . . . . . . . . . .   28
       6.1.2   LAST . . . . . . . . . . . . . . . . . . . . . . . .   31
       6.1.3   NEXT . . . . . . . . . . . . . . . . . . . . . . . .   32
     6.2   Retrieval of articles and article sections . . . . . . .   33
       6.2.1   ARTICLE  . . . . . . . . . . . . . . . . . . . . . .   34
       6.2.2   HEAD . . . . . . . . . . . . . . . . . . . . . . . .   37
       6.2.3   BODY . . . . . . . . . . . . . . . . . . . . . . . .   38
       6.2.4   STAT . . . . . . . . . . . . . . . . . . . . . . . .   40
     6.3   Article posting  . . . . . . . . . . . . . . . . . . . .   42
       6.3.1   POST . . . . . . . . . . . . . . . . . . . . . . . .   42
       6.3.2   IHAVE  . . . . . . . . . . . . . . . . . . . . . . .   44
   7.  Information commands . . . . . . . . . . . . . . . . . . . .   48
     7.1   DATE . . . . . . . . . . . . . . . . . . . . . . . . . .   48
     7.2   HELP . . . . . . . . . . . . . . . . . . . . . . . . . .   48
     7.3   NEWGROUPS  . . . . . . . . . . . . . . . . . . . . . . .   49
     7.4   NEWNEWS  . . . . . . . . . . . . . . . . . . . . . . . .   50
     7.5   Time . . . . . . . . . . . . . . . . . . . . . . . . . .   51
       7.5.1   Examples . . . . . . . . . . . . . . . . . . . . . .   52
     7.6   The LIST commands  . . . . . . . . . . . . . . . . . . .   52
       7.6.1   LIST ACTIVE  . . . . . . . . . . . . . . . . . . . .   52
       7.6.2   LIST ACTIVE.TIMES  . . . . . . . . . . . . . . . . .   54
       7.6.3   LIST DISTRIBUTIONS . . . . . . . . . . . . . . . . .   56
       7.6.4   LIST DISTRIB.PATS  . . . . . . . . . . . . . . . . .   56
       7.6.5   LIST NEWSGROUPS  . . . . . . . . . . . . . . . . . .   58




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   8.  Framework for NNTP extensions  . . . . . . . . . . . . . . .   60
     8.1   Initial IANA registry  . . . . . . . . . . . . . . . . .   62
     8.2   Standard extensions  . . . . . . . . . . . . . . . . . .   62
     8.3   The LISTGROUP extension  . . . . . . . . . . . . . . . .   62
       8.3.1   LISTGROUP  . . . . . . . . . . . . . . . . . . . . .   62
     8.4   Article metadata . . . . . . . . . . . . . . . . . . . .   64
       8.4.1   The :bytes metadata item . . . . . . . . . . . . . .   65
       8.4.2   The :lines metadata item . . . . . . . . . . . . . .   65
     8.5   The OVER extension . . . . . . . . . . . . . . . . . . .   65
       8.5.1   OVER . . . . . . . . . . . . . . . . . . . . . . . .   66
       8.5.2   LIST OVERVIEW.FMT  . . . . . . . . . . . . . . . . .   70
     8.6   The HDR extension  . . . . . . . . . . . . . . . . . . .   72
       8.6.1   HDR  . . . . . . . . . . . . . . . . . . . . . . . .   72
       8.6.2   LIST HEADERS . . . . . . . . . . . . . . . . . . . .   76
   9.  Augmented BNF Syntax for NNTP  . . . . . . . . . . . . . . .   79
     9.1   Commands . . . . . . . . . . . . . . . . . . . . . . . .   79
     9.2   Command continuation . . . . . . . . . . . . . . . . . .   81
     9.3   Responses  . . . . . . . . . . . . . . . . . . . . . . .   81
       9.3.1   Generic responses  . . . . . . . . . . . . . . . . .   81
       9.3.2   Initial response line contents . . . . . . . . . . .   82
       9.3.3   Multi-line response contents . . . . . . . . . . . .   82
     9.4   LIST EXTENSIONS responses  . . . . . . . . . . . . . . .   84
     9.5   Articles . . . . . . . . . . . . . . . . . . . . . . . .   84
     9.6   General non-terminals  . . . . . . . . . . . . . . . . .   84
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . .   87
   11.   Security Considerations  . . . . . . . . . . . . . . . . .   88
     11.1  Personal and Proprietary Information . . . . . . . . . .   88
     11.2  Abuse of Server Log Information  . . . . . . . . . . . .   88
     11.3  Weak Authentication and Access Control . . . . . . . . .   88
     11.4  DNS Spoofing . . . . . . . . . . . . . . . . . . . . . .   89
     11.5  UTF-8 issues . . . . . . . . . . . . . . . . . . . . . .   89
     11.6  Caching of LIST EXTENSIONS results . . . . . . . . . . .   90
   12.   Acknowledgements . . . . . . . . . . . . . . . . . . . . .   92
   13.   References . . . . . . . . . . . . . . . . . . . . . . . .   94
   13.1  Normative References . . . . . . . . . . . . . . . . . . .   94
   13.2  Informative References . . . . . . . . . . . . . . . . . .   94
       Author's Address . . . . . . . . . . . . . . . . . . . . . .   95
   A.  Future Directions  . . . . . . . . . . . . . . . . . . . . .   96
   B.  Interaction with other specifications  . . . . . . . . . . .   97
     B.1   Header folding . . . . . . . . . . . . . . . . . . . . .   97
     B.2   Message-IDs  . . . . . . . . . . . . . . . . . . . . . .   97
     B.3   Article posting  . . . . . . . . . . . . . . . . . . . .   98
   C.  Summary of Response Codes  . . . . . . . . . . . . . . . . .  100
   D.  Formal specification of the standard extensions  . . . . . .  104
     D.1   The LISTGROUP extension  . . . . . . . . . . . . . . . .  104
     D.2   The OVER extension . . . . . . . . . . . . . . . . . . .  104
     D.3   The HDR extension  . . . . . . . . . . . . . . . . . . .  105
       Intellectual Property and Copyright Statements . . . . . . .  106




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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
   are stored in a central database, giving subscribers the ability to
   select only those articles they wish to read.


   The Netnews model provides for indexing, cross-referencing, and
   expiration of aged messages.  For server-to-server interaction, NNTP
   is designed for efficient transmission of Netnews articles over a
   reliable full duplex communication channel.


   Every attempt is made to ensure that the protocol specification in
   this document is compatible with the version specified in RFC 977
   [RFC977].  However, this version does not support the ill-defined
   SLAVE command and permits four digit years to be specified in the
   NEWNEWS and NEWGROUPS commands.  It changes the default character set
   to UTF-8 [RFC3629] instead of US-ASCII [ANSI1986] (note that US-ASCII
   is a subset of UTF-8).  It now requires all articles to have a
   message-id, eliminating the "<0>" placeholder used in RFC 977 in some
   responses.  It also extends the newsgroup name matching capabilities
   already documented in RFC 977.


   Generally, new functionality is made available using new commands.  A
   number of such commands (including some commands taken from RFC 2980
   [RFC2980]) are now mandatory.  Part of the new functionality involves
   a mechanism to discover what new functionality is available to
   clients from a server.  This mechanism can also be used to add more
   functionality as needs merit such additions.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 term "client" or "client
   host" refers to a host making use of the NNTP service, while the term
   "server" or "server host" refers to a host that offers the NNTP
   service.





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2.  Notation


   The following notational conventions are used in this document.


     UPPERCASE     indicates literal text to be included in the
                   command;
     lowercase     indicates a token described elsewhere;
     [brackets]    indicate that the argument is optional;
     ellipsis...   indicates that the argument may be repeated any
                   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 3.4,
   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 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.


   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 a "[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 current 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 names) 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




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   defined elsewhere or needs to exist in any particular implementation.


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













































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3.  Basic Concepts


3.1  Commands and Responses


   NNTP operates over any reliable data stream 8-bit-wide channel.
   Initially, the server host starts the NNTP service by listening on a
   TCP port; when running over TCP/IP, the official port for the NNTP
   service is 119.  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.  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.


   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.  Keywords MUST consist of
   printable US-ASCII characters.  Unless otherwise noted elsewhere in
   this document, arguments SHOULD consist of printable US-ASCII
   characters.  Keywords and arguments MUST be each separated by one or
   more space or TAB characters.  Keywords MUST be at least three
   characters and MUST NOT exceed 12 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
   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
   command lines and the initial lines of responses, and SHOULD apply
   these same principles throughout.


   The term "character" means a single Unicode code point and
   implementations are not required to carry out 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, using 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




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   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.


   An NNTP server MUST implement all the commands in this specification
   except for those marked as optional and those in extensions.


   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 first or only 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.


