Network News Transfer Protocol (NNTP)

Status of This Memo

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

Copyright Notice

Copyright © The Internet Society (2006).

Abstract

The Network News Transfer Protocol (NNTP) has been in use in the Internet for a decade, and remains one of the most popular protocols (by volume) in use today. This document is a replacement for RFC 977, and officially updates the protocol specification. It clarifies some vagueness in RFC 977, includes some new base functionality, and provides a specific mechanism to add standardized extensions to NNTP.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Author's Note . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Basic Concepts  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Commands and Responses  . . . . . . . . . . . . . . . . .  6
       3.1.1.  Multi-line Data Blocks . . . . . . . . . . . . . . . . 8
     3.2.  Response Codes  . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Generic Response Codes  . . . . . . . . . . . . . . . 10
         3.2.1.1.  Examples  . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Capabilities and Extensions . . . . . . . . . . . . . . . 14
       3.3.1.  Capability Descriptions . . . . . . . . . . . . . . . 14
       3.3.2.  Standard Capabilities . . . . . . . . . . . . . . . . 15
       3.3.3.  Extensions  . . . . . . . . . . . . . . . . . . . . . 16
       3.3.4.  Initial IANA Register . . . . . . . . . . . . . . . . 18
     3.4.  Mandatory and Optional Commands . . . . . . . . . . . . . 20
       3.4.1.  Reading and Transit Servers . . . . . . . . . . . . . 21
       3.4.2.  Mode Switching  . . . . . . . . . . . . . . . . . . . 21
     3.5.  Pipelining  . . . . . . . . . . . . . . . . . . . . . . . 22
       3.5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . 23
     3.6.  Articles  . . . . . . . . . . . . . . . . . . . . . . . . 24
   4.  The WILDMAT Format  . . . . . . . . . . . . . . . . . . . . . 25
     4.1.  Wildmat Syntax  . . . . . . . . . . . . . . . . . . . . . 26
     4.2.  Wildmat Semantics . . . . . . . . . . . . . . . . . . . . 26
     4.3.  Extensions  . . . . . . . . . . . . . . . . . . . . . . . 27
     4.4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . 27
   5.  Session Administration Commands . . . . . . . . . . . . . . . 28
     5.1.  Initial Connection  . . . . . . . . . . . . . . . . . . . 28
     5.2.  CAPABILITIES  . . . . . . . . . . . . . . . . . . . . . . 29
     5.3.  MODE READER . . . . . . . . . . . . . . . . . . . . . . . 32
     5.4.  QUIT  . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   6.  Article Posting and Retrieval . . . . . . . . . . . . . . . . 35
     6.1.  Group and Article Selection . . . . . . . . . . . . . . . 36
       6.1.1.  GROUP . . . . . . . . . . . . . . . . . . . . . . . . 36
       6.1.2.  LISTGROUP . . . . . . . . . . . . . . . . . . . . . . 39
       6.1.3.  LAST  . . . . . . . . . . . . . . . . . . . . . . . . 42
       6.1.4.  NEXT  . . . . . . . . . . . . . . . . . . . . . . . . 44
     6.2.  Retrieval of Articles and Article Sections  . . . . . . . 45
       6.2.1.  ARTICLE . . . . . . . . . . . . . . . . . . . . . . . 46
       6.2.2.  HEAD  . . . . . . . . . . . . . . . . . . . . . . . . 49
       6.2.3.  BODY  . . . . . . . . . . . . . . . . . . . . . . . . 51
       6.2.4.  STAT  . . . . . . . . . . . . . . . . . . . . . . . . 53
     6.3.  Article Posting . . . . . . . . . . . . . . . . . . . . . 56
       6.3.1.  POST  . . . . . . . . . . . . . . . . . . . . . . . . 56
       6.3.2.  IHAVE . . . . . . . . . . . . . . . . . . . . . . . . 58
   7.  Information Commands  . . . . . . . . . . . . . . . . . . . . 61
     7.1.  DATE  . . . . . . . . . . . . . . . . . . . . . . . . . . 61
     7.2.  HELP  . . . . . . . . . . . . . . . . . . . . . . . . . . 62
     7.3.  NEWGROUPS . . . . . . . . . . . . . . . . . . . . . . . . 63
     7.4.  NEWNEWS . . . . . . . . . . . . . . . . . . . . . . . . . 64
     7.5.  Time  . . . . . . . . . . . . . . . . . . . . . . . . . . 65
       7.5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . 66
     7.6.  The LIST Commands . . . . . . . . . . . . . . . . . . . . 66
       7.6.1.  LIST  . . . . . . . . . . . . . . . . . . . . . . . . 67
       7.6.2.  Standard LIST Keywords  . . . . . . . . . . . . . . . 69
       7.6.3.  LIST ACTIVE . . . . . . . . . . . . . . . . . . . . . 70
       7.6.4.  LIST ACTIVE.TIMES . . . . . . . . . . . . . . . . . . 71
       7.6.5.  LIST DISTRIB.PATS . . . . . . . . . . . . . . . . . . 72
       7.6.6.  LIST NEWSGROUPS . . . . . . . . . . . . . . . . . . . 73
   8.  Article Field Access Commands . . . . . . . . . . . . . . . . 73
     8.1.  Article Metadata  . . . . . . . . . . . . . . . . . . . . 74
       8.1.1.  The :bytes Metadata Item  . . . . . . . . . . . . . . 74
       8.1.2.  The :lines Metadata Item  . . . . . . . . . . . . . . 75
     8.2.  Database Consistency  . . . . . . . . . . . . . . . . . . 75
     8.3.  OVER  . . . . . . . . . . . . . . . . . . . . . . . . . . 76
     8.4.  LIST OVERVIEW.FMT . . . . . . . . . . . . . . . . . . . . 81
     8.5.  HDR . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
     8.6.  LIST HEADERS  . . . . . . . . . . . . . . . . . . . . . . 87
   9.  Augmented BNF Syntax for NNTP . . . . . . . . . . . . . . . . 90
     9.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . 90
     9.2.  Commands  . . . . . . . . . . . . . . . . . . . . . . . . 92
     9.3.  Command Continuation  . . . . . . . . . . . . . . . . . . 93
     9.4.  Responses . . . . . . . . . . . . . . . . . . . . . . . . 93
       9.4.1.  Generic Responses . . . . . . . . . . . . . . . . . . 93
       9.4.2.  Initial Response Line Contents  . . . . . . . . . . . 94
       9.4.3.  Multi-line Response Contents  . . . . . . . . . . . . 94
     9.5.  Capability Lines  . . . . . . . . . . . . . . . . . . . . 95
     9.6.  LIST Variants . . . . . . . . . . . . . . . . . . . . . . 96
     9.7.  Articles  . . . . . . . . . . . . . . . . . . . . . . . . 97
     9.8.  General Non-terminals . . . . . . . . . . . . . . . . . . 97
     9.9.  Extensions and Validation . . . . . . . . . . . . . . . . 99
   10. Internationalisation Considerations . . . . . . . . . . . . .100
     10.1. Introduction and Historical Situation . . . . . . . . . .100
     10.2. This Specification  . . . . . . . . . . . . . . . . . . .101
     10.3. Outstanding Issues  . . . . . . . . . . . . . . . . . . .102
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .103
   12. Security Considerations . . . . . . . . . . . . . . . . . . .103
     12.1. Personal and Proprietary Information  . . . . . . . . . .104
     12.2. Abuse of Server Log Information . . . . . . . . . . . . .104
     12.3. Weak Authentication and Access Control  . . . . . . . . .104
     12.4. DNS Spoofing  . . . . . . . . . . . . . . . . . . . . . .104
     12.5. UTF-8 Issues  . . . . . . . . . . . . . . . . . . . . . .105
     12.6. Caching of Capability Lists . . . . . . . . . . . . . . .106
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .107
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .110
     14.1. Normative References  . . . . . . . . . . . . . . . . . .110
     14.2. Informative References  . . . . . . . . . . . . . . . . .110
   A.  Interaction with Other Specifications . . . . . . . . . . . .112
     A.1.  Header Folding  . . . . . . . . . . . . . . . . . . . . .112
     A.2.  Message-IDs . . . . . . . . . . . . . . . . . . . . . . .112
     A.3.  Article Posting . . . . . . . . . . . . . . . . . . . . .114
   B.  Summary of Commands . . . . . . . . . . . . . . . . . . . . .115
   C.  Summary of Response Codes . . . . . . . . . . . . . . . . . .117
   D.  Changes from RFC 977  . . . . . . . . . . . . . . . . . . . .121

1. Introduction

This document specifies the Network News Transfer Protocol (NNTP), which is used for the distribution, inquiry, retrieval, and posting of Netnews articles using a reliable stream-based mechanism. For news-reading clients, NNTP enables retrieval of news articles that are stored in a central database, giving subscribers the ability to select only those articles they wish to read.

