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IETF RFC 5537
Netnews Architecture and Protocols
Last modified on Sunday, November 29th, 2009
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Network Working Group R. Allbery, Ed.
Request for Comments: 5537 Stanford University
Obsoletes: 1036 C. Lindsey
Category: Standards Track November 2009
Netnews Architecture and Protocols
 Abstract
This document defines the architecture of Netnews systems and
specifies the correct manipulation and interpretation of Netnews
articles by software that originates, distributes, stores, and
displays them. It also specifies the requirements that must be met
by any protocol used to transport and serve Netnews articles.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Basic Concepts .............................................3
1.2. Scope ......................................................3
1.3. Requirements Notation ......................................3
1.4. Syntax Notation ............................................3
1.5. Definitions ................................................4
2. Transport .......................................................5
Allbery & Lindsey Standards Track PAGE 1 
RFC 5537 Netnews Architecture and Protocols November 2009
3. Duties of Agents ................................................6
3.1. General Principles .........................................6
3.2. The Path Header Field ......................................7
3.2.1. Constructing the Path Header Field ..................8
3.2.2. Path Header Field Example ...........................9
3.3. Article History and Duplicate Suppression .................10
3.4. Duties of a Posting Agent .................................11
3.4.1. Proto-Articles .....................................12
3.4.2. Multiple Injection of Articles .....................13
3.4.3. Followups ..........................................14
3.4.4. Construction of the References Header Field ........15
3.5. Duties of an Injecting Agent ..............................15
3.5.1. Forwarding Messages to a Moderator .................18
3.6. Duties of a Relaying Agent ................................19
3.7. Duties of a Serving Agent .................................21
3.8. Duties of a Reading Agent .................................22
3.9. Duties of a Moderator .....................................22
3.10. Duties of a Gateway ......................................24
3.10.1. Duties of an Outgoing Gateway .....................25
3.10.2. Duties of an Incoming Gateway .....................25
3.10.3. Original-Sender Header Field ......................27
3.10.4. Gateway Example ...................................28
4. Media Types ....................................................29
4.1. application/news-transmission .............................30
4.2. application/news-groupinfo ................................31
4.3. application/news-checkgroups ..............................33
5. Control Messages ...............................................35
5.1. Authentication and Authorization ..........................35
5.2. Group Control Messages ....................................36
5.2.1. The newgroup Control Message .......................36
5.2.1.1. newgroup Control Message Example ..........37
5.2.2. The rmgroup Control Message ........................38
5.2.3. The checkgroups Control Message ....................38
5.3. The cancel Control Message ................................40
5.4. The Supersedes Header Field ...............................40
5.5. The ihave and sendme Control Messages .....................41
5.6. Obsolete Control Messages .................................42
6. Security Considerations ........................................42
6.1. Compromise of System Integrity ............................42
6.2. Denial of Service .........................................44
6.3. Leakage ...................................................44
7. IANA Considerations ............................................45
8. References .....................................................45
8.1. Normative References ......................................45
8.2. Informative References ....................................46
Appendix A. Changes to the Existing Protocols ....................47
Appendix B. Acknowledgements .....................................48
Allbery & Lindsey Standards Track PAGE 2
RFC 5537 Netnews Architecture and Protocols November 2009
1. Introduction
1.1. Basic Concepts
"Netnews" is a set of protocols for generating, storing, and
retrieving news "articles" whose format is defined in [RFC 5536], and
for exchanging them amongst a readership that is potentially widely
distributed. It is organized around "newsgroups", with the
expectation that each reader will be able to see all articles posted
to each newsgroup in which he participates. These protocols most
commonly use a flooding algorithm that propagates copies throughout a
network of participating servers. Typically, only one copy is stored
per server, and each server makes it available on demand to readers
able to access that server.
"Usenet" is a particular worldwide, publicly accessible network based
on the Netnews protocols. It is only one such possible network;
there are deployments of the Netnews protocols other than Usenet
(such as ones internal to particular organizations). This document
discusses the more general Netnews architecture and protocols.
1.2. Scope
This document defines the architecture of Netnews systems and
specifies the correct manipulation and interpretation of Netnews
articles by software that originates, distributes, stores, and
displays them. It addresses protocol issues that are independent of
transport protocols such as the Network News Transfer Protocol (NNTP)
[RFC 3977], and specifies the requirements Netnews places on those
underlying transport protocols. It also specifies the handling of
control messages.
The format and syntax of Netnews articles are specified in [RFC 5536],
which should be read in conjunction with this document.
1.3. Requirements Notation
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].
1.4. Syntax Notation
Syntax defined in this document uses the Augmented Backus-Naur Form
(ABNF) notation (including the Core Rules) defined in [RFC 5234] and
constructs defined in [RFC 5536] and [RFC 5322].
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RFC 5537 Netnews Architecture and Protocols November 2009
The ABNF rules defined elsewhere and used in this document are:
CRLF = <see [RFC 5234] Appendix B.1>
DIGIT = <see [RFC 5234] Appendix B.1>
HTAB = <see [RFC 5234] Appendix B.1>
SP = <see [RFC 5234] Appendix B.1>
WSP = <see [RFC 5234] Appendix B.1>
VCHAR = <see [RFC 5234] Appendix B.1>
argument = <see [RFC 5536] Section 3.2.3>
article-locator = <see [RFC 5536] Section 3.2.14>
component = <see [RFC 5536] Section 3.1.4>
control-command = <see [RFC 5536] Section 3.2.3>
diag-keyword = <see [RFC 5536] Section 3.1.5>
diag-match = <see [RFC 5536] Section 3.1.5>
diag-other = <see [RFC 5536] Section 3.1.5>
dist-name = <see [RFC 5536] Section 3.2.4>
msg-id = <see [RFC 5536] Section 3.1.3>
newsgroup-name = <see [RFC 5536] Section 3.1.4>
path-diagnostic = <see [RFC 5536] Section 3.1.5>
path-identity = <see [RFC 5536] Section 3.1.5>
path-nodot = <see [RFC 5536] Section 3.1.5>
tail-entry = <see [RFC 5536] Section 3.1.5>
verb = <see [RFC 5536] Section 3.2.3>
display-name = <see [RFC 5322] Section 3.4>
local-part = <see [RFC 5322] Section 3.4.1>
mailbox = <see [RFC 5322] Section 3.4>
1.5. Definitions
Any term used in this document that is defined in Section 1.5 of
[RFC 5536] is used with the definition given there. In addition, the
following terms will be used:
A "hierarchy" is the set of all newsgroups whose names share a first
<component> (as defined in Section 3.1.4 of [RFC 5536]). A "sub-
hierarchy" is the set of all newsgroups whose names share several
initial components.
A "news server" is further distinguished into the roles of "injecting
agent", "relaying agent", and "serving agent". An "injecting agent"
accepts a proto-article with the goal of distributing it to relaying
and serving agents and hence to readers. A "relaying agent" accepts
articles from other relaying agents or injecting agents and
distributes them to other relaying agents or serving agents. A
"serving agent" receives an article from a relaying agent or
injecting agent and makes it available to readers.