   All multi-line responses MUST adhere to the following format:
   1.  The response consists of a sequence of one 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.  The first such line contains the response code as with a single
       line response.
   3.  If any subsequent line begins with the "termination octet" ("."
       or %x2E), that line MUST be "byte-stuffed" by pre-pending an
       additional termination octet to that line of the response.
   4.  The lines of the response MUST be followed by a terminating line
       consisting of a single termination octet followed by a CRLF pair
       in the normal way.  Thus a multi-line response is always
       terminated with the five octets CRLF "." CRLF (%x0D.0A.2E.0D.0A).
   5.  When interpreting a multi-line response, the "byte-stuffing" MUST
       be undone; i.e.  the client MUST ensure that, in any line
       beginning with the termination octet followed by octets other
       than 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 response; i.e.  the client
       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 is, of course, part of
       the last line of the response).


   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
   it MAY include octets above and below 128 mixed arbitrarily.


   This document does not place any limit on the length of a subsequent




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   line in a multi-line response.  However, the standards that define
   the format of articles may do so.


   An NNTP server MAY have an inactivity autologout timer.  Such a timer
   SHOULD be of at least three minutes duration, with the exception that
   there MAY be a shorter limit on how long the server is willing to
   wait for the first command from the client.  The receipt of any
   command from the client during the timer interval SHOULD suffice to
   reset the autologout timer.  Similarly, the receipt of any
   significant amount of data from the client while in the midst of
   sending a multi-line message to the server (such as during a POST or
   IHAVE command) SHOULD suffice to reset the autologout timer.  When
   the timer expires, the server SHOULD close the TCP connection without
   sending any response to the client.


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, set-up, 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 or not the code
   introduces a multi-line response.  Any extension MUST follow this
   principle as well, but note that, for historical reasons, the 211
   response code is an exception to this in that the response may be
   multi-line or not depending on the command (GROUP or LISTGROUP) that
   generated it.  In all other cases, the client MUST only use the




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   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.


   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 8)
   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 it is an optional command or
   extension that is not implemented by the server, the response code
   500 MUST be returned.




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   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), then 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.


   As a special case, if an argument is required to be a base64-encoded
   string [RFC3548] (there are no such arguments in this specification,
   but there may be 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 only handles a subset of
   legitimate cases (see the HDR command (Section 8.6.1) for an
   example), the response code 503 MUST be returned.  Note that where a
   command is optional (e.g.  LIST ACTIVE.TIMES) and is not provided by
   a server, this MAY be treated as an unimplemented command (response
   code 500 or 501 as appropriate) or as a working command where the
   information is not available (response code 503).


   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 start a new one
      with the appropriate authority before the command can be used.
      Note that the server MUST NOT close the TCP connection immediately
      after a 502 response except at the initial connection (Section
      5.1) and with the MODE READER (Section 5.2) command.  See also the
      latter command for historical usage of this response.
   480: the client must authenticate itself to the server (that is,
      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
      extension-label (see Section 8) of the extension (which may be a
      private extension) that provides the necessary mechanism, or




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      "MODE-READER" if it is necessary to use the MODE READER (Section
      5.2) command.


   If the server has to terminate the connection for some reason, it
   MUST give a 400 response code to the next command and then
   immediately close the TCP connection.


   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 extension:
      [C] LIST EXTENSIONS
      [S] 202 Extensions supported:
      [S] LISTGROUP
      [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


   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:




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      [C] MODE READER
      [S] 200 Reader mode, posting permitted
      [C] IHAVE   <i.am.an.article.you.will.want@example.com>
      [S] 502 Permission denied


   Example of an attempt to access a facility requiring authentication:
      [C] GROUP   secret.group
      [S] 480 Permission denied
   followed by 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 using a facility:
      [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


   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  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




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   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 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 or not
   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 receiving the
   CRLF at the end of the response from the command.  The server MAY
   ignore any data received after the command and before the CRLF at the
   end of the response is sent to the client.


   The initial connection must not be part of a pipeline; that is, the
   client MUST NOT send any command until receiving the CRLF at the end
   of the greeting.


   If the client uses blocking system calls to send commands, it MUST
   ensure that the amount of text sent in pipelining does not cause a
   deadlock between transmission and reception.  The amount of text
   involved will depend on window sizes in the transmission layer, and
   is typically 4k octets for TCP.  (Since the server only sends data in
   response to commands from the client, the converse problem does not
   occur.)


3.3.1  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] GROUP   misc.test
      [C] MODE READER
      [C] DATE




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      [C] NEXT
      [S] 211 1234 3000234 3002322 misc.test
      [S] 200 Server ready, posting allowed
      [S] 223 3000237 <668929@example.org> retrieved
   The DATE command has been thrown away by the server and so there is
   no 111 response to match it.


3.4  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 B 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 has been received from elsewhere with one of these, clients
   and servers MAY transfer it to the other without re-folding it.




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   The content of a header SHOULD be in UTF-8.  However, if a server
   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 without
   modification.  Therefore clients MUST be prepared to receive such
   headers and also data derived from them (e.g.  in the responses from
   the OVER extension (Section 8.5)) and MUST NOT assume that they are
   always UTF-8.  How the client will then process 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 "<" and 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 the article to be changed as a
   result).  See also Appendix B.2.

























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4.  The WILDMAT format


   The WILDMAT format described here is based on the version first
   developed by Rich Salz [SALZ1992], which in turn was 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.


4.1  Wildmat syntax


   A wildmat is described by the following ABNF [RFC2234] syntax (note
   that this syntax contains ambiguities and special cases described at
   the end):


      wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
      wildmat-pattern = 1*wildmat-item
      wildmat-item = wildmat-exact / wildmat-wild
      wildmat-exact = %x21-29 / %x2B / %x2D-3E / %x40-5A / %x5E-7E /
         UTF8-non-ascii ; exclude * , ? [ \ ]
      wildmat-wild = "*" / "?"


   UTF8-non-ascii is defined in Section 9.


   This syntax must be interpreted subject to the following rule:


   Where a wildmat-pattern is not immediately preceded by "!", it shall
   not begin with a "!".


   Note: the characters \ , [ and ] are not allowed in wildmats, while *
   and ? are always wildcards.  This should not be a problem since these
   characters cannot occur in newsgroup names, which is the only current
   use of wildmats.  Backslash is commonly used to suppress the special
   meaning of characters while 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 both 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.





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   For example, consider the wildmat "a*,!*b,*c*":
      the string "aaa" matches because the rightmost match is with "a*"
      the string "abb" does not match because the rightmost match is
      with "*b"
      the string "ccb" matches because the rightmost match is with "*c*"
      the string "xxx" does not match because no wildmat-pattern matches


   A wildmat-pattern matches a string if the string can be broken into
   components, each of which matches the corresponding wildmat-item in
   the pattern; the matches must be in the same order, and the whole
   string must be used in the match.  The pattern is "anchored"; that
   is, the first and last characters in the string must match the first
   and last item respectively (unless that item is an asterisk matching
   zero characters).


   A wildmat-exact matches the same character (which may be more than
   one octet in UTF-8).


   "?" matches exactly one character (which may be more than one octet).


   "*" matches zero or more characters.  It can match an empty string,
   but it cannot match a subsequence of a UTF-8 sequence that is not
   aligned to the character boundaries.


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.






















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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


   Responses
      200   Service available, posting allowed [1]
      201   Service available, posting prohibited [1]
      400   Service temporarily unavailable [1][2]
      502   Service permanently unavailable [1][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 it is not
   authorized to post articles using the POST command, the server MUST
   present a 201 greeting code.


   Otherwise the server MUST present a 400 or 502 greeting code and then
   immediately close the connection.  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.


5.1.3  Examples


   Example of a normal connection from an authorized client which then
   terminates the session (see Section 5.4):
      [Initial TCP connection set-up completed.]
      [S] 200 NNTP Service Ready, posting permitted
      [C] QUIT
      [S] 205 NNTP Service exits normally
      [Server closes connection.]




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   Example of a normal connection from an authorized client that is not
   permitted to post; it also immediately terminates the session:
      [Initial TCP 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 TCP connection set-up completed.]
      [S] 502 NNTP Service permanently unavailable
      [Server closes connection.]


   Example of a connection from a client where the server is unable to
   provide service:
      [Initial TCP connection set-up completed.]
      [S] 400 NNTP Service temporarily unavailable
      [Server closes connection.]


5.2  MODE READER


5.2.1  Usage


   This command MUST NOT be pipelined.
   Syntax
      MODE READER
   Responses
      200   Posting allowed
      201   Posting prohibited
      400   Service temporarily unavailable [1]
      502   Service permanently unavailable [1]
   [1] Following a 400 or 502 response the server MUST immediately close
      the connection.