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

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

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

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

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

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

An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for this protocol. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST requirements but not all the SHOULD requirements for NNTP is said to be "conditionally compliant".

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

1.1. Author's Note

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

No perl was used in producing this document.

2. Notation

The following notational conventions are used in this document.

     UPPERCASE     indicates literal text to be included in the
                   command.
     
     lowercase     indicates a token described elsewhere.
     
     [brackets]    indicate that the enclosed material is optional.
     
     elliptical    indicates that the argument may be repeated any
     ... marks     number of times (it must occur at least once).
     
     vertical|bar  indicates a choice of two mutually exclusive
                   arguments (exactly one must be provided).

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

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

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

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

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

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

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

3. Basic Concepts

3.1. Commands and Responses

NNTP operates over any reliable bi-directional 8-bit-wide data stream channel. When the connection is established, the NNTP server host MUST send a greeting. The client host and server host then exchange commands and responses (respectively) until the connection is closed or aborted. If the connection used is TCP, then the server host starts the NNTP service by listening on a TCP port. When a client host wishes to make use of the service, it MUST establish a TCP connection with the server host by connecting to that host on the same port on which the server is listening.

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

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

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

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

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

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

Each response MUST start with a three-digit response code that is sufficient to distinguish all responses. Certain valid responses are defined to be multi-line; for all others, the response is contained in a single line. The initial line of the response MUST NOT exceed 512 octets, which includes the response code and the terminating CRLF pair; an extension MAY specify a greater maximum for commands that it defines, but not for any other command. Single-line responses consist of an initial line only. Multi-line responses consist of an initial line followed by a multi-line data block.

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

3.1.1. Multi-line Data Blocks

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

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

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

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

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

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

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

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

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

3.2. Response Codes

Each response MUST begin with a three-digit status indicator. These are status reports from the server and indicate the response to the last command received from the client.

The first digit of the response broadly indicates the success, failure, or progress of the previous command:

1xx - Informative message
2xx - Command completed OK
3xx - Command OK so far; send the rest of it
4xx - Command was syntactically correct but failed for some reason 5xx - Command unknown, unsupported, unavailable, or syntax error

The next digit in the code indicates the function response category:

x0x - Connection, setup, and miscellaneous messages
x1x - Newsgroup selection
x2x - Article selection
x3x - Distribution functions
x4x - Posting
x8x - Reserved for authentication and privacy extensions x9x - Reserved for private use (non-standard extensions)

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

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

The server MUST respond to any command with the appropriate generic response (given in Section 3.2.1) if it represents the situation. Otherwise, each recognized command MUST return one of the response codes specifically listed in its description or in an extension. A server MAY provide extensions to this specification, including new commands, new variants or features of existing commands, and other ways of changing the internal state of the server. However, the server MUST NOT produce any other responses to a client that does not invoke any of the additional features. (Therefore, a client that restricts itself to this specification will only receive the responses that are listed.)

If a client receives an unexpected response, it SHOULD use the first digit of the response to determine the result. For example, an unexpected 2xx should be taken as success, and an unexpected 4xx or 5xx as failure.

Response codes not specified in this document MAY be used for any installation-specific additional commands also not specified. These SHOULD be chosen to fit the pattern of x9x specified above.

Neither this document nor any registered extension (see Section 3.3.3) will specify any response codes of the x9x pattern. (Implementers of extensions are accordingly cautioned not to use such responses for extensions that may subsequently be submitted for registration.)

3.2.1. Generic Response Codes

The server MUST respond to any command with the appropriate one of the following generic responses if it represents the situation.

If the command is not recognized, or if it is an optional command that is not implemented by the server, the response code 500 MUST be returned.

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

If an argument is required to be a base64-encoded string [RFC4648] (there are no such arguments in this specification, but there may be in extensions) and is not validly encoded, the response code 504 MUST be returned.

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

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

502: It is necessary to terminate the connection and to start a new

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

480: The client must authenticate itself to the server (that is, it

must provide information as to the identity of the client) before the facility can be used on this connection. This will involve the use of an authentication extension such as [NNTP-AUTH].

483: The client must negotiate appropriate privacy protection on the

connection. This will involve the use of a privacy extension such as [NNTP-TLS].

401: The client must change the state of the connection in some other

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

If the server has to terminate the connection for some reason, it MUST give a 400 response code to the next command and then immediately close the connection. Following a 400 response, clients SHOULD NOT simply reconnect immediately and retry the same actions. Rather, a client SHOULD either use an exponentially increasing delay between retries (e.g., double the waiting time after each 400 response) or present any associated text to the user for them to decide whether and when to retry.

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

3.2.1.1. Examples

Example of an unknown command:

      [C] MAIL
      [S] 500 Unknown command

Example of an unsupported command:

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

Example of an unsupported variant:

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

Example of a syntax error:

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

Example of an overlong command line:

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

Example of a bad wildmat:

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

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

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

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

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

Example of an attempt to access a facility requiring authentication:

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

Example of a successful attempt following such authentication:

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

Example of an attempt to access a facility requiring privacy:

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

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

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

Example of a temporary failure:

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

Example of the server needing to close down immediately:

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

3.3. Capabilities and Extensions

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

3.3.1. Capability Descriptions

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

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

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

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

The server MUST ensure that the capability list accurately reflects the capabilities (including extensions) currently available. If a capability is only available with the server in a certain state (for example, only after authentication), the list MUST only include the capability label when the server is in that state. Similarly, if only some of the commands in an extension will be available, or if the behaviour of the extension will change in some other manner, according to the state of the server, this MUST be indicated by different arguments in the capability line.

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

3.3.2. Standard Capabilities

The following capabilities are defined by this specification.

VERSION

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

The version number of this specification is 2.

READER

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

IHAVE

This capability indicates that the server implements the IHAVE command.

POST

This capability indicates that the server implements the POST command.

NEWNEWS

This capability indicates that the server implements the NEWNEWS command.

HDR

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

OVER

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

LIST

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

IMPLEMENTATION

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

MODE-READER

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

3.3.3. Extensions

Although NNTP is widely and robustly deployed, some parts of the Internet community might wish to extend the NNTP service. It must be emphasized that any extension to NNTP should not be considered lightly. NNTP's strength comes primarily from its simplicity. Experience with many protocols has shown that:

Protocols with few options tend towards ubiquity, whilst protocols with many options tend towards obscurity.

This means that each and every extension, regardless of its benefits, must be carefully scrutinized with respect to its implementation, deployment, and interoperability costs. In many cases, the cost of extending the NNTP service will likely outweigh the benefit.

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

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

The definition of an extension must include the following:

• a descriptive name for the extension.

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

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

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

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

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

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

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

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

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

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

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

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

• Formal syntax as described in Section 9.9.

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

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

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

3.3.4. Initial IANA Register

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

The initial content of the registry consists of these entries:

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

3.4. Mandatory and Optional Commands

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

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

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

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

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

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

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

3.4.1. Reading and Transit Servers

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

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

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

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

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

3.4.2. Mode Switching

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

A mode-switching server has two modes:

• Transit mode, which applies after the initial connection.