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RFC 5537 Netnews Architecture and Protocols November 2009
A "user agent" is further distinguished into the roles of "posting
agent" and "reading agent". A "posting agent" is software that
assists in the preparation of a proto-article and then passes it to
an injecting agent. A "reading agent" is software that retrieves
articles from a serving agent for presentation to a reader.
"Injecting" an article is the processing of a proto-article by an
injecting agent. Normally, this action is done once and only once
for a given article. "Multiple injection" is passing the same
article to multiple injecting agents, either serially or in parallel,
by one or several posting agents.
A "gateway" is software that receives news articles and converts them
to messages of some other kind (such as [RFC 5322] mail messages),
receives messages of some other kind and converts them to news
articles, or conveys articles between two separate Netnews networks.
2. Transport
The exact means used to transmit articles from one agent to another
is not specified. NNTP [RFC 3977] is the most common transport
mechanism for Netnews networks. Other methods in use include the
Unix-to-Unix Copy Protocol [UUCP] (extensively used in the early days
of Usenet) and physically delivered magnetic and optical media. Any
mechanism may be used in conjunction with this protocol provided that
it can meet the requirements specified here.
Transports for Netnews articles MUST treat news articles as
uninterpreted sequences of octets, excluding the values %d00 (which
may not occur in Netnews articles), %d13, and %d10 (which MUST only
appear in Netnews articles as a pair in that order and which,
together, denote a line separator). These octets are the US-ASCII
[ASCII] characters NUL, CR, and LF respectively.
NOTE: This corresponds to the range of octets permitted in MIME
8bit data [RFC 2045]. Transports for Netnews are not required to
support transmission of MIME binary data.
In particular, transports MUST convey all header fields unmodified
(including header fields within message/RFC 822 objects in article
bodies), even if they contain octets in the range of 128 to 255.
Furthermore, transports for relaying and serving agents MUST, and
transports for other agents SHOULD, convey lines even if they exceed
998 characters in length, especially in article bodies. (This
requirement is stricter than MIME 8bit data.) These requirements
include the transport paths between posting agents, injecting agents,
serving agents, and reading agents.
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RFC 5537 Netnews Architecture and Protocols November 2009
3. Duties of Agents
The following section specifies the duties of the agents involved in
the creation, relaying, and serving of Netnews articles. This
protocol is described by following the life of a typical Usenet
article: it is prepared by a posting agent, given to an injecting
agent, transferred through one or more relaying agents, accepted by a
serving agent, and finally retrieved by a reading agent. Articles
submitted to moderated groups go through an additional process, which
is described separately (see Section 3.5.1 and Step 7 of
Section 3.5). Finally, the additional duties and requirements of a
gateway are discussed.
At each step, each agent has a set of checks and transformations of
the article that it is required to perform. These are described as
sequences of steps to be followed, but it should be understood that
it is the effect of these sequences that is important, and
implementations may use any method that produces the same effect.
Many news servers combine the functions of injecting agent, relaying
agent, and serving agent in a single software package. For the
purposes of this specification, such combined agents should
conceptually be treated as an injecting agent that sends articles to
a serving agent and, optionally, to a relaying agent. The
requirements of all three agents MUST still be met when the news
server is performing the functions of those agents.
On news servers that accept them, control messages may have
additional effects than those described below. Those effects are
described in Section 5.
3.1. General Principles
There are two important principles that news implementors and
administrators need to keep in mind. The first is the well-known
Internet Robustness Principle:
Be liberal in what you accept, and conservative in what you send.
As applied to Netnews, this primarily means that unwanted or non-
compliant articles SHOULD be rejected as early as possible, but once
they are in general circulation, relaying and serving agents may wish
to accept them where possible rather than lose information. Posting
agents and injecting agents SHOULD therefore be maximally strict in
their application of both this protocol and [RFC 5536], and reading
agents SHOULD be robust in the presence of violations of the Netnews
article format where possible.
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RFC 5537 Netnews Architecture and Protocols November 2009
In the case of Netnews, there is an even more important principle,
derived from a much older code of practice, the Hippocratic Oath (we
may thus call this the Hippocratic Principle):
First, do no harm.
It is vital to realize that decisions that might be merely suboptimal
in a smaller context can become devastating mistakes when amplified
by the actions of thousands of hosts within a few minutes.
No Netnews agent is ever required to accept any article. It is
common for injecting, relaying, and serving agents to reject well-
formed articles for reasons of local policy (such as not wishing to
carry a particular newsgroup or attempting to filter out unwanted
articles). This document specifies how articles are to be treated if
they are accepted and specifies some cases where they must be
rejected, but an agent MAY always reject any article for other
reasons than those stated here.
A primary goal of the Netnews protocol is to ensure that all readers
receiving a particular article (as uniquely identified by the content
of its Message-ID header field) see the identical article, apart from
allowable divergence in trace headers and local metadata.
Accordingly, agents (other than moderators) MUST NOT modify articles
in ways other than described here. Unacceptable articles MUST be
rejected rather than corrected.
3.2. The Path Header Field
All news server components (injecting agents, relaying agents, and
serving agents) MUST identify themselves, when processing an article,
by prepending their <path-identity> (defined in Section 3.1.5 of
[RFC 5536]) to the Path header field. Injecting agents MUST also use
the same identity in Injection-Info header fields that they add, and
serving and relaying agents SHOULD use the same identity in any Xref
header fields they add.
The <path-identity> used by an agent may be chosen via one of the
following methods (in decreasing order of preference):
1. The fully qualified domain name (FQDN) of the system on which the
agent is running.
2. A fully qualified domain name (FQDN) within a domain affiliated
with the administrators of the agent and guaranteed to be unique
by the administrators of that domain. For example, the
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RFC 5537 Netnews Architecture and Protocols November 2009
uniqueness of server.example.org could be guaranteed by the
administrator of example.org even if there is no DNS record for
server.example.org itself.
3. Some other (arbitrary) name in the form of a <path-nodot>,
believed to be unique and registered at least with all the other
news servers to which that relaying agent or injecting agent
sends articles. This option SHOULD NOT be used unless the
earlier options are unavailable or unless the name is of
longstanding usage.
Some existing implementations treat <path-identity> as case-
sensitive, some as case-insensitive. The <path-identity> therefore
SHOULD be all lowercase and implementations SHOULD compare identities
case-insensitively.
3.2.1. Constructing the Path Header Field
If a relaying or serving agent receives an article from an injecting
or serving agent that is part of the same news server, it MAY leave
the Path header field of the article unchanged. Otherwise, every
injecting, relaying, or serving agent that accepts an article MUST
update the Path header field as follows. Note that the Path header
field content is constructed from right to left by prepending
elements.
1. The agent MUST prepend "!" to the Path header field content.
2. An injecting agent SHOULD prepend the <path-diagnostic>
"!.POSTED", optionally followed by "." and the FQDN or IP address
of the source, to the Path header field content.