5.2.2  Description


   MODE READER SHOULD be sent by any client that intends to use any
   command in this specification (including Section 8) other than IHAVE,
   HEAD, STAT, LIST ACTIVE, or LIST EXTENSIONS; other extensions MAY
   also require MODE READER to be used.  Servers MAY require that this
   command be issued before any commands other than the above are sent
   and MAY reject such commands until after a MODE READER command has
   been sent.  Such rejections SHOULD use response code 401 with
   argument "MODE-READER", but for historical reasons response code 502
   MAY be used, even though this situation does not meet the conditions
   for that response.


   Once MODE READER is sent, IHAVE (and any related extensions) MAY no




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   longer be permitted, even if it were permitted before the MODE READER
   command.  The results of LIST EXTENSIONS MAY be different following a
   MODE READER command than prior to the issuing of that command.


   The server MUST return a response using the same codes as the initial
   greeting (as described in Section 5.1.1) to indicate its ability to
   provide reading service to the client.  Note that the response need
   not be the same as the one presented during the initial greeting.


   Servers are encouraged to not require this command even though
   clients SHOULD send it when appropriate.  It is present to support
   some news architectures that switch between modes based on whether a
   given connection is a peer-to-peer connection with another server or
   a news reading client.


5.2.3  Examples


   Example of use of the MODE READER command by an authorized client
   which then terminates the session (see Section 5.4):
      [C] MODE READER
      [S] 200 NNTP Service Ready, posting permitted
      [C] QUIT
      [S] 205 NNTP Service exits normally
      [Server closes connection.]


   Example of use of the MODE READER command by an authorized client
   that is not permitted to post; it also immediately terminates the
   session:
      [C] MODE READER
      [S] 201 NNTP Service Ready, posting prohibited
      [C] QUIT
      [S] 205 NNTP Service exits normally
      [Server closes connection.]


   Example of use of MODE READER by a client not authorized to receive
   service from the server as a news reader:
      [C] MODE READER
      [S] 502 NNTP Service permanently unavailable
      [Server closes connection.]


   Example of a connection from any client where the server is
   temporarily unable to provide news reader service:
      [C] MODE READER
      [S] 400 NNTP Service temporarily unavailable
      [Server closes connection.]


   Example of a facility that requires MODE READER before use, using the
   preferred response:




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      [C] GROUP   misc.test
      [S] 401 MODE-READER currently in peering mode
      [C] MODE READER
      [S] 200 NNTP Service Ready, posting permitted
      [C] GROUP   misc.test
      [S] 211 1234 3000234 3002322 misc.test


   Example of a facility that requires MODE READER before use, using the
   historical but deprecated response:
      [C] GROUP   misc.test
      [S] 502 Not available in peering mode
      [C] MODE READER
      [S] 200 NNTP Service Ready, posting permitted
      [C] GROUP   misc.test
      [S] 211 1234 3000234 3002322 misc.test


   Example of a facility that cannot be used after MODE READER:
      [C] IHAVE   <i.am.an.article.you.have@example.com>
      [S] 435 Duplicate
      [C] MODE READER
      [S] 200 Reader mode, posting permitted
      [C] IHAVE   <i.am.an.article.you.have@example.com>
      [S] 502 Permission denied


5.3  LIST EXTENSIONS


5.3.1  Usage


   This command is optional.
   Syntax
      LIST EXTENSIONS
   Responses
      202   Extension list follows (multiline)
      402   Server has no extensions


5.3.2  Description


   The LIST EXTENSIONS command allows a client to determine which
   extensions are supported by the server at any given time.  See
   Section 8 for further discussion of extensions.


   This command MUST be implemented by any server that implements any
   registered extension, and is optional otherwise.  A server MUST NOT
   generate the generic response 401, 480, 483, or 502 (all of which
   indicate "not permitted") to this command.


   This command MAY be issued at anytime during a session.  It is not
   required that the client issues this command before attempting to




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   make use of any extension.  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).  An NNTP
   client is only able to get the current and correct information
   concerning available extensions at any point during a session by
   issuing a LIST EXTENSIONS command at that point of that session and
   processing the response, and the server MUST ensure that those
   extensions currently listed in the returned information are
   available.  Therefore, if an extension (including those in Section 8)
   is only available before or after a MODE READER command, the LIST
   EXTENSIONS command MUST only include the extension in that situation.
   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, before or after a MODE READER command, this MUST be
   indicated by different arguments to the extension-label in the
   results of LIST EXTENSIONS in each situation.


   While some extensions are likely to be always available or never
   available, others will "appear" and "disappear" depending on server
   state changes within the session or 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 11.6
   for further discussion of this topic.


   The list of extensions is returned as a multi-line response following
   the 202 response code.  Each extension is listed on a separate line;
   the line MUST begin with an extension-label and optionally one or
   more arguments (separated by one or more spaces).  The
   extension-label and the meaning of the arguments are specified as
   part of the definition of the extension.  The extension-label is a
   string of 1 to 12 US-ASCII letters and MUST be in uppercase (that is,
   %x41-5A).  Arguments are strings 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).


   The server MUST NOT list the same extension twice in the response,
   MUST list all supported registered extensions, and SHOULD list all
   supported private extensions.  The order in which the extensions are
   listed is not significant.  The server need not even consistently
   return the same order.  If the server does not support any
   extensions, it MUST return an empty list.  The 402 response code is
   documented for historic reasons only; clients SHOULD handle it
   gracefully, but servers MUST NOT generate it.




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   Following a generic failure response, such as 403, an extension might
   still be available, and the client MAY attempt to use it.


5.3.3  Examples


   Example of a successful response:
      [C] LIST EXTENSIONS
      [S] 202 Extensions supported:
      [S] OVER MSGID
      [S] HDR
      [S] LISTGROUP
      [S] .
   The particular extensions shown here are simply examples of what
   might be defined in other places, and no particular meaning should be
   attributed to them.


   Example where no extensions are available:
      [C] LIST EXTENSIONS
      [S] 202 Extensions supported:
      [S] .


   Example from a non-conforming server which indicates "no extensions
   available" using the 402 response code:
      [C] LIST EXTENSIONS
      [S] 402 Server has no extensions


5.4  QUIT


5.4.1  Usage


   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 its transactions with the NNTP
   server.


   If a client simply disconnects (or the connection times out or some
   other fault occurs), the server MUST gracefully cease its attempts to
   service the client, disconnecting from its end if necessary.


   The server MUST NOT generate any response code to the QUIT command




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   other than 205 or, if any arguments are provided, 501.


5.4.3  Examples


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













































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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.  One key is the message-id of an article.
   Another key is composed of the newsgroup name and the article number
   within that newsgroup.  That key MUST be unique to a particular
   server (there will be only one article with that number within a
   particular newsgroup), but is not required to be globally unique.
   Additionally, because the same article can be cross-posted to
   multiple newsgroups, there may be multiple keys that point to the
   same article on the same server.  The final key is the arrival
   timestamp, giving the time that the article arrived at the server.


   The server MUST ensure that article numbers are issued in order of
   arrival timestamp; that is, articles arriving later MUST have higher
   numbers than those that arrive earlier.  The server SHOULD allocate
   the next sequential unused number to each new article.


   Article numbers MUST lie between 1 and 4,294,967,295 inclusive.  The
   client and server 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.


6.1  Group and article selection


   The following commands are used to set the "current 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


   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






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6.1.1.2  Description


   The required argument is the name of the newsgroup to be selected
   (e.g.  "news.software.b").  A list of valid newsgroups may be
   obtained by using the LIST ACTIVE command (see Section 7.6.1).


   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
   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
      non-zero, the same requirements apply as for a non-empty group.


   The set of articles in a group may change after the GROUP command is
   carried out.  That is:
   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 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




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   response for that newsgroup in any session, and 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
   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 current
   selected newsgroup MUST be set to that group and the current article
   number MUST be set to the first article in the group.  If an empty
   newsgroup is selected, the current article pointer is made invalid.
   If an invalid group is specified, the current selected newsgroup and
   current article number MUST NOT be changed.


   The GROUP command (or the LISTGROUP command, if implemented) MUST be
   used by a client and a successful response received before any other
   command is used that depends on the value of the current 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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6.1.2  LAST


6.1.2.1  Usage


   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


6.1.2.2  Description


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


6.1.2.3  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




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      [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 current 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 current 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


6.1.3  NEXT


6.1.3.1  Usage


   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


6.1.3.2  Description


   If the current 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




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   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 6.1.2).


6.1.3.3  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


   Example of an attempt to retrieve an article without having selected
   a group (via the GROUP command) first:
      [Assumes current 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 current 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 commands are described in
   terms of the differences.  As specified in Section 3.4, 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


   Syntax
      ARTICLE message-id
      ARTICLE number
      ARTICLE
   Responses
      First form (message-id specified)
         220 0|n message-id   Article follows (multiline)
         430                  No article with that message-id
      Second form (article number specified)
         220 n message-id     Article follows (multiline)
         412                  No newsgroup selected
         423                  No article with that number
      Third form (current article number used)
         220 n message-id     Article follows (multiline)
         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 based on the arguments and
   presents the entire article (that is, the headers, an empty line, and
   the body in that order).  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 current 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.


