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
Although the protocol specification in this document is largely
compatible with the version specified in RFC 977 [RFC977], a number
of changes are summarised in Appendix D. In particular:
o the default character set is changed from US-ASCII [ANSI1986] to
UTF-8 [RFC3629] (note that US-ASCII is a subset of UTF-8);
o a number of commands that were optional in RFC 977 or that have
been taken from RFC 2980 [RFC2980] are now mandatory; and
o a CAPABILITIES command has been added to allow clients to
determine what functionality is available from a server.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
An implementation is not compliant if it fails to satisfy one or more
of the MUST requirements for this protocol. An implementation that
satisfies all the MUST and all the SHOULD requirements for its
protocols is said to be "unconditionally compliant"; one that
satisfies all the MUST requirements but not all the SHOULD
requirements for NNTP is said to be "conditionally compliant".
For the remainder of this document, the terms "client" and "client
host" refer to a host making use of the NNTP service, while the terms
"server" and "server host" refer to a host that offers the NNTP
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.
The following notational conventions are used in this document.
UPPERCASE indicates literal text to be included in the
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
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
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
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
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
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
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
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
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
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
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
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
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).
Example of an unknown command:
[S] 500 Unknown command
Example of an unsupported command:
[S] 101 Capability list:
[S] VERSION 2
[S] LIST ACTIVE NEWSGROUPS
[S] 500 Unknown command
Example of an unsupported variant:
[C] MODE POSTER
[S] 501 Unknown MODE option
Example of a syntax error:
[C] ARTICLE email@example.com
[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
[C] MODE READER
[S] 200 Reader mode, posting permitted
[C] IHAVE <firstname.lastname@example.org>
[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
[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
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
o The capability label, which is a keyword indicating the
capability. A capability label may be defined by this
specification or a successor, or by an extension.
o The label is then followed by zero or more tokens, which are
arguments of the capability. The form and meaning of these tokens
is specific to each capability.
The server MUST ensure that the capability list accurately reflects
the capabilities (including extensions) currently available. If a
capability is only available with the server in a certain state (for
example, only after authentication), the list MUST only include the
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.
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.
This capability indicates that the server implements the various
commands useful for reading clients.
This capability indicates that the server implements the IHAVE
This capability indicates that the server implements the POST
This capability indicates that the server implements the NEWNEWS
This capability indicates that the server implements the header
access commands (HDR and LIST HEADERS).
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.
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.
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.)
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.
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:
o a descriptive name for the extension.
o the capability label or labels defined by the extension (the
capability label of a registered extension MUST NOT begin with
o The syntax, values, and meanings of any arguments for each
capability label defined by the extension.
o Any new NNTP commands associated with the extension (the names of
commands associated with registered extensions MUST NOT begin with
o The syntax and possible values of arguments associated with the
new NNTP commands.
o The response codes and possible values of arguments for the
responses of the new NNTP commands.
o Any new arguments the extension associates with any other
pre-existing NNTP commands.
o Any increase in the maximum length of commands and initial
response lines over the value specified in this document.
o A specific statement about the effect on pipelining that this
extension may have (if any).
o A specific statement about the circumstances when use of this
extension can alter the contents of the capabilities list (other
than the new capability labels it defines).
o A specific statement about the circumstances under which the
extension can cause any pre-existing command to produce a 401,
480, or 483 response.
o A description of how the use of MODE READER on a mode-switching
server interacts with the extension.
o A description of how support for the extension affects the
behaviour of a server and NNTP client in any other manner not
o Formal syntax as described in Section 9.9.
A private extension MAY or MAY NOT be included in the capabilities
list. If it is, the capability label MUST begin with "X". A server
MAY provide additional keywords (for new commands and also for new
variants of existing commands) as part of a private extension. To
avoid the risk of a clash with a future registered extension, these
keywords SHOULD begin with "X".
If the server advertises a capability defined by a registered
extension, it MUST implement the extension so as to fully conform
with the specification (for example, it MUST implement all the
commands that the extension describes as mandatory). If it does not
implement the extension as specified, it MUST NOT list the extension
in the capabilities list under its registered name. In that case, it
MAY, but SHOULD NOT, provide a private extension (not listed, or
listed with a different name) that implements part of the extension
or implements the commands of the extension with a different meaning.
A server MUST NOT send different response codes to basic NNTP
commands documented here or to commands documented in registered
extensions in response to the availability or use of a private
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
3.4. Mandatory and Optional Commands
For a number of reasons, not all the commands in this specification
are mandatory. However, it is equally undesirable for every command
to be optional, since this means that a client will have no idea what
facilities are available. Therefore, as a compromise, some of the
commands in this specification are mandatory (they must be supported
by all servers) while the remainder are not. The latter are then
subdivided into bundles, each indicated by a single capability label.
o If the label is included in the capability list returned by the
server, the server MUST support all commands in that bundle.
o If the label is not included, the server MAY support none or some
of the commands but SHOULD NOT support all of them. In general,
there will be no way for a client to determine which commands are
supported without trying them.
The bundles have been chosen to provide useful functionality, and
therefore server authors are discouraged from implementing only part
of a bundle.
The description of each command will either indicate that it is
mandatory, or will give, using the term "indicating capability", the
capability label indicating whether the bundle including this command
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
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"
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
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
A mode-switching server has two modes:
o Transit mode, which applies after the initial connection.
* It MUST advertise the MODE-READER capability.
* It MUST NOT advertise the READER capability.
However, the server MAY cease to advertise the MODE-READER
capability after the client uses any command except CAPABILITIES.
o Reading mode, after a successful MODE READER command (see Section
* It MUST NOT advertise the MODE-READER capability.
* It MUST advertise the READER capability.
* It MAY NOT advertise the IHAVE capability, even if it was
advertising it in transit mode.
A client SHOULD only issue a MODE READER command to a server if it is
advertising the MODE-READER capability. If the server does not
support CAPABILITIES (and therefore does not conform to this
specification), the client MAY use the following heuristic:
o If the client wishes to use any "reader" commands, it SHOULD use
the MODE READER command immediately after the initial connection.
o Otherwise, it SHOULD NOT use the MODE READER command.
In each case, it should be prepared for some commands to be
unavailable that would have been available if it had made the other
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
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.)
Example of correct use of pipelining:
[C] GROUP misc.test
[S] 211 1234 3000234 3002322 misc.test
[S] 223 3000234 <email@example.com> retrieved
[S] 223 3000237 <firstname.lastname@example.org> retrieved
Example of incorrect use of pipelining (the MODE READER command may
not be pipelined):
[C] MODE READER
[S] 200 Server ready, posting allowed
[S] 223 3000237 <email@example.com> retrieved
The DATE command has been thrown away by the server, so there is no
111 response to match it.
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
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:
o A message-id MUST begin with "<", end with ">", and MUST NOT
contain the latter except at the end.
o A message-id MUST be between 3 and 250 octets in length.
o A message-id MUST NOT contain octets other than printable US-ASCII
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.