3. A relaying or serving agent SHOULD prepend a <path-diagnostic> to
the Path header field content, where the <path-diagnostic> is
chosen as follows:
* If the expected <path-identity> of the source of the article
matches the leftmost <path-identity> of the Path header
field's content, use "!" (<diag-match>), resulting in two
consecutive "!"s.
* If the expected <path-identity> of the source of the article
does not match, use "!.MISMATCH." followed by the expected
<path-identity> of the source or its IP address.
* If the relaying or serving agent is not willing or able to
check the <path-identity>, use "!.SEEN." followed by the FQDN,
IP address, or expected <path-identity> of the source.
Allbery & Lindsey Standards Track PAGE 8
RFC 5537 Netnews Architecture and Protocols November 2009
The "expected <path-identity> of the source of the article" is a
<path-identity> for the injecting or relaying agent that passed
the article to this relaying or serving agent, determined by
properties of the connection via which the article was received
(for example, an authentication identity or a peer IP address).
Be aware that [RFC 1036] did not include <path-diagnostic>.
Implementations that predate this specification will add only
single "!" characters between <path-identity> strings.
4. The agent MAY then prepend to the Path header field content "!"
or "!!" followed by an additional <path-identity> for itself
other than its primary one. Using "!!", and thereby adding a
<diag-match> since the <path-identity> clearly is verified, is
RECOMMENDED. This step may be repeated any number of times.
This is permitted for agents that have multiple <path-identity>s
(such as during a transition from one to another). Each of these
<path-identity>s MUST meet the requirements set out in
Section 3.2.
5. Finally, the agent MUST prepend its primary <path-identity> to
the Path header field content. The primary <path-identity> is
the <path-identity> it normally advertises to its peers for their
use in generating <path-diagnostic>s as described above.
Any agent that modifies the Path header field MAY fold it by
inserting FWS (folding white space) immediately after any <path-
identity> or <diag-other> it added (see Section 3.1.5 of [RFC 5536]
for allowable locations for FWS).
3.2.2. Path Header Field Example
Here is an example of a Path header field created by following the
rules for injecting and relaying agents.
Path: foo.isp.example!.SEEN.isp.example!foo-news
!.MISMATCH.2001:DB8:0:0:8:800:200C:417A!bar.isp.example
!!old.site.example!barbaz!!baz.isp.example
!.POSTED.dialup123.baz.isp.example!not-for-mail
This article was injected by baz.isp.example as indicated by the
<diag-keyword> "POSTED". The injector has recorded that it received
the article from dialup123.baz.isp.example. "not-for-mail" is a
common <tail-entry>.
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RFC 5537 Netnews Architecture and Protocols November 2009
The article was relayed to the relaying agent known, at least to
old.site.example, as "barbaz". That relaying agent confirmed to its
satisfaction that "baz.isp.example" was an expected <path-identity>
for the source of the article and therefore used <diag-match> ("!")
for its <path-diagnostic>.
barbaz relayed it to old.site.example, which does not support <diag-
keyword> and therefore used the old "!" delimiter. This indicates
that the identity of "barbaz" was not verified and may have been
forged.
old.site.example relayed it to a news server using the <path-
identity> of bar.isp.example and claiming (by using the "!" <path-
diagnostic>) to have verified that it came from old.site.example.
bar.isp.example relayed it to foo-news, which, not being convinced
that it truly came from bar.isp.example, inserted the <diag-keyword>
"MISMATCH" and then stated that it received the article from the IPv6
address [2001:DB8:0:0:8:800:200C:417A]. (This is not to say that
bar.isp.example was not a correct <path-identity> for that source but
simply that the identity did not match the expectations of foo-news.)
foo-news then passed the article to foo.isp.example, which declined
to validate its <path-identity> and instead appended the <diag-
keyword> "SEEN" to indicate it knows the source of the article as
isp.example. This may be either an expected <path-identity> or the
FQDN of the system from which it received the article. Presumably,
foo.isp.example is a serving agent that then delivered the article to
a reading agent.
baz.isp.example, bar.isp.example, and foo-news folded the Path header
field.
3.3. Article History and Duplicate Suppression
Netnews normally uses a flood-fill algorithm for propagation of
articles in which each news server offers the articles it accepts to
multiple peers, and each news server may be offered the same article
from multiple other news servers. Accordingly, duplicate suppression
is key; if a news server accepted every article it was offered, it
may needlessly accept (and then potentially retransmit) dozens of
copies of every article.
Relaying and serving agents therefore MUST keep a record of articles
they have already seen and use that record to reject additional
offers of the same article. This record is called the "history" file
or database.
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RFC 5537 Netnews Architecture and Protocols November 2009
Each article is uniquely identified by its message identifier, so a
relaying or serving agent could satisfy this requirement by storing a
record of every message identifier that agent has ever seen. Such a
history database would grow without bound, however, so it is common
and permitted to optimize based on the Injection-Date or Date header
field of an article as follows. (In the following discussion, the
"date" of an article is defined to be the date represented by its
Injection-Date header field, if present; otherwise, by its Date
header field.)
o Agents MAY select a cutoff interval and reject any article with a
date farther in the past than that cutoff interval. If this
interval is shorter than the time it takes for an article to
propagate through the network, the agent might reject an article
it had not yet seen, so it ought not to be aggressively short.
For Usenet, for example, a cutoff interval of no less than seven
days is conventional.
o Agents that enforce such a cutoff MAY then drop records of
articles that had dates older than the cutoff from their history
databases. If such an article were offered to the agent again, it
would be rejected due to the cutoff date, so the history record is
no longer required to suppress the duplicate.
o Alternatively, agents MAY drop history records according to the
date when the article was first seen by that agent rather than the
date of the article. In this case, the history retention interval
MUST be at least 24 hours longer than the cutoff interval to allow
for articles dated in the future. This interval matches the
allowable error in the date of the article (see Section 3.5).
These are just two implementation strategies for article history,
albeit the most common ones. Relaying and serving agents are not
required to use these strategies, only to meet the requirement of not
accepting an article more than once. However, these strategies are
safe and widely deployed, and implementors are encouraged to use one
of them, especially if they do not have extensive experience with
Netnews and the subtle effects of its flood-fill algorithm.
3.4. Duties of a Posting Agent
A posting agent is the component of a user agent that assists a
poster in creating a valid proto-article and forwarding it to an
injecting agent.
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RFC 5537 Netnews Architecture and Protocols November 2009
Posting agents SHOULD ensure that proto-articles they create are
valid according to [RFC 5536] and any other applicable policies. They
MUST NOT create any Injection-Info header field; this header field
may only be added by the injecting agent.
If the proto-article already contains both Message-ID and Date header
fields, posting agents MAY add an Injection-Date header field to that
proto-article immediately before passing that proto-article to an
injection agent. They SHOULD do so if the Date header field
(representing the composition time of the proto-article) is more than
a day in the past at the time of injection. They MUST do so if the
proto-article is being submitted to more than one injecting agent;
see Section 3.4.2.