   In the response, the article number MUST be replaced with zero,
   except that if there is a current selected group and the article is
   present in that group, the server MAY use that article number.  (The
   server is not required to determine whether the article is in the
   current 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




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


   The server MUST NOT change the current 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 response 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 current selected newsgroup is invalid, a 412 response MUST be
   returned.  If the argument is a number and that article does not
   exist in the current 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.


6.2.1.3  Examples


   Example of a successful retrieval of an article (using no 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




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      [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: <411@example.net>
      [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: <411@example.net>
      [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


   Example of an unsuccessful retrieval of an article by number because
   no newsgroup was selected first:
      [Assumes current selected newsgroup is invalid.]
      [C] ARTICLE   300256
      [S] 412 No newsgroup selected


   Example of an attempt to retrieve an article when the current
   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






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6.2.2  HEAD


6.2.2.1  Usage


   Syntax
      HEAD message-id
      HEAD number
      HEAD
   Responses
      First form (message-id specified)
         221 0|n message-id   Headers follow (multiline)
         430                  No article with that message-id
      Second form (article number specified)
         221 n message-id     Headers follow (multiline)
         412                  No newsgroup selected
         423                  No article with that number
      Third form (current article number used)
         221 n message-id     Headers follow (multiline)
         412                  No newsgroup selected
         420                  Current article number is invalid
   Parameters
      number     = Requested article number
      n          = Returned article number
      message-id = Article message-id


6.2.2.2  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 (using
   no 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: <411@example.net>
      [S] .




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   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: <411@example.net>
      [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


   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 current 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
   current 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


   Syntax
      BODY message-id
      BODY number
      BODY






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   Responses
      First form (message-id specified)
         222 0|n message-id   Body follows (multiline)
         430                  No article with that message-id
      Second form (article number specified)
         222 n message-id     Body follows (multiline)
         412                  No newsgroup selected
         423                  No article with that number
      Third form (current article number used)
         222 n message-id     Body follows (multiline)
         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 (using no
   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


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




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   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 current 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
   current 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


   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
      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




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   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 (using no 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


   Example of STAT on an article by number when no newsgroup was
   selected first:
      [Assumes current selected newsgroup is invalid.]
      [C] STAT   300256
      [S] 412 No newsgroup selected


   Example of STAT on an article when the current 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 which 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>




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      [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 current selected group, the fifth shows that the number, if
   provided, must be that relating to the current selected group, and
   the last one shows that the current selected article is still not
   changed by the use of STAT with a message-id even if it returns an
   article number.


6.3  Article posting


   Article posting is done in one of two modes: 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


   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




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   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 3.4 and MUST be sent by the client to the server in the
   manner specified (in Section 3.1) for multi-line responses (except
   that there is no initial line containing a response code).  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.  Others are returned
   following the sending of the article.


   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 and not accepted and
   silently discarded.  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 B.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]




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      [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


   Example of an attempt to post when posting is not allowed:
      [C] MODE READER
      [S] 201 NNTP Service Ready, posting prohibited
      [C] POST
      [S] 440 Posting not permitted


6.3.2  IHAVE


6.3.2.1  Usage


   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




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   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.


   If transmission of the article is requested, the client MUST send the
   entire article, including headers and body, in the format defined
   above (Section 3.1) for multi-line responses (except that there is no
   initial line containing a response code).  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 either 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 such problems 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 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 be able immediately to
   determine whether or not an article is suitable for posting or
   forwarding, an NNTP server MAY acknowledge the successful transfer of
   the article (with a 235 response) but later silently discard it.


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>




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      [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


   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





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   Example of sending an article to a site that requests the article be
   tried again later:
      [C] IHAVE   <i.am.an.article.you.defer@example.com>
      [S] 436 Retry later
















































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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


   Syntax
      DATE
   Responses
      111 yyyymmddhhmmss   server date and time
   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).  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


   Syntax
      HELP
   Responses
      100   Help text follows (multiline)







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7.2.2  Description


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


   This text is not guaranteed to be in any particular format and MUST
   NOT be used by clients as a replacement for the LIST EXTENSIONS
   command described in Section 5.3


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


   Syntax
      NEWGROUPS date time [GMT]
   Responses
      231   List of new newsgroups follows (multiline)
   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 7.6.1).  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 results SHOULD be consistent with those of the LIST
   ACTIVE.TIMES command (Section 7.6.2), except that if the specified
   date and time is earlier than the oldest entry in the latter then the
   results of this command may include extra groups.


   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




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   of the year.  If the first two digits of the year are not specified
   (this is supported only for backwards compatibility), the year is to
   be taken from the current century if yy is smaller than or equal to
   the current year, otherwise the year is from the previous century.


   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 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


   Syntax
      NEWNEWS wildmat date time [GMT]
   Responses
      230   List of new articles follows (multiline)
   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 so, 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.


   The DATE command MUST return a timestamp from the same clock as is
   used for determining article arrival and group creation times.  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.


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




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


7.6.1  LIST ACTIVE


7.6.1.1  Usage


   Syntax
      LIST ACTIVE [wildmat]
   Responses
      215   Information follows (multiline)
   Parameters
      wildmat = groups of interest






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7.6.1.2  Description


   The LIST ACTIVE command with no arguments returns a list of valid
   newsgroups and associated information.  The server MUST include every
   group that the client is permitted to select with the GROUP (Section
   6.1.1) command.  Each newsgroup is sent as a line of text in the
   following format:
   group high low status
   where:
   "group" is the name of the newsgroup;
   "high" is the reported high water mark for the group;
   "low" is the reported low water mark for the group;
   "status" is the current status of the group on this server.
   Each field in the line is separated from its neighbouring fields by
   one or more spaces.


   Note that an empty list is a possible valid response, and indicates
   that there are currently no valid newsgroups.


   The reported high and low water marks are as described in the GROUP
   command (see Section 6.1.1).


   The status field is typically one of:
   "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".


   If the optional wildmat argument is specified, the response is
   limited to only the groups (if any) whose names match the wildmat.
   If no wildmat is specified, the keyword ACTIVE MAY be omitted without
   altering the effect of the command.


7.6.1.3  Examples


   Example of LIST ACTIVE returning a list of newsgroups:




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      [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] .


   The same output 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] .


   Example of LIST ACTIVE omitting the second keyword and returning no
   newsgroups:
      [C] LIST
      [S] 215 list of newsgroups follows
      [S] .


   Example of LIST ACTIVE with a wildmat:
      [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] .


7.6.2  LIST ACTIVE.TIMES


7.6.2.1  Usage


   This command is optional.
   Syntax
      LIST ACTIVE.TIMES [wildmat]
   Responses
      215   Information follows (multiline)
   Parameters
      wildmat = groups of interest


7.6.2.2  Description


   The active.times list is maintained by some news transfer systems to
   contain information about who created a particular newsgroup and




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   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].


   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 LIST ACTIVE.  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 (subject to the
   caveats in the description of that command).


   If the information is available, it is returned as a multi-line
   response following the 215 response code.  If 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 (whether or not
   a wildmat is specified) and indicates that there are no groups
   meeting the above criteria.


7.6.2.3  Examples


   Example of LIST ACTIVE.TIMES returning a list of newsgroups:
      [C] LIST ACTIVE.TIMES
      [S] 215 information follows
      [S] misc.test 930445408 <creatme@isc.org>
      [S] alt.rfc-writers.recovery 930562309 <m@example.com>
      [S] tx.natives.recovery 930678923 <sob@academ.com>
      [S] .


   Example of LIST ACTIVE.TIMES returning an error where the command is
   recognized but the software does not maintain this information:
      [C] LIST ACTIVE.TIMES
      [S] 503 program error, function not performed


   Example of LIST ACTIVE.TIMES sent to a server that does not recognize
   this command:
      [C] LIST ACTIVE.TIMES
      [S] 501 Syntax Error






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7.6.3  LIST DISTRIBUTIONS


7.6.3.1  Usage


   This command is optional.
   Syntax
      LIST DISTRIBUTIONS
   Responses
      215   Information follows (multiline)


7.6.3.2  Description


   The distributions list is maintained by some news transfer systems to
   contain information about valid values for the content of the
   Distribution header in a news article and about what the various
   values mean.  Each line of this list consists of two fields separated
   from each other by one or more spaces.  The first field is a value
   and the second is a short explanation of the meaning of that value.


   If the information is available, it is returned as a multi-line
   response following the 215 response code.


7.6.3.3  Examples


   Example of LIST DISTRIBUTIONS returning a list of distributions:
      [C] LIST DISTRIBUTIONS
      [S] 215 information follows
      [S] usa United States of America
      [S] na North America
      [S] world All over the World
      [S] .