• It MUST advertise the MODE-READER capability.

• It MUST NOT advertise the READER capability.

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

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

• It MUST NOT advertise the MODE-READER capability.

• It MUST advertise the READER capability.

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

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

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

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

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

3.5. Pipelining

NNTP is designed to operate over a reliable bi-directional connection, such as TCP. Therefore, if a command does not depend on the response to the previous one, it should not matter if it is sent before that response is received. Doing this is called "pipelining". However, certain server implementations throw away all text received from the client following certain commands before sending their response. If this happens, pipelining will be affected because one or more commands will have been ignored or misinterpreted, and the client will be matching the wrong responses to each command. Since there are significant benefits to pipelining, but also circumstances where it is reasonable or common for servers to behave in the above manner, this document puts certain requirements on both clients and servers.

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

If the specific description of a command says it "MUST NOT be pipelined", that command MUST end any pipeline of commands. That is, the client MUST NOT send any following command until it receives the CRLF at the end of the response from the command. The server MAY ignore any data received after the command and before the CRLF at the end of the response is sent to the client.

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

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

3.5.1. Examples

Example of correct use of pipelining:

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

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

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

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

3.6. Articles

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

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

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

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

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

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

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

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

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

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

4. The WILDMAT Format

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

4.1. Wildmat Syntax

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

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

Note: the characters ",", "\", "[", and "]" are not allowed in wildmats, while * and ? are always wildcards. This should not be a problem, since these characters cannot occur in newsgroup names, which is the only current use of wildmats. Backslash is commonly used to suppress the special meaning of characters, whereas brackets are used to introduce sets. However, these usages are not universal, and interpretation of these characters in the context of UTF-8 strings is potentially complex and differs from existing practice, so they were omitted from this specification. A future extension to this specification may provide semantics for these characters.

4.2. Wildmat Semantics

A wildmat is tested against a string and either matches or does not match. To do this, each constituent <wildmat-pattern> is matched against the string, and the rightmost pattern that matches is identified. If that <wildmat-pattern> is not preceded with "!", the whole wildmat matches. If it is preceded by "!", or if no <wildmat- pattern> matches, the whole wildmat does not match.

For example, consider the wildmat "a*,!*b,*c*":

• 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.

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

5. Session Administration Commands

5.1. Initial Connection

5.1.1. Usage

This command MUST NOT be pipelined.

Responses [1]

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

[1] These are the only valid response codes for the initial greeting;

the server MUST not return any other generic response code.

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

close the connection.

5.1.2. Description

There is no command presented by the client upon initial connection to the server. The server MUST present an appropriate response code as a greeting to the client. This response informs the client whether service is available and whether the client is permitted to post.

If the server will accept further commands from the client including POST, the server MUST present a 200 greeting code. If the server will accept further commands from the client, but the client is not authorized to post articles using the POST command, the server MUST present a 201 greeting code.

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

Note: the distinction between the 200 and 201 response codes has turned out in practice to be insufficient; for example, some servers do not allow posting until the client has authenticated, while other clients assume that a 201 response means that posting will never be possible even after authentication. Therefore, clients SHOULD use the CAPABILITIES command (Section 5.2) rather than rely on this response.

5.1.3. Examples

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

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

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

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

Example of a normal connection from an unauthorized client:

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

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

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

5.2. CAPABILITIES

5.2.1. Usage

This command is mandatory.

Syntax

     CAPABILITIES [keyword]

Responses

     101    Capability list follows (multi-line)

Parameters

     keyword    additional feature, see description

5.2.2. Description

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

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

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

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

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

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

5.2.3. Examples

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

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

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

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

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

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

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

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

5.3. MODE READER

5.3.1. Usage

   Indicating capability: MODE-READER

This command MUST NOT be pipelined.

Syntax

     MODE READER

Responses

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

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

connection.

5.3.2. Description

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

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

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

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

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

5.3.3. Examples

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

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

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

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

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

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

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

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

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

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

5.4. QUIT

5.4.1. Usage

This command is mandatory.

Syntax

     QUIT

Responses

     205    Connection closing

5.4.2. Description

The client uses the QUIT command to terminate the session. The server MUST acknowledge the QUIT command and then close the connection to the client. This is the preferred method for a client to indicate that it has finished all of its transactions with the NNTP server.

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

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

5.4.3. Examples

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

6. Article Posting and Retrieval

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

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

Article numbers MUST lie between 1 and 2,147,483,647, inclusive. The client and server MAY use leading zeroes in specifying article numbers but MUST NOT use more than 16 digits. In some situations, the value zero replaces an article number to show some special situation.

Note that it is likely that the article number limit of 2,147,483,647 will be increased by a future revision or extension to this specification. While servers MUST NOT send article numbers greater than this current limit, client and server developers are advised to use internal structures and datatypes capable of handling larger values in anticipation of such a change.

6.1. Group and Article Selection

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

6.1.1. GROUP

6.1.1.1. Usage

   Indicating capability: READER

Syntax

     GROUP group

Responses

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

Parameters

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

6.1.1.2. Description

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

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

The successful selection response will return the article numbers of the first and last articles in the group at the moment of selection (these numbers are referred to as the "reported low water mark" and the "reported high water mark") and an estimate of the number of articles in the group currently available.

If the group is not empty, the estimate MUST be at least the actual number of articles available and MUST be no greater than one more than the difference between the reported low and high water marks. (Some implementations will actually count the number of articles 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.

• The high water mark will be one less than the low water mark, and the estimated article count will be zero. Servers SHOULD use this method to show an empty group. This is the only time that the high water mark can be less than the low water mark.

• All three numbers will be zero.

• The high water mark is greater than or equal to the low water mark. The estimated article count might be zero or non-zero; if it is non-zero, the same requirements apply as for a non-empty group.

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

• Articles may be removed from the group.

• Articles may be reinstated in the group with the same article number, but those articles MUST have numbers no less than the reported low water mark (note that this is a reinstatement of the previous article, not a new article reusing the number).

• New articles may be added with article numbers greater than the reported high water mark. (If an article that was the one with the highest number has been removed and the high water mark has been adjusted accordingly, the next new article will not have the number one greater than the reported high water mark.)

Except when the group is empty and all three numbers are zero, whenever a subsequent GROUP command for the same newsgroup is issued, either by the same client or a different client, the reported low water mark in the response MUST be no less than that in any previous response for that newsgroup in this session, and it SHOULD be no less than that in any previous response for that newsgroup ever sent to any client. Any failure to meet the latter condition SHOULD be transient only. The client may make use of the low water mark to remove all remembered information about articles with lower numbers, as these will never recur. This includes the situation when the high water mark is one less than the low water mark. No similar assumption can be made about the high water mark, as this can decrease if an article is removed and then increase again if it is reinstated or if new articles arrive.

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

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

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

6.1.1.3. Examples

Example for a group known to the server:

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test

Example for a group unknown to the server:

      [C] GROUP example.is.sob.bradner.or.barber
      [S] 411 example.is.sob.bradner.or.barber is unknown

Example of an empty group using the preferred response:

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

Example of an empty group using an alternative response:

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

Example of an empty group using a different alternative response:

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

Example reselecting the currently selected newsgroup:

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

6.1.2. LISTGROUP

6.1.2.1. Usage

   Indicating capability: READER

Syntax

     LISTGROUP [group [range]]

Responses

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

Parameters

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

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

6.1.2.2. Description

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

On success, a list of article numbers is returned as a multi-line data block following the 211 response code (the arguments on the initial response line are the same as for the GROUP command). The list contains one number per line and is in numerical order. It lists precisely those articles that exist in the group at the moment of selection (therefore, an empty group produces an empty list). If the optional range argument is specified, only articles within the range are included in the list (therefore, the list MAY be empty even if the group is not).

The range argument may be any of the following:

• An article number.