The Injection-Date header field is new in this revision of the
Netnews protocol and is designed to allow the Date header field to
hold the composition date (as recommended in Section 3.6.1 of
[RFC 5322]), even if the proto-article is not to be injected for some
time after its composition. However, note that all implementations
predating this specification ignore the Injection-Date header field
and use the Date header field in its stead for rejecting articles
older than their cutoff (see Section 3.3), and injecting agents
predating this specification do not add an Injection-Date header.
Articles with a Date header field substantially in the past will
still be rejected by implementations predating this specification,
regardless of the Injection-Date header field, and hence may suffer
poorer propagation.
Contrary to [RFC 5322], which implies that the mailbox or mailboxes in
the From header field should be that of the poster or posters, a
poster who does not, for whatever reason, wish to use his own mailbox
MAY use any mailbox ending in the top-level domain ".invalid"
[RFC 2606].
Posting agents meant for use by ordinary posters SHOULD reject any
attempt to post an article that cancels or supersedes (via the
Supersedes header field) another article of which the poster is not
the author or sender.
3.4.1. Proto-Articles
A proto-article is an article in the format used by a posting agent
when offering that article to an injecting agent. It may omit
certain header fields that can be better supplied by the injecting
agent and will not contain header fields that are added by the
injecting agent. A proto-article is only for transmission to an
injecting agent and SHOULD NOT be transmitted to any other agent.
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RFC 5537 Netnews Architecture and Protocols November 2009
A proto-article has the same format as a normal article except that
the Injection-Info and Xref header fields MUST NOT be present, the
Path header field SHOULD NOT contain a "POSTED" <diag-keyword>, and
any of the following mandatory header fields MAY be omitted:
Message-ID, Date, and Path. In all other respects, a proto-article
MUST be a valid Netnews article. In particular, the header fields
that may be omitted MUST NOT be present with invalid content.
If a posting agent intends to offer the same proto-article to
multiple injecting agents, the header fields Message-ID, Date, and
Injection-Date MUST be present and identical in all copies of the
proto-article. See Section 3.4.2.
3.4.2. Multiple Injection of Articles
Under some circumstances (for example, when posting to multiple,
supposedly disjoint, networks, when using injecting agents with
spotty connectivity, or when desiring additional redundancy), a
posting agent may wish to offer the same article to multiple
injecting agents. In this unusual case, the goal is not to create
multiple independent articles but rather to inject the same article
at multiple points and let the normal duplicate suppression facility
of Netnews (see Section 3.3) ensure that any given agent accepts the
article only once, even if supposedly disjoint networks have
unexpected links.
Whenever possible, multiple injection SHOULD be done by offering the
same proto-article to multiple injecting agents. The posting agent
MUST supply the Message-ID, Date, and Injection-Date header fields,
and the proto-article as offered to each injecting agent MUST be
identical.
In some cases, offering the same proto-article to all injecting
agents may not be possible (such as when gatewaying, after injection,
articles found on one Netnews network to another supposedly
unconnected one). In this case, the posting agent MUST remove any
Xref header field and rename or remove any Injection-Info, Path, and
other trace header fields before passing it to another injecting
agent. (This converts the article back into a proto-article.) It
MUST retain unmodified the Message-ID, Date, and Injection-Date
header fields. It MUST NOT add an Injection-Date header field if it
is missing from the existing article.
NOTE: Multiple injection inherently risks duplicating articles.
Multiple injection after injection, by converting an article back
to a proto-article and injecting it again, additionally risks
loops, loss of trace information, unintended repeat injection into
the same network, and other problems. It should be done with care
Allbery & Lindsey Standards Track PAGE 13
RFC 5537 Netnews Architecture and Protocols November 2009
and only when there is no alternative. The requirement to retain
Message-ID, Date, and Injection-Date header fields minimizes the
possibility of a loop and ensures that the newly injected article
is not treated as a new, separate article.
Multiple injection of an article that lists one or more moderated
newsgroups in its Newsgroups header field SHOULD only be done by a
moderator and MUST only be done after the proto-article has been
approved for all moderated groups to which it is to be posted and
after an Approved header field has been added (see Section 3.9).
Multiple injection of an unapproved article intended for moderated
newsgroups will normally only result in the moderator receiving
multiple copies, and if the newsgroup status is not consistent across
all injecting agents, may result in duplication of the article or
other problems.
3.4.3. Followups
A followup is an article that contains a response to the contents of
an earlier article, its precursor. In addition to its normal duties,
a posting agent preparing a followup is also subject to the following
requirements. Wherever in the following it is stated that, by
default, a header field is said to be inherited from one of those
header fields in the precursor, it means that its initial content is
to be a copy of the content of that precursor header field (with
changes in folding permitted). However, posters MAY then override
that default before posting.
Despite the historic practice of some posting agents, the Keywords
header field SHOULD NOT be inherited by default from the precursor
article.
1. If the Followup-To header field of the precursor article consists
of "poster", the followup MUST NOT be posted by default but, by
default, is to be emailed to the address given in the precursor's
Reply-To or From header field following the rules for an email
reply [RFC 5322]. This action MAY be overridden by the poster, in
which case the posting agent should continue as if the
Followup-To header field in the precursor did not exist.
2. The Newsgroups header field SHOULD, by default, be inherited from
the precursor's Followup-To header field if present; otherwise,
it is inherited from the precursor's Newsgroups header field.
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RFC 5537 Netnews Architecture and Protocols November 2009
3. The Subject header field SHOULD, by default, be inherited from
that of the precursor. The case-sensitive string "Re: "
(including the space after the colon) MAY be prepended to the
content of its Subject header field unless it already begins with
that string.
NOTE: Prepending "Re: " serves no protocol function and hence
is not required, but it is widely expected and not doing so
would be surprising.
4. The Distribution header field SHOULD, by default, be inherited
from the precursor's Distribution header field, if present.
5. The followup MUST have a References header field referring to its
precursor, constructed in accordance with Section 3.4.4.
3.4.4. Construction of the References Header Field
The following procedure is to be used whenever some previous article
(the "parent") is to be referred to in the References header field of
a new article, whether because the new article is a followup and the
parent is its precursor or for some other reason.
The content of the new article's References header field MUST be
formed from the content of the parent's References header field if
present, followed by the content of the Message-ID header field of
the parent. If the parent had a References header, FWS as defined in
[RFC 5536] MUST be added between its content and the Message-ID header
field content.
If the resulting References header field would, after unfolding,
exceed 998 characters in length (including its field name but not the
final CRLF), it MUST be trimmed (and otherwise MAY be trimmed).
Trimming means removing any number of message identifiers from its
content, except that the first message identifier and the last two
MUST NOT be removed.
An essential property of the References header field, guaranteed by
the above procedure and REQUIRED to be maintained by any extensions
to this protocol, is that an article MUST NOT precede one of its
parents.