   Example of LIST DISTRIBUTIONS returning an error where the command is
   recognized but the software does not maintain this information:
      [C] LIST DISTRIBUTIONS
      [S] 503 program error, function not performed


   Example of LIST DISTRIBUTIONS sent to a server that does not
   recognize this command:
      [C] LIST DISTRIBUTIONS
      [S] 501 Syntax Error


7.6.4  LIST DISTRIB.PATS


7.6.4.1  Usage







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   This command is optional.
   Syntax
      LIST DISTRIB.PATS
   Responses
      215   Information follows (multiline)


7.6.4.2  Description


   The distrib.pats list is maintained by some news transfer systems 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
   group name), and the third field is a value for the Distribution
   header content.


   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.


   If the information is available, it is returned as a multi-line
   response following the 215 response code.


7.6.4.3  Examples


   Example of LIST DISTRIB.PATS returning a list of newsgroups:
      [C] LIST DISTRIB.PATS
      [S] 215 information follows
      [S] 10:local.*:local
      [S] 5:*:world
      [S] 20:local.here.*:thissite
      [S] .


   Example of LIST DISTRIB.PATS returning an error where the command is
   recognized but the software does not maintain this information:
      [C] LIST DISTRIB.PATS
      [S] 503 program error, function not performed


   Example of LIST DISTRIB.PATS sent to a server that does not recognize
   this command:
      [C] LIST DISTRIB.PATS
      [S] 501 Syntax Error







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7.6.5  LIST NEWSGROUPS


7.6.5.1  Usage


   This command is optional.
   Syntax
      LIST NEWSGROUPS [wildmat]
   Responses
      215   Information follows (multiline)
   Parameters
      wildmat = groups of interest


7.6.5.2  Description


   The newsgroups list is maintained by some news transfer systems 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 (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.


   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.


   If the information is available, it is returned as a multi-line
   response following the 215 response code.  If 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 (whether or not
   a wildmat is specified) and indicates that there are no groups
   meeting the above criteria.


7.6.5.3  Examples


   Example of LIST NEWSGROUPS returning a list of newsgroups:
      [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] .


   Example of LIST NEWSGROUPS returning an error where the command is




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   recognized but the software does not maintain this information:
      [C] LIST NEWSGROUPS
      [S] 503 program error, function not performed


   Example of LIST NEWSGROUPS sent to a server that does not recognize
   this command:
      [C] LIST NEWSGROUPS
      [S] 501 Syntax error












































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8.  Framework for NNTP extensions


   Although NNTP is widely and robustly deployed, some parts of the
   Internet community might wish to extend the NNTP service.  This
   document defines a means whereby an extended NNTP client can query
   the server to determine the service extensions that it supports.


   It must be emphasized that any extension to the NNTP service should
   not be considered lightly.  NNTP's strength comes primarily from its
   simplicity.  Experience with many protocols has shown that:
      Protocols with few options tend towards ubiquity, whilst protocols
      with many options tend towards obscurity.
   This means that each and every extension, regardless of its benefits,
   must be carefully scrutinized with respect to its implementation,
   deployment, and interoperability costs.  In many cases, the cost of
   extending the NNTP service will likely outweigh the benefit.


   Given this environment, the framework for extensions described in
   this document consists of:
   o  a mechanism for clients to determine a server's available
      extensions
   o  a registry of NNTP service extensions


   The LIST EXTENSIONS command is described in this document (see
   Section 5.3) and is the mechanism for clients to use to determine
   what extensions are available.  Except where stated otherwise, the
   commands in this document are understood (even if not supported) by
   all servers and are not described in the list of features returned by
   the LIST EXTENSIONS command.


   The IANA shall maintain a registry of NNTP service extensions.


   An extension is identified by a unique extension-label, which is a
   string of 1 to 12 uppercase US-ASCII letters.  The extension-label
   will often be the name of a new command that the extension adds.
   However this is not a requirement: an extension might not add any new
   commands or keywords.


   An extension is either a private extension or else it is included in
   the IANA registry and is defined in an RFC (in which case it is a
   "standard extension" or "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:
   o  a descriptive name for the extension;
   o  the extension-label (which is returned by LIST EXTENSIONS to
      indicate to the client that the server supports this particular




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      extension) - the extension-label of a registered extension MUST
      NOT begin with "X";
   o  the syntax, values, and meanings of any arguments following the
      extension-label in the output of LIST EXTENSIONS;
   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  how support for the extension affects the behaviour of a server
      and NNTP client;
   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 this extension
      may have (if any);
   o  a specific statement about the circumstances when use of this
      extension can alter the output from LIST EXTENSIONS;
   o  the circumstances under which the extension can cause any
      pre-existing command to produce a 401, 480, or 483 response;
   o  whether the extension can be used before or after the MODE READER
      command, and what changes (if any) the latter has on the
      extension.


   A private extension need not be included in the output of LIST
   EXTENSIONS.  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, the names of private extensions and commands
   defined by them SHOULD begin with "X" and MUST NOT be the same as the
   name of a registered extension.


   If the server provides a registered extension (indicated by listing
   it in the output of LIST EXTENSIONS), it MUST implement all of the
   commands in the specification of the extension except for those
   marked as optional.  If it does not implement the extension as
   specified, it MUST NOT list the extension in the output of LIST
   EXTENSIONS under its registered name; in this 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 commands documented in registered
   extensions in response to the availability or use of a private




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   extension.


8.1  Initial IANA registry


   The IANA's initial registry of NNTP service extensions consists of
   these entries:


   +-------------------------+--------------+--------------------------+
   | Extension               | Label        | Added behaviour          |
   +-------------------------+--------------+--------------------------+
   | Specific article        | LISTGROUP    | Defined in this document |
   | numbers                 |              |                          |
   |                         |              |                          |
   | Overview support        | OVER         | Defined in this document |
   |                         |              |                          |
   | Batched header          | HDR          | Defined in this document |
   | retrieval               |              |                          |
   +-------------------------+--------------+--------------------------+



8.2  Standard extensions


   N.B.  while these extensions are standard extensions, the term
   includes all extensions in the IANA registry, not just these three.


   Each of the following sections describes an extension that a server
   MAY provide.  If the server provides the extension, it MUST include
   the appropriate extension label in the response to LIST EXTENSIONS.
   If it does not provide it, it MUST NOT include the appropriate
   extension label.  The descriptions of facilities in each section are
   written as if the extension is provided.  If it is not provided, the
   entire section should be ignored.


   The formal definitions of these extensions are provided in Appendix
   D.


8.3  The LISTGROUP extension


   This extension provides one command and has the extension label
   LISTGROUP.


8.3.1  LISTGROUP


8.3.1.1  Usage








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   Syntax
      LISTGROUP [group]
   Responses
      211 number low high group   Article numbers follow (multiline)
      411                         No such newsgroup
      412                         No newsgroup selected [1]
   Parameters
      group  = name of newsgroup
      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.


8.3.1.2  Description


   The LISTGROUP command is used to get a listing of all the article
   numbers in a particular newsgroup.  As a side effect, it also selects
   the group in the same way as the GROUP command (see Section 6.1.1).


   The optional argument is the name of the newsgroup to be selected
   (e.g.  "news.software.misc").  A list of valid newsgroups may be
   obtained from the LIST ACTIVE command.  If no group is specified, the
   current selected newsgroup is used.


   On success, the list of article numbers is returned as a multi-line
   response 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, is in numerical order, and lists
   precisely those articles that exist in the group.


   When a valid group is selected by means of this command, the current
   selected newsgroup MUST be set to that group and the current article
   number MUST be set to the first article in the group.  If an empty
   newsgroup is selected, the current article pointer is made invalid.
   If an invalid group is specified, the current selected newsgroup and
   current article number MUST NOT be changed.


   The LISTGROUP command MAY be used by a client as a replacement for
   the GROUP command in establishing a valid current selected newsgroup
   and current article number.


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


8.3.1.3  Examples


   Example of LISTGROUP on an empty group:




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


   Example of LISTGROUP on a valid current 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 failing because no group has been selected:
      [Assumes current selected newsgroup is invalid.]
      [C] LISTGROUP
      [S] 412 no current group
      [C] GROUP   example.is.sob.bradner.or.barber
      [S] 411 no such group
      [C] LISTGROUP
      [S] 412 no current group


8.4  Article metadata


   The OVER and HDR extensions refer to the concept of "article
   metadata".  This is data about articles that does not occur within
   the article itself.  Each metadata item has a name which MUST begin
   with a colon (and which 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-".




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8.4.1  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 "byte-stuffing" purposes).


   Note to client implementers: some existing servers return a value
   different to that above.  The commonest reasons for this are:
   o  counting a CRLF pair as one octet;
   o  including the "." character used for byte-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.


8.4.2  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).


8.5  The OVER extension


   This extension provides two commands, OVER and LIST OVERVIEW.FMT.
   The label for this extension is OVER.