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

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

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

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

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

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

6.1.2.3. Examples

Example of LISTGROUP being used to select a group:

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

Example of LISTGROUP on an empty group:

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

Example of LISTGROUP on a valid, currently selected newsgroup:

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

Example of LISTGROUP with a range:

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

Example of LISTGROUP with an empty range:

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

Example of LISTGROUP with an invalid range:

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

6.1.3. LAST

6.1.3.1. Usage

   Indicating capability: READER

Syntax

     LAST

Responses

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

Parameters

     n             Article number
     message-id    Article message-id

6.1.3.2. Description

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

There MAY be no previous article in the group, although the current article number is not the reported low water mark. There MUST NOT be a previous article when the current article number is the reported low water mark.

Because articles can be removed and added, the results of multiple LAST and NEXT commands MAY not be consistent over the life of a particular NNTP session.

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

6.1.3.3. Examples

Example of a successful article retrieval using LAST:

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

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

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

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

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] LAST
      [S] 422 No previous article to retrieve

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

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

6.1.4. NEXT

6.1.4.1. Usage

   Indicating capability: READER

Syntax

     NEXT

Responses

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

Parameters

     n             Article number
     message-id    Article message-id

6.1.4.2. Description

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

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

6.1.4.3. Examples

Example of a successful article retrieval using NEXT:

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

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

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

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

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

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

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

6.2. Retrieval of Articles and Article Sections

The ARTICLE, BODY, HEAD, and STAT commands are very similar. They differ only in the parts of the article that are presented to the client and in the successful response code. The ARTICLE command is described here in full, while the other three commands are described in terms of the differences. As specified in Section 3.6, an article consists of two parts: the article headers and the article body.

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

6.2.1. ARTICLE

6.2.1.1. Usage

   Indicating capability: READER

Syntax

ARTICLE message-id
ARTICLE number
ARTICLE

   Responses

First form (message-id specified)

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

Second form (article number specified)

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

Third form (current article number used)

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

Parameters

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

6.2.1.2. Description

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

In the first form, a message-id is specified, and the server presents the article with that message-id. In this case, the server MUST NOT alter the currently selected newsgroup or current article number. This is both to facilitate the presentation of articles that may be referenced within another article being read, and because of the semantic difficulties of determining the proper sequence and membership of an article that may have been cross-posted to more than one newsgroup.

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

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

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

Note that a previously valid article number MAY become invalid if the article has been removed. A previously invalid article number MAY become valid if the article has been reinstated, but this article number MUST be no less than the reported low water mark for that group.

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

Since the message-id is unique for each article, it may be used by a client to skip duplicate displays of articles that have been posted more than once, or to more than one newsgroup.

The article is returned as a multi-line data block following the 220 response code.

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

6.2.1.3. Examples

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

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] ARTICLE
      [S] 220 3000234 <45223423@example.com>
      [S] Path: pathost!demo!whitehouse!not-for-mail
      [S] From: "Demo User" <nobody@example.net>
      [S] Newsgroups: misc.test
      [S] Subject: I am just a test article
      [S] Date: 6 Oct 1998 04:38:40 -0500
      [S] Organization: An Example Net, Uncertain, Texas
      [S] Message-ID: <45223423@example.com>
      [S]
      [S] This is just a test article.
      [S] .

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

      [C] ARTICLE <45223423@example.com>
      [S] 220 0 <45223423@example.com>
      [S] Path: pathost!demo!whitehouse!not-for-mail
      [S] From: "Demo User" <nobody@example.net>
      [S] Newsgroups: misc.test
      [S] Subject: I am just a test article
      [S] Date: 6 Oct 1998 04:38:40 -0500
      [S] Organization: An Example Net, Uncertain, Texas
      [S] Message-ID: <45223423@example.com>
      [S]
      [S] This is just a test article.
      [S] .

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

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

Example of an unsuccessful retrieval of an article by number:

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

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

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

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

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

6.2.2. HEAD

6.2.2.1. Usage

This command is mandatory.

Syntax

HEAD message-id
HEAD number
HEAD

   Responses

First form (message-id specified)

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

Second form (article number specified)

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

Third form (current article number used)

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

Parameters

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

6.2.2.2. Description

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

6.2.2.3. Examples

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

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] HEAD
      [S] 221 3000234 <45223423@example.com>
      [S] Path: pathost!demo!whitehouse!not-for-mail
      [S] From: "Demo User" <nobody@example.net>
      [S] Newsgroups: misc.test
      [S] Subject: I am just a test article
      [S] Date: 6 Oct 1998 04:38:40 -0500
      [S] Organization: An Example Net, Uncertain, Texas
      [S] Message-ID: <45223423@example.com>
      [S] .

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

      [C] HEAD <45223423@example.com>
      [S] 221 0 <45223423@example.com>
      [S] Path: pathost!demo!whitehouse!not-for-mail
      [S] From: "Demo User" <nobody@example.net>
      [S] Newsgroups: misc.test
      [S] Subject: I am just a test article
      [S] Date: 6 Oct 1998 04:38:40 -0500
      [S] Organization: An Example Net, Uncertain, Texas
      [S] Message-ID: <45223423@example.com>
      [S] .

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

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

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

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

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

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

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

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

6.2.3. BODY

6.2.3.1. Usage

   Indicating capability: READER

Syntax

BODY message-id
BODY number
BODY

   Responses

First form (message-id specified)

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

Second form (article number specified)

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

Third form (current article number used)

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

Parameters

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

6.2.3.2. Description

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

6.2.3.3. Examples

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

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] BODY
      [S] 222 3000234 <45223423@example.com>
      [S] This is just a test article.
      [S] .

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

      [C] BODY <45223423@example.com>
      [S] 222 0 <45223423@example.com>
      [S] This is just a test article.
      [S] .

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

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

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

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

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

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

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

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

6.2.4. STAT

6.2.4.1. Usage

This command is mandatory.

Syntax

STAT message-id
STAT number
STAT

   Responses

First form (message-id specified)

     223 0|n message-id    Article exists
     430                   No article with that message-id

Second form (article number specified)

     223 n message-id      Article exists
     412                   No newsgroup selected
     423                   No article with that number

Third form (current article number used)

     223 n message-id      Article exists
     412                   No newsgroup selected
     420                   Current article number is invalid

Parameters

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

6.2.4.2. Description

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

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

6.2.4.3. Examples

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

6.3. Article Posting

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

6.3.1. POST

6.3.1.1. Usage

   Indicating capability: POST

This command MUST NOT be pipelined.

Syntax

     POST
   
   Responses

Initial responses

     340    Send article to be posted
     440    Posting not permitted

Subsequent responses

     240    Article received OK
     441    Posting failed

6.3.1.2. Description

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

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

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

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

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

6.3.1.3. Examples

Example of a successful posting:

      [C] POST
      [S] 340 Input article; end with <CR-LF>.<CR-LF>
      [C] From: "Demo User" <nobody@example.net>
      [C] Newsgroups: misc.test
      [C] Subject: I am just a test article
      [C] Organization: An Example Net
      [C]
      [C] This is just a test article.
      [C] .
      [S] 240 Article received OK

Example of an unsuccessful posting:

      [C] POST
      [S] 340 Input article; end with <CR-LF>.<CR-LF>
      [C] From: "Demo User" <nobody@example.net>
      [C] Newsgroups: misc.test
      [C] Subject: I am just a test article
      [C] Organization: An Example Net
      [C]
      [C] This is just a test article.
      [C] .
      [S] 441 Posting failed

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

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

6.3.2. IHAVE

6.3.2.1. Usage

   Indicating capability: IHAVE

This command MUST NOT be pipelined.

Syntax

     IHAVE message-id
   
   Responses

Initial responses

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

Subsequent responses

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

Parameters

     message-id    Article message-id

6.3.2.2. Description

The IHAVE command informs the server that the client has an article with the specified message-id. If the server desires a copy of that article, a 335 response MUST be returned, instructing the client to send the entire article. If the server does not want the article (if, for example, the server already has a copy of it), a 435 response MUST be returned, indicating that the article is not wanted. Finally, if the article isn't wanted immediately but the client should retry later if possible (if, for example, another client is in the process of sending the same article to the server), a 436 response MUST be returned.