3.5. Duties of an Injecting Agent
An injecting agent takes a proto-article from a posting agent and
either forwards it to a moderator or passes it to a relaying or
serving agent or agents. An injecting agent bears the primary
responsibility for ensuring that any article it injects conforms with
Allbery & Lindsey Standards Track PAGE 15
RFC 5537 Netnews Architecture and Protocols November 2009
the rules of the Netnews standards. The administrator of an
injecting agent is also expected to bear some responsibility towards
the rest of the Netnews network to which it is connected for the
articles the injecting agent accepts.
Injecting agents, when rejecting articles, are encouraged to
communicate the reason for rejection to the posting agent by using
whatever facility is provided by the underlying transport. The
injecting agent is in a unique position to communicate the reason for
rejection; relaying agents and serving agents normally have to reject
messages silently. The injecting agent therefore bears much of the
burden of diagnosing broken posting agents or communicating policy
violations to posters.
An injecting agent MUST have available a list (possibly empty) of
moderated groups for which it accepts articles and the corresponding
submission addresses. It SHOULD have available a list of valid
newsgroups to catch articles not posted to a valid newsgroup and
therefore likely to be silently discarded by relaying and serving
agents. Usually, an injecting agent is deployed in conjunction with
a serving agent and maintains these lists based on control messages
received by the serving agent.
An injecting agent processes proto-articles as follows:
1. It SHOULD verify that the article is from a trusted source (for
example, by relying on the authorization capability of the
underlying transport used to talk to the posting agent).
2. It MUST reject any proto-article that does not have the proper
mandatory header fields for a proto-article, that has Injection-
Info or Xref header fields, that has a Path header field
containing the "POSTED" <diag-keyword>, or that is not
syntactically valid as defined by [RFC 5536]. It SHOULD reject
any proto-article that contains a header field deprecated for
Netnews (see, for example, [RFC 3798]). It MAY reject any proto-
article that contains trace header fields (e.g., NNTP-Posting-
Host) indicating that it was already injected by an injecting
agent that did not add Injection-Info or Injection-Date.
3. It SHOULD reject any article whose Injection-Date or Date header
field is more than 24 hours into the future (and MAY use a
margin less than 24 hours). It SHOULD reject any article whose
Injection-Date header field is too far in the past (older than
the cutoff interval of a relaying agent that the injecting agent
is using, for example). It SHOULD similarly reject any article
whose Date header field is too far in the past, since not all
news servers support Injection-Date and only the injecting agent
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RFC 5537 Netnews Architecture and Protocols November 2009
can provide a useful error message to the posting agent. In
either case, this interval SHOULD NOT be any shorter than 72
hours into the past.
4. It SHOULD reject any proto-article whose Newsgroups header field
does not contain at least one <newsgroup-name> for a valid
group, or that contains a <newsgroup-name> reserved for specific
purposes by Section 3.1.4 of [RFC 5536] unless that specific
purpose or local agreement applies to the proto-article being
processed. Crossposting to unknown newsgroups is not precluded
provided that at least one of the newsgroups in the Newsgroups
header is valid.
5. The Message-ID and Date header fields with appropriate contents
MUST be added when not present in the proto-article.
6. The injecting agent MUST NOT alter the body of the article in
any way (including any change of Content-Transfer-Encoding). It
MAY add other header fields not already provided by the poster,
but injecting agents are encouraged to use the Injection-Info
header for such information and to minimize the addition of
other headers. It SHOULD NOT alter, delete, or reorder any
existing header field except the Path header field. It MUST NOT
alter or delete any existing Message-ID header field.
7. If the Newsgroups header contains one or more moderated groups
and the proto-article does not contain an Approved header field,
the injecting agent MUST either forward it to a moderator as
specified in Section 3.5.1 or, if that is not possible, reject
it. This forwarding MUST be done after adding the Message-ID
and Date headers if required, and before adding the Injection-
Info and Injection-Date headers.
8. Otherwise, a Path header field with a <tail-entry> MUST be added
if not already present.
9. The injecting agent MUST then update the Path header field as
described in Section 3.2.1.
10. An Injection-Info header field SHOULD be added that identifies
the source of the article and possibly other trace information
as described in Section 3.2.8 of [RFC 5536].
11. If the proto-article already had an Injection-Date header field,
it MUST NOT be modified or replaced. If the proto-article had
both a Message-ID header field and a Date header field, an
Injection-Date header field MUST NOT be added, since the proto-
article may have been multiply injected by a posting agent that
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RFC 5537 Netnews Architecture and Protocols November 2009
predates this standard. Otherwise, the injecting agent MUST add
an Injection-Date header field containing the current date and
time.
12. Finally, the injecting agent forwards the article to one or more
relaying agents, and the injection process is complete.
3.5.1. Forwarding Messages to a Moderator
An injecting agent MUST forward the proto-article to the moderator of
the leftmost moderated group listed in the Newsgroups header field,
customarily via email. There are two standard ways in which it may
do this:
1. The complete proto-article is encapsulated, header fields and
all, within the email. This SHOULD be done by creating an email
message with a Content-Type of application/news-transmission with
the usage parameter set to "moderate". The body SHOULD NOT
contain any content other than the message. This method has the
advantage of removing any possible conflict between Netnews and
email header fields and any changes to those fields during
transport through email.
2. The proto-article is sent as an email with the addition of any
header fields required for an email as defined in [RFC 5322], and
possibly with the addition of other header fields conventional in
email, such as To and Received. The existing Message-ID header
field SHOULD be retained.
Although both of these methods have been used in the past and the
first has clear technical advantages, the second is in more common
use and many moderators are not prepared to deal with messages in the
first format. Accordingly, the first method SHOULD NOT be used
unless the moderator to which it is being forwarded is known to be
able to handle this method.
NOTE: Deriving the email address of the moderator of a group is
outside the scope of this document. It is worth mentioning,
however, that a common method is to use a forwarding service that
handles submissions for many moderated groups. For maximum
compatibility with existing news servers, such forwarding services
generally form the submission address for a moderated group by
replacing each "." in the <newsgroup-name> with "-" and then using
that value as the <local-part> of a <mailbox> formed by appending
a set domain.
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RFC 5537 Netnews Architecture and Protocols November 2009
Forwarding proto-articles to moderators via email is the most general
method and the most common in large Netnews networks such as Usenet,
but any means of forwarding the article that preserves it without
injecting it MAY be used. For example, if the injecting agent has
access to a database used by the moderator to store proto-articles
awaiting processing, it may place the proto-article directly into
that database. Such methods may be more appropriate for smaller
Netnews networks.
3.6. Duties of a Relaying Agent
A relaying agent accepts injected articles from injecting and other
relaying agents and passes them on to relaying or serving agents. To
avoid bypass of injecting agent policies and forgery of Path and
Injection-Info headers, relaying agents SHOULD accept articles only
from trusted agents.