   The OVER extension provides access to the "overview database", which
   is a database of headers extracted from incoming articles.  Only
   certain headers are included in the database.  The database also
   includes some article metadata.


   The information stored in the 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 records a metadata item for some articles but not 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




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   (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 which are not stored in the database.  Therefore if a
   header appears in the LIST OVERVIEW.FMT output but not the OVER
   output for a given article, that header does not appear in the
   article, and similarly for metadata items.


   These rules assume the fields being stored in the database remain
   constant for long periods of time, with the database therefore being
   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: 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.


   Support for the message-id form of the OVER command is optional.  If
   an implementation supports this form, it MUST use the argument
   "MSGID" following the extension label in the output of LIST
   EXTENSIONS; if not, it MUST NOT use any argument.


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


8.5.1  OVER


8.5.1.1  Usage


   Syntax
      OVER message-id
      OVER range
      OVER
   Responses
      First form (message-id specified)
         224   Overview information follows (multiline)
         430   No article with that message-id
      Second form (range specified)
         224   Overview information follows (multiline)
         412   No newsgroup selected
         423   No articles in that range
      Third form (current article number used)
         224   Overview information follows (multiline)
         412   No newsgroup selected
         420   Current article number is invalid







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   Parameters
      range      = number(s) of articles
      message-id = message-id of article


8.5.1.2  Description


   The OVER command returns the contents of the headers and metadata in
   the database for an article specified by message-id, or from a
   specified article or range of articles in the current 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.  Support
   for the first (message-id) form is optional.  If it is not supported,
   the generic response code 503 MUST be returned.


   If the information is available, it is returned as a multi-line
   response 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 only be
   one line but it will still be in multi-line format).  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 current selected group and the article is present in
   that group, the server MAY use that article number (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




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   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.


   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, or 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 current 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 current 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.


8.5.1.3  Examples


   In the first three 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 (using no article number):




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      [C] GROUP   misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] OVER
      [S] 224 Overview information follows
      [S] 300234|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] .


   Example of a successful retrieval of overview information for an
   article by message-id:
      [C] LIST EXTENSIONS
      [S] 202 extensions supported:
      [S] OVER MSGID
      [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] LIST EXTENSIONS
      [S] 202 extensions supported:
      [S] OVER
      [S] .
      [C] OVER   <45223423@example.com>
      [S] 503 Overview by message-id unsupported


   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] 300234|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|




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      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 field(s) 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 unsuccessful retrieval of overview information by
   number because no newsgroup was selected first:
      [Assumes current selected newsgroup is invalid.]
      [C] OVER
      [S] 412 No newsgroup selected


   Example of an attempt to retrieve information when the current
   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


8.5.2  LIST OVERVIEW.FMT


8.5.2.1  Usage


   This command is optional.
   Syntax
      LIST OVERVIEW.FMT
   Responses
      215   Information follows (multiline)


8.5.2.2  Description


   The LIST OVERVIEW.FMT command returns a description of the fields in
   the database for which it is consistent (as described above).


   If the information is available, it is returned as a multi-line
   response following the 215 response code.  The information contains
   one line per field in the order they are returned by the OVER
   command; the first 7 lines MUST (except for the case of letters) be
   exactly:





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


    except that, for compatibility with existing implementations, the
   last two lines MAY instead be:


       Bytes:
       Lines:


    even though they refer to metadata, not headers.


   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 used more than once in
   a session.


8.5.2.3  Examples


   Example of LIST OVERVIEW.FMT output corresponding to the example OVER
   output above, using 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] .


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




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      [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] .


   Example of LIST OVERVIEW.FMT returning an error where the command is
   recognized but the software does not maintain this information:
      [C] LIST OVERVIEW.FMT
      [S] 503 overview.fmt not available


8.6  The HDR extension


   This extension provides two new commands: HDR and LIST HEADERS.  The
   label for this extension is HDR.


   The HDR extension provides access to specific headers and metadata
   items (collectively "fields") of articles or groups of articles.  In
   the case of headers, an implementation MAY restrict the use of this
   extension to a specific list of headers or MAY allow it to be used
   with any header; it may behave differently when the HDR command is
   used with a message-id argument and when it is used with a range or
   no argument.


   The HDR command may take information from a database rather than
   directly from the articles.  If so, the same issues of consistency
   and inconsistency apply as with the OVER extension (Section 8.5) and
   the LIST HEADERS command SHOULD take the same approach as the LIST
   OVERVIEW.FMT command in resolving them.


8.6.1  HDR


8.6.1.1  Usage


   Syntax
      HDR header message-id
      HDR header range
      HDR header







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   Responses
      First form (message-id specified)
         225   Headers follow (multiline)
         430   No article with that message-id
      Second form (range specified)
         225   Headers follow (multiline)
         412   No newsgroup selected
         423   No articles in that range
      Third form (current article number used)
         225   Headers follow (multiline)
         412   No newsgroup selected
         420   Current article number is invalid
   Parameters
      header     = name of header, without the colon
      range      = number(s) of articles
      message-id = message-id of article


8.6.1.2  Description


   The HDR command retrieves specific headers from an article specified
   by message-id, or from a specified article or range of articles in
   the current selected newsgroup.  It can also return certain metadata
   about the article or articles.


   The required header argument is the name of a header (e.g.
   "subject") in an article, or the name of a metadata item, and is
   case-insensitive.  Names of metadata items always begin with a colon.
   Except where stated otherwise, metadata items are treated as if they
   were header contents, and references to headers in this description
   apply equally to metadata items.


   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
   response 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 at most one line but it
   will still be in multi-line format).  The line consists of the
   article number, a space, and then the contents of the header or
   metadata item.  In the case of a header, the header name, colon, and
   the first space after the colon are all omitted.





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   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 current selected group and the article is present in
   that group, the server MAY use that article number (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 extension
   (Section 8.5.1.2), and the same comment concerning NUL, CR, and LF
   applies).


   The header content is in all cases taken from the article.  This
   means that, for example, a request for the header "Lines" returns the
   contents of the "Lines" header of the specified articles, if any, not
   the line count metadata or any other server-generated value.  If the
   header occurs in a given article multiple times, only the content of
   the first occurrence is returned by HDR.


   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 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 current 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 current selected
   newsgroup, 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 headers and
   metadata items; it may behave differently in this respect for the
   first (message-id) form than for the other forms.  If so, it MUST
   respond with the generic 503 response to attempts to request other
   headers, rather than returning erroneous results such as a successful
   empty response.


   If HDR uses a separate database and it is inconsistent for the
   requested header or metadata item, 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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8.6.1.3  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 3000234-300238
      [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 3000234-300238
      [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] .


   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




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   Example of an unsuccessful retrieval of headers from articles by
   number because no newsgroup was selected first:
      [Assumes current selected newsgroup is invalid.]
      [C] HDR   subject 300256-
      [S] 412 No newsgroup selected


   Example of an unsuccessful retrieval of headers because the current
   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 3000234-300238
      [S] 503 HDR not permitted on Content-Type


8.6.2  LIST HEADERS


8.6.2.1  Usage


   Syntax
      LIST HEADERS [MSGID|RANGE]
   Responses
      215   Header and metadata list follows (multiline)
   Parameters
      MSGID = requests list for access by message-id
      RANGE = requests list for access by range


8.6.2.2  Description


   The LIST HEADERS command returns a list of headers and metadata items
   that may be retrieved using the HDR command.


   The information is returned as a multi-line response following the
   215 response code and contains one line for each header or metadata
   item name (excluding the colon in the former case).  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 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 extension).





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   An implementation that also supports the OVER extension 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 to the other two forms (range specified or
   current article number used) in respect of which headers or metadata
   items are available, then:
   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 always produce the same results and ignore any argument.


8.6.2.3  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 8.5.2.3:
      [C] LIST EXTENSIONS
      [S] 202 extensions supported:
      [S] OVER
      [S] HDR
      [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] .




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   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] .


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











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9.  Augmented BNF Syntax for NNTP


   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 [RFC2234] strings are
   case-insensitive.  Non-terminals used in several places are defined
   in a separate section at the end.


   The non-terminals "command-line", "command-continuation", and
   "response" between them specify the text that flows between client
   and server.  For each command, the sequence is:
   o  the client sends an instance of "command-line";
   o  the server sends an instance of "response";
   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 server sends an instance of "response".