If transmission of the article is requested, the client MUST send the entire article, including headers and body, to the server as a multi-line data block (see Section 3.1.1). Thus, a single dot (".") on a line indicates the end of the text, and lines starting with a dot in the original text have that dot doubled during transmission. The server MUST return a 235 response, indicating that the article was successfully transferred; a 436 response, indicating that the transfer failed but should be tried again later; or a 437 response, indicating that the article was rejected.

This function differs from the POST command in that it is intended for use in transferring already-posted articles between hosts. It SHOULD NOT be used when the client is a personal news-reading program, since use of this command indicates that the article has already been posted at another site and is simply being forwarded from another host. However, despite this, the server MAY elect not to post or forward the article if, after further examination of the article, it deems it inappropriate to do so. Reasons for such subsequent rejection of an article may include problems such as inappropriate newsgroups or distributions, disc space limitations, article lengths, garbled headers, and the like. These are typically restrictions enforced by the server host's news software and not necessarily by the NNTP server itself.

The client SHOULD NOT assume that the article has been successfully transferred unless it receives an affirmative response from the server. A lack of response (such as a dropped network connection or a network timeout) SHOULD be treated the same as a 436 response.

Because some news server software may not immediately be able to determine whether an article is suitable for posting or forwarding, an NNTP server MAY acknowledge the successful transfer of the article (with a 235 response) but later silently discard it.

6.3.2.3. Examples

Example of successfully sending an article to another site:

      [C] IHAVE <i.am.an.article.you.will.want@example.com>
      [S] 335 Send it; end with <CR-LF>.<CR-LF>
      [C] Path: pathost!demo!somewhere!not-for-mail
      [C] From: "Demo User" <nobody@example.com>
      [C] Newsgroups: misc.test
      [C] Subject: I am just a test article
      [C] Date: 6 Oct 1998 04:38:40 -0500
      [C] Organization: An Example Com, San Jose, CA
      [C] Message-ID: <i.am.an.article.you.will.want@example.com>
      [C]
      [C] This is just a test article.
      [C] .
      [S] 235 Article transferred OK

Example of sending an article to another site that rejects it. Note that the message-id in the IHAVE command is not the same as the one in the article headers; while this is bad practice and SHOULD NOT be done, it is not forbidden.

      [C] IHAVE <i.am.an.article.you.will.want@example.com>
      [S] 335 Send it; end with <CR-LF>.<CR-LF>
      [C] Path: pathost!demo!somewhere!not-for-mail
      [C] From: "Demo User" <nobody@example.com>
      [C] Newsgroups: misc.test
      [C] Subject: I am just a test article
      [C] Date: 6 Oct 1998 04:38:40 -0500
      [C] Organization: An Example Com, San Jose, CA
      [C] Message-ID: <i.am.an.article.you.have@example.com>
      [C]
      [C] This is just a test article.
      [C] .
      [S] 437 Article rejected; don't send again

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

      [C] IHAVE <i.am.an.article.you.will.want@example.com>
      [S] 335 Send it; end with <CR-LF>.<CR-LF>
      [C] Path: pathost!demo!somewhere!not-for-mail
      [C] From: "Demo User" <nobody@example.com>
      [C] Newsgroups: misc.test
      [C] Subject: I am just a test article
      [C] Date: 6 Oct 1998 04:38:40 -0500
      [C] Organization: An Example Com, San Jose, CA
      [C] Message-ID: <i.am.an.article.you.will.want@example.com>
      [C]
      [C] This is just a test article.
      [C] .
      [S] 436 Transfer failed

Example of sending an article to a site that already has it:

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

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

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

7. Information Commands

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

7.1. DATE

7.1.1. Usage

   Indicating capability: READER

Syntax

     DATE

Responses

     111 yyyymmddhhmmss    Server date and time

Parameters

     yyyymmddhhmmss    Current UTC date and time on server

7.1.2. Description

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

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

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

7.1.3. Examples

      [C] DATE
      [S] 111 19990623135624

7.2. HELP

7.2.1. Usage

This command is mandatory.

Syntax

     HELP

Responses

     100    Help text follows (multi-line)

7.2.2. Description

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

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

7.2.3. Examples

[C] HELP
[S] 100 Help text follows
[S] This is some help text. There is no specific [S] formatting requirement for this test, though
[S] it is customary for it to list the valid commands
[S] and give a brief definition of what they do.
[S] .

7.3. NEWGROUPS

7.3.1. Usage

   Indicating capability: READER

Syntax

NEWGROUPS date time [GMT]

Responses

     231    List of new newsgroups follows (multi-line)

Parameters

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

7.3.2. Description

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

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

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

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

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

7.3.3. Examples

Example where there are new groups:

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

Example where there are no new groups:

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

7.4. NEWNEWS

7.4.1. Usage

   Indicating capability: NEWNEWS

Syntax

     NEWNEWS wildmat date time [GMT]

Responses

     230    List of new articles follows (multi-line)

Parameters

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

7.4.2. Description

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

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

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

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

7.4.3. Examples

Example where there are new articles:

      [C] NEWNEWS news.*,sci.* 19990624 000000 GMT
      [S] 230 list of new articles by message-id follows
      [S] <i.am.a.new.article@example.com>
      [S] <i.am.another.new.article@example.com>
      [S] .

Example where there are no new articles:

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

7.5. Time

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

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

First session:

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

Subsequent sessions:

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

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

7.5.1. Examples

First session:

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

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

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

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

7.6. The LIST Commands

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

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

7.6.1. LIST

7.6.1.1. Usage

   Indicating capability: LIST

Syntax

     LIST [keyword [wildmat|argument]]

Responses

     215    Information follows (multi-line)

Parameters

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

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

7.6.1.2. Description

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

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

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

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

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

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

7.6.1.3. Examples

Example of LIST with the ACTIVE keyword:

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

Example of LIST with no keyword:

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

The output is identical to that of the previous example.

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

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

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

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

Example of LIST where the keyword is not recognised:

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

7.6.2. Standard LIST Keywords

This specification defines the following LIST keywords:

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

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

7.6.3. LIST ACTIVE

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

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

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

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

The status field is typically one of the following:

"y" Posting is permitted.

"n" Posting is not permitted.

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

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

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

For example:

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

or, on an implementation that includes leading zeroes:

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

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

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

7.6.4. LIST ACTIVE.TIMES

This keyword is optional.

The active.times list is maintained by some NNTP servers to contain information about who created a particular newsgroup and when. Each line of this list consists of three fields separated from each other by one or more spaces. The first field is the name of the newsgroup. The second is the time when this group was created on this news server, measured in seconds since the start of January 1, 1970. The third is plain text intended to describe the entity that created the newsgroup; it is often a mailbox as defined in RFC 2822 [RFC2822]. For example:

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

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

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

7.6.5. LIST DISTRIB.PATS

This keyword is optional.

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

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

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

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

7.6.6. LIST NEWSGROUPS

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

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

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

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

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

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

8. Article Field Access Commands

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

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

8.1. Article Metadata

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

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

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

8.1.1. The :bytes Metadata Item

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

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

• Counting a CRLF pair as one octet.

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

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

• 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.

8.1.2. The :lines Metadata Item

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

8.2. Database Consistency

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

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

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

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

8.3. OVER

8.3.1. Usage

   Indicating capability: OVER

Syntax

OVER message-id
OVER range
OVER

   Responses

First form (message-id specified)

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

Second form (range specified)

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

Third form (current article number used)

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

Parameters

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

8.3.2. Description

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

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

• An article number.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

8.3.3. Examples

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

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

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] OVER
      [S] 224 Overview information follows
      [S] 3000234|I am just a test article|"Demo User"
          <nobody@example.com>|6 Oct 1998 04:38:40 -0500|
          <45223423@example.com>|<45454@example.net>|1234|
          17|Xref: news.example.com misc.test:3000363
      [S] .