An article SHOULD NOT be relayed unless the sending agent has been
configured to supply, and the receiving agent to receive, at least
one of the <newsgroup-name>s in its Newsgroups header field and at
least one of the <dist-name>s in its Distribution header field (if
present). Exceptionally, control messages creating or removing
newsgroups (newgroup or rmgroup control messages, for example) SHOULD
be relayed if the affected group appears in its Newsgroups header
field and both the sending and receiving relaying agents are
configured to relay a newsgroup of that name (whether or not such a
newsgroup exists).
In order to avoid unnecessary relaying attempts, an article SHOULD
NOT be relayed if the <path-identity> of the receiving agent (or some
known alias thereof) appears as a <path-identity> (excluding within
the <tail-entry> or following a "POSTED" <diag-keyword>) in its Path
header field.
A relaying agent processes an article as follows:
1. It MUST reject any article without a Newsgroups header field or
Message-ID header field, or without either an Injection-Date or
Date header field.
2. It MUST examine the Injection-Date header field or, if absent,
the Date header field, and reject the article if that date is
more than 24 hours into the future. It MAY reject articles with
dates in the future with a smaller margin than 24 hours.
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RFC 5537 Netnews Architecture and Protocols November 2009
3. It MUST reject any article that has already been accepted. If it
implements one of the mechanisms described in Section 3.3, this
means that it MUST reject any article whose date falls outside
the cutoff interval since it won't know whether or not such
articles had been accepted previously.
4. It SHOULD reject any article that does not include all the
mandatory header fields. It MAY reject any article that contains
header fields that do not have valid contents.
5. It SHOULD reject any article that matches an already-received
cancel control message or the contents of the Supersedes header
field of an accepted article, provided that the relaying agent
has chosen (on the basis of local site policy) to honor that
cancel control message or Supersedes header field.
6. It MAY reject any article without an Approved header field posted
to a newsgroup known to be moderated. This practice is strongly
encouraged, but the information necessary to do so is not
required to be maintained by a relaying agent.
7. It MUST update the Path header field as described in
Section 3.2.1.
8. It MAY delete any Xref header field already present. It MAY add
a new Xref header field for its own use (but recall that
[RFC 5536] permits at most one such header field).
9. Finally, it passes the article on to other relaying and serving
agents to which it is configured to send articles.
Relaying agents SHOULD, where possible in the underlying transport,
inform the agent that passed the article to the relaying agent if the
article was rejected. Relaying agents MUST NOT inform any other
external entity of the rejection of an article unless that external
entity has explicitly requested that it be informed of such errors.
Relaying agents MUST NOT alter, delete, or rearrange any part of an
article except for the Path and Xref header fields. They MUST NOT
modify the body of articles in any way. If an article is not
acceptable as is, the article MUST be rejected rather than modified.
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RFC 5537 Netnews Architecture and Protocols November 2009
3.7. Duties of a Serving Agent
A serving agent accepts articles from a relaying agent or injecting
agent, stores them, and makes them available to reading agents.
Articles are normally indexed by newsgroup and <article-locator>
(Section 3.2.14 of [RFC 5536]), usually in the form of a decimal
number.
If the serving agent stores articles by newsgroup, control messages
SHOULD NOT be stored in the newsgroups listed in the control
message's Newsgroups header field. Instead, they SHOULD be stored in
a newsgroup in the hierarchy "control", which is reserved for this
purpose. Conventionally, control messages are stored in newsgroups
named for the type of control message (such as "control.cancel" for
cancel control messages).
A serving agent MUST have available a list (possibly empty) of
moderated groups for which it accepts articles so that it can reject
unapproved articles posted to moderated groups. Frequently, a
serving agent is deployed in combination with an injecting agent and
can use the same list as the injecting agent.
A serving agent processes articles as follows:
1. It MUST reject any article that does not include all the
mandatory header fields or any article that contains header
fields that do not have valid contents.
2. It MUST examine the Injection-Date header field or, if absent,
the Date header field, and reject the article if that date is
more than 24 hours into the future. It MAY reject articles with
dates in the future with a smaller margin than 24 hours.
3. It MUST reject any article that has already been accepted. If it
implements one of the mechanisms described in Section 3.3, this
means that it MUST reject any article whose date falls outside
the cutoff interval since it won't know whether or not such
articles had been accepted previously.
4. It SHOULD reject any article that matches an already-received and
honored cancel message or Supersedes header field, following the
same rules as a relaying agent (Section 3.6).
5. It MUST reject any article without an Approved header field
posted to any newsgroup listed as moderated.
6. It MUST update the Path header field as described in
Section 3.2.1.
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RFC 5537 Netnews Architecture and Protocols November 2009
7. It MUST remove any Xref header field from each article (except
when specially configured to preserve the <article-locator>s set
by the sending site). It then MAY (and usually will) add a new
Xref header field (but recall that [RFC 5536] permits at most one
such header field).
8. Finally, it stores the article and makes it available for reading
agents.
Serving agents MUST NOT create new newsgroups simply because an
unrecognized <newsgroup-name> occurs in a Newsgroups header field.
Newsgroups are normally created via control messages (Section 5.2.1).
Serving agents MUST NOT alter, delete, or rearrange any part of an
article except for the Path and Xref header fields. They MUST NOT
modify the body of the articles in any way. If an article is not
acceptable as is, the article MUST be rejected rather than modified.
3.8. Duties of a Reading Agent
Since a reading agent is only a passive participant in a Netnews
network, there are no specific protocol requirements placed on it.
See [USEAGE] for best-practice recommendations.
3.9. Duties of a Moderator
A moderator receives news articles, customarily by email, decides
whether to approve them and, if so, either passes them to an
injecting agent or forwards them to further moderators.
Articles are normally received by the moderator in email, either
encapsulated as an object of Content-Type application/
news-transmission (or possibly encapsulated but without an explicit
Content-Type header field) or else directly as an email already
containing all the header fields appropriate for a Netnews article
(see Section 3.5.1). Moderators who may receive articles via email
SHOULD be prepared to accept articles in either format.
There are no protocol restrictions on what criteria are used for
accepting or rejecting messages or on what modifications a moderator
may make to a message (both header fields and body) before injecting
it. Recommended best practice, however, is to make the minimal
required changes. Moderators need to be aware that modifications
made to articles may invalidate signatures created by the poster or
previous moderators. See [USEAGE] for further best-practice
recommendations.
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RFC 5537 Netnews Architecture and Protocols November 2009
Moderators process articles as follows:
1. They decide whether to approve or reject a proto-article and, if
approved, prepare the proto-article for injection. If the proto-
article was received as an unencapsulated email message, this
will require converting it back to a valid Netnews proto-article.
If the article is rejected, it is normally rejected for all
newsgroups to which it was posted and nothing further is done.
If it is approved, the moderator proceeds with the following
steps.