9.1  Commands


   This syntax defines the non-terminal "command-line", which represents
   what is sent from the client to the server.


     command-line = command EOL
     command = article-command /
           body-command /
           date-command /
           group-command /
           hdr-command /
           head-command /
           help-command /
           ihave-command /
           last-command /
           list-active-command /
           list-active-times-command /
           list-distrib-pats-command /
           list-distributions-command /
           list-extensions-command /
           list-headers-command /
           list-newsgroups-command /
           list-overview-fmt-command /
           listgroup-command /
           mode-reader-command /
           newgroups-command /
           newnews-command /
           next-command /
           over-command /




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           post-command /
           quit-command /
           stat-command /
           x-command


     article-command = "ARTICLE" [article-ref]
     body-command = "BODY" [article-ref]
     date-command = "DATE"
     group-command = "GROUP" WS newsgroup-name
     hdr-command = "HDR" WS header-meta-name [range-ref]
     head-command = "HEAD" [article-ref]
     help-command = "HELP"
     ihave-command = "IHAVE" WS message-id
     last-command = "LAST"
     list-active-command = "LIST" [WS "ACTIVE" [WS wildmat]]
     list-active-times-command = "LIST" WS "ACTIVE.TIMES" [WS wildmat]
     list-distrib-pats-command = "LIST" WS "DISTRIB.PATS"
     list-distributions-command = "LIST" WS "DISTRIBUTIONS"
     list-extensions-command = "LIST" WS "EXTENSIONS"
     list-headers-command = "LIST" WS "HEADERS" WS ["MSGID" / "RANGE"]
     list-newsgroups-command = "LIST" WS "NEWSGROUPS" [WS wildmat]
     list-overview-fmt-command = "LIST" WS "OVERVIEW.FMT"
     listgroup-command = "LISTGROUP" [WS newsgroup-name]
     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]
     post-command = "POST"
     quit-command = "QUIT"
     stat-command = "STAT" [article-ref]
     x-command = x-command-name *(WS x-argument)
         ; This is the generic syntax for an extension command.
         ; Each extension command is specified fully elsewhere


     article-ref = WS (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
     metadata-name = ":" 1*A-NOTCOLON
     range = article-number ["-" [article-number]]
     range-ref = WS (range / message-id)
     time = 2DIGIT 2DIGIT 2DIGIT
     x-command-name = 3*12A-CHAR
     x-argument = 1*P-CHAR





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9.2  Command continuation


   This syntax defines the further material sent by the client in the
   case of multi-stage commands.


     command-continuation = ihave-continuation /
           post-continuation


     ihave-continuation = encoded-article
     post-continuation = encoded-article


     encoded-article = content-lines termination
       ; after undoing the "byte-stuffing", this MUST match "article"



9.3  Responses


9.3.1  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 / multiline-response
     multiline-response = simple-response content-lines termination


     simple-response =
           simple-response-content [SP trailing-comment] CRLF
     simple-response-content = 3DIGIT arguments
     trailing-comment = *U-CHAR
     arguments = *(SP argument) ; How many depends on the response
     argument = 1*A-CHAR




















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9.3.2  Initial response line contents


   This syntax defines the specific initial response lines for the
   various commands and extensions in this specification.  Only those
   response codes with arguments are listed.


     simple-response-content =/ response-111-content
           response-211-content
           response-22x-content
           response-401-content


     response-111-content = "111" SP date4y time
     response-211-content = "211" 3(SP article-number) SP newsgroup-name
     response-22x-content = ("220" / "221" / "222" / "223")
           SP article-number SP message-id
     response-401-content = "401" SP extension-label



9.3.3  Multi-line response contents


   This syntax defines the content of the various multi-line responses
   (more precisely, the part of the response in "content-lines"), in
   each case after any "byte-stuffing" has been undone.


     multiline-response-content = article-response /
           body-response /
           hdr-response /
           head-response /
           help-response /
           list-active-response /
           list-active-times-response /
           list-distrib-pats-response /
           list-distributions-response /
           list-extensions-response /
           list-headers-response /
           list-newsgroups-response /
           list-overview-fmt-response /
           listgroup-response /
           newgroups-response /
           newnews-response /
           over-response


     article-response = article
     body-response = body
     hdr-response = *(article-number SP hdr-content CRLF)
     head-response = 1*header
     help-response = *(*B-CHAR CRLF)
     list-active-response = *(newsgroup-name SPA article-number




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           SPA article-number SPA newsgroup-status CRLF)
     list-active-times-response =
           *(newsgroup-name SPA 1*DIGIT SPA newsgroup-creator CRLF)
     list-distrib-pats-response =
           *(1*DIGIT ":" wildmat ":" distribution CRLF)
     list-distributions-response =
           *(distribution SPA distribution-description CRLF)
     list-extensions-response =
           *(extension-descriptor CRLF)
     list-headers-response = *(header-meta-name CRLF) /
           *((metadata-name / ":") CRLF)
     list-newsgroups-response =
           *(newsgroup-name WS newsgroup-description CRLF)
     list-overview-fmt-response = list-overview-fmt-text
     listgroup-response = *(article-number CRLF)
     newgroups-response = list-active-response
     newnews-response = *(message-id CRLF)
     over-response = *(article-number over-content CRLF)


     list-overview-fmt-text =
           "Subject:" CRLF
           "From:" CRLF
           "Date:" CRLF
           "Message-ID:" CRLF
           "References:" CRLF
           ( ":bytes" CRLF ":lines" / "Bytes:" CRLF "Lines:") CRLF
           *((header-name ":full" / metadata-name) CRLF)


     distribution = 1*P-CHAR
     distribution-description = U-TEXT
     hdr-content = *S-NONTAB
     hdr-n-content = [(header-name ":" / metadata-name) SP hdr-content]
     newsgroup-creator = U-TEXT
     newsgroup-description = S-TEXT
     newsgroup-status = %x79 / %x6E / %x6D / private-status
     over-content = 1*6(TAB hdr-content) /
           7(TAB hdr-content) *(TAB hdr-n-content)
     private-status = 1*P-CHAR ; except the values in newsgroup-status














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9.4  LIST EXTENSIONS responses


   This syntax defines the generic form of a LIST EXTENSIONS response
   line.


     extension-argument = 1*P-CHAR
     extension-descriptor = extension-generic-descriptor
     extension-generic-descriptor =
           extension-label *(SPA extension-argument)
     extension-label = 1*12UPPER


   This syntax defines the specific LIST EXTENSIONS response lines for
   the various extensions in this specification.


     extension-descriptor =/ hdr-extension /
           listgroup-extension /
           over-extension


     hdr-extension = %x48.44.52                         ; "HDR"
     listgroup-extension = %x4C.49.53.54.47.52.4F.55.50 ; "LISTGROUP"
     over-extension = %x4F.56.45.52 [SPA "MSGID"]       ; "OVER"



9.5  Articles


   This syntax defines the non-terminal "article", which represents the
   format of an article as described in Section 3.4.


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



9.6  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.


     article-number = 1*16DIGIT
     content-lines = *([content-text] CRLF)
     content-text = (".." / B-NONDOT) *B-CHAR
     header-name = 1*A-NOTCOLON
     message-id = "<" 1*248A-NOTGT ">"
     newsgroup-name = 1*wildmat-exact
     termination = "." CRLF




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     wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
     wildmat-pattern = 1*wildmat-item
     wildmat-item = wildmat-exact / wildmat-wild
     wildmat-exact = %x21-29 / %x2B / %x2D-3E / %x40-5A / %x5E-7E /
          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
     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)





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   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.


   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


   Implementations SHOULD only generate content that meets this syntax:


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




































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10.  IANA Considerations


   This specification requires IANA to keep a registry of
   extension-labels.  The initial contents of this registry are
   specified in Section 8.1.  As described in Section 8, names 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
   extension-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.



































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11.  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.


11.1  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 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.


11.2  Abuse of Server Log Information


   A server is in the position to save session data about a user's
   requests that might identify their reading patterns or subjects of
   interest.  This information is clearly confidential in nature and its
   handling can be constrained by law in certain countries.  People
   using the NNTP protocol to provide data are responsible for ensuring
   that such material is not distributed without the permission of any
   individuals that are identifiable by the published results.


11.3  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 the NNTP protocol itself 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.





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11.4  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
   generally prone to security attacks based on the deliberate
   misassociation of IP addresses and DNS names.  Clients and servers
   need to be cautious in assuming the continuing validity of an IP
   number/DNS name association.


   In particular, NNTP clients and servers SHOULD rely on their name
   resolver for confirmation of an IP number/DNS name association,
   rather than caching the result of previous host name lookups.  Many
   platforms already can cache host name lookups locally when
   appropriate, and they SHOULD be configured to do so.  It is proper
   for these lookups to be cached, however, only when the TTL (Time To
   Live) information reported by the name server makes it likely that
   the cached information will remain useful.


   If NNTP clients or servers cache the results of host name lookups in
   order to achieve a performance improvement, they MUST observe the TTL
   information reported by DNS.  If NNTP clients or servers do not
   observe this rule, they could be spoofed when a previously accessed
   server's IP address changes.  As network renumbering is expected to
   become increasingly common, the possibility of this form of attack
   will grow.  Observing this requirement thus reduces this potential
   security vulnerability.


   This requirement also improves the load-balancing 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.