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

      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] OVER MSGID
      [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
      [S] .
      [C] OVER <45223423@example.com>
      [S] 224 Overview information follows
      [S] 0|I am just a test article|"Demo User"
          <nobody@example.com>|6 Oct 1998 04:38:40 -0500|
          <45223423@example.com>|<45454@example.net>|1234|
          17|Xref: news.example.com misc.test:3000363
      [S] .

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

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

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

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

      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] OVER 3000234-3000240
      [S] 224 Overview information follows
      [S] 3000234|I am just a test article|"Demo User"
          <nobody@example.com>|6 Oct 1998 04:38:40 -0500|
          <45223423@example.com>|<45454@example.net>|1234|
          17|Xref: news.example.com misc.test:3000363
      [S] 3000235|Another test article|nobody@nowhere.to
          (Demo User)|6 Oct 1998 04:38:45 -0500|<45223425@to.to>||
          4818|37||Distribution: fi
      [S] 3000238|Re: I am just a test article|somebody@elsewhere.to|
          7 Oct 1998 11:38:40 +1200|<kfwer3v@elsewhere.to>|
          <45223423@to.to>|9234|51
      [S] .

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

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

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

Example of an invalid range:

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

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

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

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

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

8.4. LIST OVERVIEW.FMT

8.4.1. Usage

   Indicating capability: OVER

Syntax

     LIST OVERVIEW.FMT

Responses

     215    Information follows (multi-line)

8.4.2. Description

See Section 7.6.1 for general requirements of the LIST command.

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

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

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

Bytes:
Lines:

even though they refer to metadata, not headers.

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

There are no leading or trailing spaces in the output.

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

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

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

8.4.3. Examples

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

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

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

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

8.5. HDR

8.5.1. Usage

   Indicating capability: HDR

Syntax

HDR field message-id
HDR field range
HDR field

   Responses

First form (message-id specified)

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

Second form (range specified)

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

Third form (current article number used)

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

Parameters

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

8.5.2. Description

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

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

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

• An article number.

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

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

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

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

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

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

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

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

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

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

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

8.5.3. Examples

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

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

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

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

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

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

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

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

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

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

Example of an unsuccessful retrieval of headers from articles by number because no newsgroup was selected first:

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

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

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

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

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

8.6. LIST HEADERS

8.6.1. Usage

   Indicating capability: HDR

Syntax

     LIST HEADERS [MSGID|RANGE]

Responses

     215    Field list follows (multi-line)

Parameters

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

8.6.2. Description

See Section 7.6.1 for general requirements of the LIST command.

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

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

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

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

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

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

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

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

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

8.6.3. Examples

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

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

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

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

Example of an implementation providing access to all headers:

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

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

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

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

9. Augmented BNF Syntax for NNTP

9.1. Introduction

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

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

For each command, the sequence is as follows:

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

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

• The server sends an instance of <response>.

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

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

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

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

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

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

9.2. Commands

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

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

command =/ article-command /

           body-command /
           capabilities-command /
           date-command /
           group-command /
           hdr-command /
           head-command /
           help-command /
           ihave-command /
           last-command /
           list-command /
           listgroup-command /
           mode-reader-command /
           newgroups-command /
           newnews-command /
           next-command /
           over-command /
           post-command /
           quit-command /
           stat-command
     
     article-command = "ARTICLE" [WS article-ref]
     body-command = "BODY" [WS article-ref]
     capabilities-command = "CAPABILITIES" [WS keyword]
     date-command = "DATE"
     group-command = "GROUP" [WS newsgroup-name]
     hdr-command = "HDR" WS header-meta-name [WS range-ref]
     head-command = "HEAD" [WS article-ref]
     help-command = "HELP"
     ihave-command = "IHAVE" WS message-id
     last-command = "LAST"
     list-command = "LIST" [WS list-arguments]
     listgroup-command = "LISTGROUP" [WS newsgroup-name [WS range]]
     mode-reader-command = "MODE" WS "READER"
     newgroups-command = "NEWGROUPS" WS date-time
     newnews-command = "NEWNEWS" WS wildmat WS date-time
     next-command = "NEXT"
     over-command = "OVER" [WS range-ref]
     post-command = "POST"
     quit-command = "QUIT"
     stat-command = "STAT" [WS article-ref]
     
     article-ref = article-number / message-id
     date = date2y / date4y
     date4y = 4DIGIT 2DIGIT 2DIGIT
     date2y = 2DIGIT 2DIGIT 2DIGIT
     date-time = date WS time [WS "GMT"]
     header-meta-name = header-name / metadata-name
     list-arguments = keyword [WS token]
     metadata-name = ":" 1*A-NOTCOLON
     range = article-number ["-" [article-number]]
     range-ref = range / message-id
     time = 2DIGIT 2DIGIT 2DIGIT

9.3. Command Continuation

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

     command-datastream = UNDEFINED
       ; not used, provided as a hook for extensions
     command-continuation = ihave-335-continuation /
           post-340-continuation
     
     ihave-335-continuation = encoded-article
     post-340-continuation = encoded-article
     
     encoded-article = multi-line-data-block
       ; after undoing the "dot-stuffing", this MUST match <article>

9.4. Responses

9.4.1. Generic Responses

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

     response = simple-response / multi-line-response
     simple-response = initial-response-line
     multi-line-response = initial-response-line multi-line-data-block
     
     initial-response-line =
           initial-response-content [SP trailing-comment] CRLF
     initial-response-content = X-initial-response-content
     X-initial-response-content = 3DIGIT *(SP response-argument)
     response-argument = 1*A-CHAR
     trailing-comment = *U-CHAR

9.4.2. Initial Response Line Contents

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

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

9.4.3. Multi-line Response Contents

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

     multi-line-response-content = article-220-ml-content /
           body-222-ml-content /
           capabilities-101-ml-content /
           hdr-225-ml-content /
           head-221-ml-content /
           help-100-ml-content /
           list-215-ml-content /
           listgroup-211-ml-content /
           newgroups-231-ml-content /
           newnews-230-ml-content /
           over-224-ml-content
     
     article-220-ml-content = article
     body-222-ml-content = body
     capabilities-101-ml-content = version-line CRLF
           *(capability-line CRLF)
     hdr-225-ml-content = *(article-number SP hdr-content CRLF)
     head-221-ml-content = 1*header
     help-100-ml-content = *(*U-CHAR CRLF)
     list-215-ml-content = list-content
     listgroup-211-ml-content = *(article-number CRLF)
     newgroups-231-ml-content = active-groups-list
     newnews-230-ml-content = *(message-id CRLF)
     over-224-ml-content = *(article-number over-content CRLF)
     
     active-groups-list = *(newsgroup-name SPA article-number
           SPA article-number SPA newsgroup-status CRLF)
     hdr-content = *S-NONTAB
     hdr-n-content = [(header-name ":" / metadata-name) SP hdr-content]
     list-content = body
     newsgroup-status = %x79 / %x6E / %x6D / private-status
     over-content = 1*6(TAB hdr-content) /
           7(TAB hdr-content) *(TAB hdr-n-content)
     private-status = token ; except the values in newsgroup-status

9.5. Capability Lines

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

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

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

capability-entry =/

           hdr-capability /
           ihave-capability /
           implementation-capability /
           list-capability /
           mode-reader-capability /
           newnews-capability /
           over-capability /
           post-capability /
           reader-capability
     
     hdr-capability = "HDR"
     ihave-capability = "IHAVE"
     implementation-capability = "IMPLEMENTATION" *(WS token)
     list-capability = "LIST" 1*(WS keyword)
     mode-reader-capability = "MODE-READER"
     newnews-capability = "NEWNEWS"
     over-capability = "OVER" [WS "MSGID"]
     post-capability = "POST"
     reader-capability = "READER"
     
     version-line = "VERSION" 1*(WS version-number)
     version-number = nzDIGIT *5DIGIT

9.6. LIST Variants

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

     ; active
     list-arguments =/ "ACTIVE" [WS wildmat]
     list-content =/ list-active-content
     list-active-content = active-groups-list
     