2. If the Newsgroups header field contains further moderated
newsgroups for which approval has not already been given, they
may either reach some agreement with the other moderators on the
disposition of the article or, more generally, add an indication
(identifying both the moderator and the name of the newsgroup)
that they approve the article and then forward it to the
moderator of the leftmost unapproved newsgroup. This forwarding
SHOULD be done following the procedure in Section 3.5.1. It MAY
be done by rotating the <newsgroup-name>s in the Newsgroups
header field so that the leftmost unapproved newsgroup is the
leftmost moderated newsgroup in that field and then posting it,
letting the injecting agent do the forwarding. However, when
using this mechanism, they MUST first ensure that the article
contains no Approved header field.
3. If the Newsgroups header field contains no further unapproved
moderated groups, they add an Approved header field (see Section
3.2.1 of [RFC 5536]) identifying the moderator and, insofar as is
possible, all the other moderators who have approved the article.
The moderator who takes this step assumes responsibility for
ensuring that the article was approved by the moderators of all
moderated newsgroups to which it was posted.
4. Moderators are encouraged to retain the Message-ID header field
unless it is invalid or the article was significantly changed
from its original form. Moderators are also encouraged to retain
the Date header field unless it appears to be stale (72 hours or
more in the past) for reasons understood by the moderator (such
as delays in the moderation process), in which case they MAY
substitute the current date. Any Injection-Date, Injection-Info,
or Xref header fields already present MUST be removed.
5. Any Path header field MUST either be removed or truncated to only
those entries following its "POSTED" <diag-keyword>, if any.
6. The moderator then passes the article to an injecting agent,
having first observed all the duties of a posting agent.
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RFC 5537 Netnews Architecture and Protocols November 2009
3.10. Duties of a Gateway
A gateway transforms an article into the native message format of
another medium, or translates the messages of another medium into
news articles, or transforms articles into proto-articles for
injection into a separate Netnews network. Encapsulation of a news
article into a message of MIME type application/news-transmission, or
the subsequent undoing of that encapsulation, is not gatewaying since
it involves no transformation of the article.
There are two basic types of gateway, the outgoing gateway that
transforms a news article into a different type of message, and the
incoming gateway that transforms a message from another network into
a news proto-article and injects it into a Netnews network. These
are handled separately below.
Transformation of an article into another medium stands a very high
chance of discarding or interfering with the protection inherent in
the news system against duplicate articles. The most common problem
caused by gateways is loops that repeatedly reinject previously
posted articles. To prevent this, a gateway MUST take precautions
against loops, as detailed below.
The transformations applied to the message SHOULD be as minimal as
possible while still accomplishing the gatewaying. Every change made
by a gateway potentially breaks a property of one of the media or
loses information, and therefore only those transformations made
necessary by the differences between the media should be applied.
If bidirectional gatewaying (both an incoming and an outgoing
gateway) is being set up between Netnews and some other medium, the
incoming and outgoing gateways SHOULD be coordinated to avoid
unintended reinjection of gated articles. Circular gatewaying
(gatewaying a message into another medium and then back into Netnews)
SHOULD NOT be done; encapsulation of the article SHOULD be used
instead where this is necessary.
Safe bidirectional gatewaying between a mailing list and a newsgroup
is far easier if the newsgroup is moderated. Posts to the moderated
group and submissions to the mailing list can then go through a
single point that does the necessary gatewaying and then sends the
message out to both the newsgroup and the mailing list at the same
time, eliminating most of the possibility of loops. Bidirectional
gatewaying between a mailing list and an unmoderated newsgroup, in
contrast, is difficult to do correctly and is far more fragile.
Newsgroups intended to be bidirectionally gated to a mailing list
SHOULD therefore be moderated where possible, even if the moderator
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RFC 5537 Netnews Architecture and Protocols November 2009
is a simple gateway and injecting agent that correctly handles
crossposting to other moderated groups and otherwise passes all
traffic.
3.10.1. Duties of an Outgoing Gateway
From the perspective of Netnews, an outgoing gateway is just a
special type of reading agent. The exact nature of what the outgoing
gateway will need to do to articles depends on the medium to which
the articles are being gated. Because it raises the danger of loops
due to the possibility of one or more corresponding incoming gateways
back from that medium to Netnews, the operation of the outgoing
gateway is subject to additional constraints.
The following practices are encouraged for all outgoing gateways,
regardless of whether there is known to be a related incoming
gateway, both as precautionary measures and as guidelines to quality
of implementation:
1. The message identifier of the news article should be preserved if
at all possible, preferably as or within the corresponding unique
identifier of the other medium. However, if it is not preserved
in this way, then at least it should be preserved as a comment in
the message. This helps greatly with preventing loops.
2. The Date and Injection-Date of the news article should also be
preserved if possible, for similar reasons.
3. The message should be tagged in some way so as to prevent its
reinjection into Netnews. This may be impossible to do without
knowledge of potential incoming gateways, but it is better to try
to provide some indication even if not successful; at the least,
a human-readable indication that the article should not be gated
back to Netnews can help locate a human problem.
4. Netnews control messages should not be gated to another medium
unless they would somehow be meaningful in that medium.
3.10.2. Duties of an Incoming Gateway
The incoming gateway has the responsibility of ensuring that all of
the requirements of this protocol are met by the articles that it
forms. In addition to its special duties as a gateway, it bears all
of the duties and responsibilities of a posting agent, and it has the
same responsibility of a relaying agent to reject articles that it
has already gatewayed.
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RFC 5537 Netnews Architecture and Protocols November 2009
An incoming gateway MUST NOT gate the same message twice. It may not
be possible to ensure this in the face of mangling or modification of
the message, but at the very least a gateway, when given a copy of a
message that it has already gated and that is identical except for
trace header fields (like Received in Email or Path in Netnews), MUST
NOT gate the message again. An incoming gateway SHOULD take
precautions against having this rule bypassed by modifications of the
message that can be anticipated.
News articles prepared by gateways MUST be valid news proto-articles
(see Section 3.4.1). This often requires the gateway to synthesize a
conforming article from non-conforming input. The gateway MUST then
pass the article to an injecting agent, not directly to a relaying
agent.
Incoming gateways MUST NOT pass control messages (articles containing
a Control or Supersedes header field) without removing or renaming
that header field. Gateways MAY, however, generate cancel control
messages for messages they have gatewayed. If a gateway receives a
message that it can determine is a valid equivalent of a cancel
control message in the medium it is gatewaying, it SHOULD discard
that message without gatewaying it, generate a corresponding cancel
control message of its own, and inject that cancel control message.
NOTE: It is not unheard of for mail-to-news gateways to be used to
post control messages, but encapsulation should be used for these
cases instead. Gateways by their very nature are particularly
prone to loops. Spews of normal articles are bad enough; spews of
control messages with special significance to the news system,
possibly resulting in high processing load or even in emails being
sent for every message received, are catastrophic. It is far
preferable to construct a system specifically for posting control
messages that can do appropriate consistency checks and
authentication of the originator of the message.