11.5  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:
   o  generating a 501 response code.





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   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);
   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 latter 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 close the
   connection if it is no longer possible to parse it sensibly.


11.6  Caching of LIST EXTENSIONS results


   The LIST EXTENSIONS command provides information about the extensions
   currently available from 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 results from this command in a given server
   state will not change from session to session; 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 which permits the use of cleartext
   passwords on links that are encrypted but not otherwise:
      [Initial TCP connection set-up completed.]
      [S] 200 NNTP Service Ready, posting permitted
      [C] LIST EXTENSIONS
      [S] 202 Extensions supported:
      [S] XENCRYPT
      [S] .
      [C] XENCRYPT
      [Client and server negotiate encryption on the link]
      [S] 283 Encrypted link established
      [C] LIST EXTENSIONS
      [S] 202 Extensions supported:




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      [S] XSECRET
      [S] .
      [C] XSECRET   fred flintstone
      [S] 290 Password for fred accepted


   If the client caches the last LIST EXTENSIONS result, then on the
   next session it will attempt to use XSECRET on an unencrypted link:
      [Initial TCP connection set-up completed.]
      [S] 200 NNTP Service Ready, posting permitted
      [C] XSECRET   fred flintstone
      [S] 483 Only permitted on secure links
   exposing 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 LIST EXTENSIONS results 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.






























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12.  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:
   o  The work of the NNTP committee chaired by Eliot Lear.  The
      organization of this document was influenced by the last available
      draft from this working group.  A special thanks to Eliot for
      generously providing the original machine-readable sources for
      that document.
   o  The work of the DRUMS working group, specifically RFC 1869
      [RFC1869], which is the basis of the NNTP 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, which includes those listed below:
   Rick Adams
      Original author of the NNTP extensions to the RN news reader and
      last maintainer of Bnews
   Stan Barber
      Original author of the NNTP extensions to the news readers that
      are part of Bnews
   Geoff Collyer
      Original author of the OVERVIEW database proposal and one of the
      original authors of CNEWS
   Dan Curry
      Original author of the xvnews news reader






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   Wayne Davison
      Author of the first threading extensions to the RN news reader
      (commonly called TRN)
   Geoff Huston
      Original author of ANU NEWS
   Phil Lapsey
      Original author of the UNIX reference implementation for NNTP
   Iain Lea
      Original maintainer of the TIN news reader
   Chris Lewis
      First known implementer of the AUTHINFO GENERIC extension
   Rich Salz
      Original author of INN
   Henry Spencer
      One of the original authors of CNEWS
   Kim Storm
      Original author of the NN news reader


   Other people who contributed to this document include:


      Matthias Andree
      Greg Andruk
      Maurizio Codogno
      Andrew Gierth
      Juergen Helbing
      Scott Hollenbeck
      Charles Lindsey
      Ade Lovett
      Ken Murchinson
      Francois Petillon
      Peter Robinson
      Rob Siemborski
      Howard Swinehart
      Ruud van Tol
      Jeffrey Vinocur


   The author thanks them all and apologises to anyone omitted.


   Finally, the present author gratefully acknowledges the vast amount
   of work put into previous drafts by the previous author:
      Stan Barber <sob@academ.com>











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13.  References


13.1  Normative References


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


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


   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.


   [RFC3548]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.


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


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


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


13.2  Informative References


   [NNTP-AUTH]
              Vinocur, J., Newman, C. and K. Murchinson, "NNTP
              Authentication", draft-ietf-nntpext-authinfo-02 (work in
              progress), July 2004.


   [NNTP-TLS]
              Vinocur, J. and C. Newman, "Using TLS with NNTP",
              draft-ietf-nntpext-tls-nntp-01 (work in progress), October
              2003.


   [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", RFC 1305, March 1992.


   [RFC1869]  Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.




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              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              November 1995.


   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Nielsen, 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.



Author's Address


   Clive D.W. Feather
   Thus plc
   322 Regents Park Road
   London  N3 2QQ
   GB


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








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Appendix A.  Future Directions


   It has been proposed that the response code range 6xx be used for
   multiline responses.  While existing commands and extensions do not
   use this, it would at least limit the problem clients would face in
   dealing with an unknown response.














































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Appendix B.  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.


B.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 that in Section 9.5.


     header = header-name ":" SP [header-content] CRLF
     header-content = [WS] 1*P-CHAR *( [CRLF] WS 1*P-CHAR )



B.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 of old articles being reinjected
   into the global Usenet system, RFC 1036 [RFC1036] requires that
   message-ids are globally unique for all time.


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




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      <abcd@example.com>
      <"abcd"@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 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 draft
   successor 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 based on the date and time that the article
   arrived.  When handling email or Netnews articles, a Message-ID
   header SHOULD be added to ensure global consistency and uniqueness.


B.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 have already done any
   necessary validation (or has in turn obtained the article from a




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   third party which 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, 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, fold or limit
   lines, or otherwise process the incoming text.


   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 being duplicates.  In the case of
   email or Netnews articles, therefore, the posted article SHOULD
   contain a header with 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 the same message-id allocated, whether or
   not those contents are suitable for use as the message-id.




























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Appendix C.  Summary of Response Codes


   This section contains a list of every response code defined in this
   document, 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 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 202 (multi-line)
      Generated by: LIST EXTENSIONS
      Meaning: extension list follows.
   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:
         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.
   Response code 215 (multi-line)
      Generated by: LIST ACTIVE, LIST ACTIVE.TIMES, LIST DISTRIB.PATS,
      LIST DISTRIBUTIONS, LIST HEADERS, LIST NEWSGROUPS,
      LIST OVERVIEW.FMT
      Meaning: information follows.
   Response code 220 (multi-line)
      Generated by: ARTICLE
      2 arguments: n message-id
      Meaning: article follows.







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   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.
   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: extension-label
      Meaning: the server is in the wrong mode; the indicated extension
      should be used to change the mode.







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   Response code 402
      Generated by: LIST EXTENSIONS
      Meaning: server has no extensions.
   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, 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.
   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.




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   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.
   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 [RFC3548] of an argument





























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Appendix D.  Formal specification of the standard extensions


   This section gives a formal definition of each of the extensions in
   Section 8.2 as required by Section 8 for the IANA registry.


D.1  The LISTGROUP extension


   o  This extension provides information about specific article
      numbers.
   o  The extension-label is "LISTGROUP".
   o  The extension-label has no arguments.
   o  The extension defines one new command: LISTGROUP, whose behaviour,
      arguments, and responses are defined in Section 8.3.
   o  The extension does not associate any new responses with
      pre-existing NNTP commands.
   o  The extension does not affect the behaviour of a server or client
      other than via the new command.
   o  The extension does not affect the maximum length of commands and
      initial response lines.
   o  The extension does not alter pipelining, and the LISTGROUP command
      can be pipelined.
   o  Use of this extension does not alter the output from LIST
      EXTENSIONS.
   o  The extension does not cause any pre-existing command to produce a
      401, 480, or 483 response.
   o  The LISTGROUP command can only be used after the MODE READER
      command.


D.2  The OVER extension


   o  This extension provides support for an overview of newsgroups.
   o  The extension-label is "OVER".
   o  The extension-label has the optional argument "MSGID", indicating
      that the message-id variant of the OVER command is supported.
   o  The extension defines two new commands: OVER and LIST
      OVERVIEW.FMT, whose behaviour, arguments, and responses are
      defined in Section 8.5.
   o  The extension does not associate any new responses with
      pre-existing NNTP commands.
   o  The extension requires the server to maintain an overview database
      and article metadata, as described in Section 8.4.
   o  The extension does not affect the maximum length of commands and
      initial response lines.
   o  The extension does not alter pipelining, and the OVER and LIST
      OVERVIEW.FMT commands can be pipelined.
   o  Use of this extension does not alter the output from LIST
      EXTENSIONS.





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   o  The extension does not cause any pre-existing command to produce a
      401, 480, or 483 response.
   o  The OVER and LIST OVERVIEW.FMT commands can only be used after the
      MODE READER command.


D.3  The HDR extension


   o  This extension provides batched header retrieval.
   o  The extension-label is "HDR".
   o  The extension-label has no arguments.
   o  The extension defines two new commands: HDR and LIST HEADERS,
      whose behaviour, arguments, and responses are defined in Section
      8.6.
   o  The extension does not associate any new responses with
      pre-existing NNTP commands.
   o  The extension requires the server to maintain article metadata, as
      described in Section 8.4.
   o  The extension does not affect the maximum length of commands and
      initial response lines.
   o  The extension does not alter pipelining, and the HDR and LIST
      HEADERS commands can be pipelined.
   o  Use of this extension does not alter the output from LIST
      EXTENSIONS.
   o  The extension does not cause any pre-existing command to produce a
      401, 480, or 483 response.
   o  The HDR and LIST HEADERS commands can only be used after the MODE
      READER command.

























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