     ; active.times
     list-arguments =/ "ACTIVE.TIMES" [WS wildmat]
     list-content =/ list-active-times-content
     list-active-times-content =
           *(newsgroup-name SPA 1*DIGIT SPA newsgroup-creator CRLF)
     newsgroup-creator = U-TEXT
     
     ; distrib.pats
     list-arguments =/ "DISTRIB.PATS"
     list-content =/ list-distrib-pats-content
     list-distrib-pats-content =
           *(1*DIGIT ":" wildmat ":" distribution CRLF)
     distribution = token
     
     ; headers
     list-arguments =/ "HEADERS" [WS ("MSGID" / "RANGE")]
     list-content =/ list-headers-content
     list-headers-content = *(header-meta-name CRLF) /
           *((metadata-name / ":") CRLF)
     
     ; newsgroups
     list-arguments =/ "NEWSGROUPS" [WS wildmat]
     list-content =/ list-newsgroups-content
     list-newsgroups-content =
           *(newsgroup-name WS newsgroup-description CRLF)
     newsgroup-description = S-TEXT
     
     ; overview.fmt
     list-arguments =/ "OVERVIEW.FMT"
     list-content =/ list-overview-fmt-content
     list-overview-fmt-content = "Subject:" CRLF
           "From:" CRLF
           "Date:" CRLF
           "Message-ID:" CRLF
           "References:" CRLF
           ( ":bytes" CRLF ":lines" / "Bytes:" CRLF "Lines:") CRLF
           *((header-name ":full" / metadata-name) CRLF)

9.7. Articles

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

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

9.8. General Non-terminals

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

     multi-line-data-block = content-lines termination
     content-lines = *([content-text] CRLF)
     content-text = (".." / B-NONDOT) *B-CHAR
     termination = "." CRLF
     
     article-number = 1*16DIGIT
     header-name = 1*A-NOTCOLON
     keyword = ALPHA 2*(ALPHA / DIGIT / "." / "-")
     message-id = "<" 1*248A-NOTGT ">"
     newsgroup-name = 1*wildmat-exact
     token = 1*P-CHAR
     
     wildmat = wildmat-pattern *("," ["!"] wildmat-pattern)
     wildmat-pattern = 1*wildmat-item
     wildmat-item = wildmat-exact / wildmat-wild
     wildmat-exact = %x22-29 / %x2B / %x2D-3E / %x40-5A / %x5E-7E /
          UTF8-non-ascii  ; exclude ! * , ? [ \ ]
     wildmat-wild = "*" / "?"
     
     base64 = *(4base64-char) [base64-terminal]
     base64-char = UPPER / LOWER / DIGIT / "+" / "/"
     base64-terminal = 2base64-char "==" / 3base64-char "="
     
     ; Assorted special character sets
     ;   A- means based on US-ASCII, excluding controls and SP
     ;   P- means based on UTF-8, excluding controls and SP
     ;   U- means based on UTF-8, excluding NUL CR and LF
     ;   B- means based on bytes, excluding NUL CR and LF
     A-CHAR     = %x21-7E
     A-NOTCOLON = %x21-39 / %x3B-7E  ; exclude ":"
     A-NOTGT    = %x21-3D / %x3F-7E  ; exclude ">"
     P-CHAR     = A-CHAR / UTF8-non-ascii
     U-CHAR     = CTRL / TAB / SP / A-CHAR / UTF8-non-ascii
     U-NONTAB   = CTRL /       SP / A-CHAR / UTF8-non-ascii
     U-TEXT     = P-CHAR *U-CHAR
     B-CHAR     = CTRL / TAB / SP / %x21-FF
     B-NONDOT   = CTRL / TAB / SP / %x21-2D / %x2F-FF  ; exclude "."
     
     ALPHA = UPPER / LOWER   ; use only when case-insensitive
     CR = %x0D
     CRLF = CR LF
     CTRL = %x01-08 / %x0B-0C / %x0E-1F
     DIGIT = %x30-39
     nzDIGIT = %x31-39
     EOL = *(SP / TAB) CRLF
     LF = %x0A
     LOWER = %x61-7A
     SP = %x20
     SPA = 1*SP
     TAB = %x09
     UPPER = %x41-5A
     UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4
     UTF8-2    = %xC2-DF UTF8-tail
     UTF8-3    = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2UTF8-tail /
                 %xED %x80-9F UTF8-tail / %xEE-EF 2UTF8-tail
     UTF8-4    = %xF0 %x90-BF 2UTF8-tail / %xF1-F3 3UTF8-tail /
                 %xF4 %x80-8F 2UTF8-tail
     UTF8-tail = %x80-BF
     WS = 1*(SP / TAB)

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

Implementations MUST accept any content that meets this syntax:

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

and MAY pass such content on unaltered.

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

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

9.9. Extensions and Validation

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

command

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

command-datastream

for each new command that immediately streams data;

command-continuation

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

initial-response-content

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

multi-line-response-content

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

capability-entry

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

list-arguments

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

list-content

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

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

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

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

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

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

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

10. Internationalisation Considerations

10.1. Introduction and Historical Situation

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

Since then, Usenet and NNTP have spread throughout the world. In the absence of standards for handling the issues of language and character sets, countries, newsgroup hierarchies, and individuals have found a variety of solutions that work for them but that are not necessarily appropriate elsewhere. For example, some have adopted a default 8-bit character set appropriate to their needs (such as ISO/IEC 8859-1 in Western Europe or KOI-8 in Russia), others have used ASCII (either US-ASCII or national variants) in headers but local 16-bit character sets in article bodies, and still others have gone for a combination of MIME [RFC2045] and UTF-8. With the increased use of MIME in email, it is becoming more common to find NNTP articles containing MIME headers that identify the character set of the body, but this is far from universal.

The resulting confusion does not help interoperability.

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

10.2. This Specification

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

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

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

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

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

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

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

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

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

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

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

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

10.3. Outstanding Issues

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

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

Restricting newsgroup names to UTF-8 is not a complete solution. In particular, when new newsgroup names are created or a user is asked to enter a newsgroup name, some scheme of canonicalisation will need to take place. This specification does not attempt to define that canonicalization; further work is needed in this area, in conjunction with the article format specifications. Until such specifications are published, implementations SHOULD match newsgroup names octet by octet. It is anticipated that any approved scheme will be applied "at the edges", and therefore octet-by-octet comparison will continue to apply to most, if not all, uses of newsgroup names in NNTP.

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

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

11. IANA Considerations

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

Different entries in the registry MUST use different capability labels.

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

12. Security Considerations

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

12.1. Personal and Proprietary Information

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

12.2. Abuse of Server Log Information

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

12.3. Weak Authentication and Access Control

There is no user-based or token-based authentication in the basic NNTP specification. Access is normally controlled by server configuration files. Those files specify access by using domain names or IP addresses. However, this specification does permit the creation of extensions to NNTP for such purposes; one such extension is [NNTP-AUTH]. While including such mechanisms is optional, doing so is strongly encouraged.

Other mechanisms are also available. For example, a proxy server could be put in place that requires authentication before connecting via the proxy to the NNTP server.

12.4. DNS Spoofing

Many existing NNTP implementations authorize incoming connections by checking the IP address of that connection against the IP addresses obtained via DNS lookups of lists of domain names given in local configuration files. Servers that use this type of authentication and clients that find a server by doing a DNS lookup of the server name rely very heavily on the Domain Name Service, and are thus generally prone to security attacks based on the deliberate misassociation of IP addresses and DNS names. Clients and servers need to be cautious in assuming the continuing validity of an IP number/DNS name association.

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

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

This requirement also improves the load-balancing behaviour of clients for replicated servers using the same DNS name and reduces the likelihood of a user's experiencing failure in accessing sites that use that strategy.

12.5. UTF-8 Issues

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

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

• Generating a 501 response code.

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

• Closing the connection.

• 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 correct encoding in a command line, this needs to happen before the command line is parsed into individual arguments. If the replacement came after parsing, it would be possible to generate an argument with an embedded space, which is forbidden. Use of the "replacement character" does not have this problem, since it is permitted wherever non-US-ASCII characters are. Implementations SHOULD use one of the first two solutions where the general structure of the NNTP stream remains intact and SHOULD close the connection if it is no longer possible to parse it sensibly.

12.6. Caching of Capability Lists

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

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

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

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

      [Initial connection set-up completed.]
      [S] 200 NNTP Service Ready, posting permitted
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] NEWNEWS
      [S] POST
      [S] XENCRYPT
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] XENCRYPT
      [Client and server negotiate encryption on the link]
      [S] 283 Encrypted link established
      [C] CAPABILITIES
      [S] 101 Capability list:
      [S] VERSION 2
      [S] READER
      [S] NEWNEWS
      [S] POST
      [S] XSECRET
      [S] LIST ACTIVE NEWSGROUPS
      [S] .
      [C] XSECRET fred flintstone
      [S] 290 Password for fred accepted

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

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

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

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

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

13. Acknowledgements

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

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

The author gratefully acknowledges the following:

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

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

• 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.

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

         Russ Allbery <rra@stanford.edu>
         Wayne Davison <davison@armory.com>
         Chris Lewis <clewis@bnr.ca>
         Tom Limoncelli <tal@mars.superlink.net>
         Eric Schnoebelen <eric@egsner.cirr.com>
         Rich Salz <rsalz@osf.org>

This work was motivated by the work of various news reader authors and news server authors, including those listed below:

Rick Adams

Original author of the NNTP extensions to the RN news reader and last maintainer of Bnews.

Stan Barber

Original author of the NNTP extensions to the news readers that are part of Bnews.

Geoff Collyer

Original author of the OVERVIEW database proposal and one of the original authors of CNEWS.

Dan Curry

Original author of the xvnews news reader.

Wayne Davison

Author of the first threading extensions to the RN news reader (commonly called TRN).

Geoff Huston

Original author of ANU NEWS.

Phil Lapsey

Original author of the UNIX reference implementation for NNTP.

Iain Lea

Original maintainer of the TIN news reader.

Chris Lewis

First known implementer of the AUTHINFO GENERIC extension.

Rich Salz

Original author of INN.

Henry Spencer

One of the original authors of CNEWS.

Kim Storm

Original author of the NN news reader.

Other people who contributed to this document include:

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

The author thanks them all and apologises to anyone omitted.

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

      Stan Barber <sob@academ.com>

14. References

14.1. Normative References

   [ANSI1986]    American National Standards Institute, "Coded Character
                 Set - 7-bit American Standard Code for Information
                 Interchange", ANSI X3.4, 1986.
   
   [RFC977]      Kantor, B. and P. Lapsley, "Network News Transfer
                 Protocol", RFC 977, February 1986.
   
   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.
   
   [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
                 Extensions) Part Three: Message Header Extensions for
                 Non-ASCII Text", RFC 2047, November 1996.
   
   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                 10646", STD 63, RFC 3629, November 2003.
   
   [RFC4234]     Crocker, D., Ed. and P. Overell, "Augmented BNF for
                 Syntax Specifications: ABNF", RFC 4234, October 2005.
   
   [RFC4648]     Josefsson, S., "The Base16, Base32, and Base64 Data
                 Encodings", RFC 4648, October 2006.
   
   [TF.686-1]    International Telecommunications Union - Radio,
                 "Glossary, ITU-R Recommendation TF.686-1",
                 ITU-R Recommendation TF.686-1, October 1997.

14.2. Informative References

   [NNTP-AUTH]   Vinocur, J., Murchison, K., and C. Newman, "Network
                 News Transfer Protocol (NNTP) Extension for
                 Authentication",
                 RFC 4643, October 2006.
   
   [NNTP-STREAM] Vinocur, J. and K. Murchison, "Network News Transfer
                 Protocol (NNTP) Extension for Streaming Feeds",
                 RFC 4644, October 2006.
   
   [NNTP-TLS]    Murchison, K., Vinocur, J., and C. Newman, "Using
                 Transport Layer Security (TLS) with Network News
                 Transfer Protocol (NNTP)", RFC 4642, October 2006.
   
   [RFC1036]     Horton, M. and R. Adams, "Standard for interchange of
                 USENET messages", RFC 1036, December 1987.
   
   [RFC1305]     Mills, D., "Network Time Protocol (Version 3)
                 Specification, Implementation and Analysis", RFC 1305,
                 March 1992.
   
   [RFC1869]     Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
                 Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
                 November 1995.
   
   [RFC2616]     Fielding,  R., Gettys, J., Mogul, J., Frystyk, H.,
                 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
   
   [RFC2629]     Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
                 June 1999.
   
   [RFC2822]     Resnick, P., "Internet Message Format", RFC 2822, April
                 2001.
   
   [RFC2980]     Barber, S., "Common NNTP Extensions", RFC 2980, October
                 2000.
   
   [ROBE1995]    Robertson, R., "FAQ: Overview database / NOV General
                 Information", January 1995.

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

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

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

Appendix A. Interaction with Other Specifications

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

A.1. Header Folding

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

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

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

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

A.2. Message-IDs

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

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

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

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

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

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

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

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

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

      [C] HEAD <45223423@example.com>
      [S] 221 0 <45223423@example.com>
      [S] Path: pathost!demo!whitehouse!not-for-mail
      [S] Message-ID: <1234@example.net>
      [S] From: "Demo User" <nobody@example.net>
      [S] Newsgroups: misc.test
      [S] Subject: I am just a test article
      [S] Date: 6 Oct 1998 04:38:40 -0500
      [S] Organization: An Example Net, Uncertain, Texas
      [S] .

A.3. Article Posting

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

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

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

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

Appendix B. Summary of Commands

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

Ordered by command name:

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

Ordered by indicating capability:

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

Appendix C. Summary of Response Codes

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

Response code 100 (multi-line)

Generated by: HELP
Meaning: help text follows.

Response code 101 (multi-line)

Generated by: CAPABILITIES
Meaning: capabilities list follows.

Response code 111

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

Response code 200

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

Response code 201

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

Response code 205

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

Response code 211

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

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

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

Response code 215 (multi-line)

Generated by: LIST
Meaning: information follows.

Response code 220 (multi-line)

Generated by: ARTICLE
2 arguments: n message-id
Meaning: article follows.

Response code 221 (multi-line)

Generated by: HEAD
2 arguments: n message-id
Meaning: article headers follow.

Response code 222 (multi-line)

Generated by: BODY
2 arguments: n message-id
Meaning: article body follows.

Response code 223

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

Response code 224 (multi-line)

Generated by: OVER
Meaning: overview information follows.

Response code 225 (multi-line)

Generated by: HDR
Meaning: headers follow.

Response code 230 (multi-line)

Generated by: NEWNEWS
Meaning: list of new articles follows.

Response code 231 (multi-line)

Generated by: NEWGROUPS
Meaning: list of new newsgroups follows.

Response code 235

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

Response code 240

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

Response code 335

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

Response code 340

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

Response code 400

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

Response code 401

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

Response code 403

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

Response code 411

Generated by: GROUP, LISTGROUP
Meaning: no such newsgroup.

Response code 412

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

Response code 420

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

Response code 421

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

Response code 422

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

Response code 423

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

Response code 430

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

Response code 435

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

Response code 436

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

Response code 437

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

Response code 440

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

Response code 441

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

Response code 480

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

Response code 483

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

Response code 500

Generic response
Meaning: unknown command.

Response code 501

Generic response
Meaning: syntax error in command.

Response code 502

Generic response and generated by initial connection

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

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

Response code 503

Generic response
Meaning: feature not supported.

Response code 504

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

Appendix D. Changes from RFC 977

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Author's Address

   Clive D.W. Feather
   THUS plc
   322 Regents Park Road
   London
   N3  2QQ
   United Kingdom
   
   Phone: +44 20 8495 6138
   Fax:   +44 870 051 9937
   EMail: clive@demon.net
   URI:   http://www.davros.org/

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