If there is a message identifier that fills a role similar to that of
the Message-ID header field in news, it SHOULD be used in the
formation of the message identifier of the news article, perhaps with
transformations required to meet the uniqueness requirement of
Netnews and with the removal of any comments so as to comply with the
syntax in Section 3.1.3 of [RFC 5536]. Such transformations SHOULD be
designed so that two messages with the same identifier generate the
same Message-ID header field.
NOTE: Message identifiers play a central role in the prevention of
duplicates, and their correct use by gateways will do much to
prevent loops. Netnews does, however, require that message
identifiers be unique, and therefore message identifiers from
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RFC 5537 Netnews Architecture and Protocols November 2009
other media may not be suitable for use without modification. A
balance must be struck by the gateway between preserving
information used to prevent loops and generating unique message
identifiers.
Exceptionally, if there are multiple incoming gateways for a
particular set of messages, each to a different newsgroup(s), each
one SHOULD generate a message identifier unique to that gateway.
Each incoming gateway nonetheless MUST ensure that it does not gate
the same message twice.
NOTE: Consider the example of two gateways of a given mailing list
into two separate Usenet newsgroups, both of which preserve the
email message identifier. Each newsgroup may then receive a
portion of the messages (different sites seeing different
portions). In these cases, where there is no one "official"
gateway, some other method of generating message identifiers has
to be used to avoid collisions. It would obviously be preferable
for there to be only one gateway that crossposts, but this may not
be possible to coordinate.
If no date information is available, the gateway MAY supply a Date
header field with the gateway's current date. If only partial
information is available (such as date but not time), this SHOULD be
fleshed out to a full Date by adding default values rather than by
discarding this information. Only in very exceptional circumstances
should Date information be discarded, as it plays an important role
in preventing reinjection of old messages.
An incoming gateway MUST add a Sender header field to the news
article it forms by containing the <mailbox> of the administrator of
the gateway. Problems with the gateway may be reported to this
<mailbox>. The <display-name> portion of this <mailbox> SHOULD
indicate that the entity responsible for injection of the message is
a gateway. If the original message already had a Sender header
field, it SHOULD be renamed to Original-Sender so that its contents
can be preserved. See Section 3.10.3 for the specification of that
header field.
3.10.3. Original-Sender Header Field
The Original-Sender header field holds the content of a Sender header
field in an original message received by an incoming gateway,
preserving it while the incoming gateway adds its own Sender header
field. The content syntax makes use of syntax defined in [RFC 5536]
and [RFC 5322].
Allbery & Lindsey Standards Track PAGE 27
RFC 5537 Netnews Architecture and Protocols November 2009
header =/ Original-Sender-header
Original-Sender-header
= "Original-Sender" ":" SP
Original-Sender-content
Original-Sender-content
= mailbox
The Permanent Message Header Field Repository entry for this header
field is:
Header field name: Original-Sender
Applicable protocol: Netnews
Status: standard
Author/Change controller: IETF
Specification document(s): RFC 5537
3.10.4. Gateway Example
To illustrate the type of precautions that should be taken against
loops, here is an example of the measures taken by one particular
combination of mail-to-news and news-to-mail gateways designed to
handle bidirectional gatewaying between mailing lists and unmoderated
groups:
1. The news-to-mail gateway preserves the message identifier of the
news article in the generated email message. The mail-to-news
gateway likewise preserves the email message identifier, provided
that it is syntactically valid for Netnews. This allows the news
system's built-in suppression of duplicates to serve as the first
line of defense against loops.
2. The news-to-mail gateway adds an X-* header field to all messages
it generates. The mail-to-news gateway discards any incoming
messages containing this header field. This is robust against
mailing list managers that replace the message identifier and
against any number of email hops, provided that the other message
header fields are preserved.
3. The mail-to-news gateway prepends the host name from which it
received the email message to the content of the Path header
field. The news-to-mail gateway refuses to gateway any message
that contains the list server name in its Path header field
(including in the tail section). This is robust against any
amount of munging of the message header fields by the mailing
list, provided that the email only goes through one hop.
Allbery & Lindsey Standards Track PAGE 28
RFC 5537 Netnews Architecture and Protocols November 2009
4. The mail-to-news gateway is designed never to generate bounces to
the envelope sender. Instead, articles that are rejected by the
news server (for reasons not warranting silent discarding of the
message) result in a bounce message sent to an errors address
that is known not to forward to any mailing lists. In this way,
they can be handled by the news administrators.
These precautions have proven effective in practice at preventing
loops for this particular application (bidirectional gatewaying
between mailing lists and locally distributed newsgroups where both
gateways can be designed together). General gatewaying to world-wide
newsgroups poses additional difficulties; one must be very wary of
strange configurations, such as a newsgroup gated to a mailing list
that is in turn gated to a different newsgroup.
4. Media Types
This document defines several media types, which have been registered
with IANA as provided for in [RFC 4288].
The media type message/news, as previously registered with IANA, is
hereby declared obsolete. The intent of this media type was to
define a standard way of transmitting news articles via mail for
human reading. However, it was never widely implemented, and its
default treatment as application/octet-stream by agents that did not
recognize it was counter-productive. The media type message/RFC 822
(defined in Section 5.2.1 of [RFC 2046]) SHOULD be used in its place.
The updated MIME media type definition of message/news is:
MIME type name: message
MIME subtype name: news
Required parameters: none
Optional parameters: none
Encoding considerations: same as message/RFC 822
Security considerations: News articles may constitute "control
messages", which can have effects on a
host's news system beyond just addition
of information. Since control messages
may occur in normal news flow, most hosts
are suitably defended against undesired
effects already, but transmission of news
articles via mail may bypass
Allbery & Lindsey Standards Track PAGE 29
RFC 5537 Netnews Architecture and Protocols November 2009
firewall-type defenses. Reading a news
article transmitted by mail involves no
hazards beyond those of mail, but
submitting it to news software for
processing should be done with care.
Interoperability considerations:
Rarely used, and therefore often
handled as application/octet-stream.
Disposition should by default be inline.
Published specification: RFC 5537
Applications that use this media type:
Some old mail and news user agents.
Intended usage: OBSOLETE
Author: Henry Spencer
Change controller: IETF
4.1. application/news-transmission
The media type application/news-transmission is intended for the
encapsulation of complete news articles where the intention is that
the recipient should then inject them into Netnews. This application
type provides one of the methods for mailing articles to moderators
(see Section 3.5.1) and may be used to convey messages to an
injecting agent. This encapsulation removes the need to transform an
email message into a Netnews proto-article and provides a way to send
a Netnews article using MIME through a transport medium that does not
support 8bit data.
The MIME media type definition of application/news-transmission is:
MIME type name: application
MIME subtype name: news-transmission
Required parameters: none
Optional parameters: One and only one of "usage=moderate",
"usage=inject", or "usage=relay".
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RFC 5537 Netnews Architecture and Protocols November 2009
Encoding considerations: A transfer-encoding different from that
of the article transmitted MAY be
supplied to ensure correct transmission
over some 7bit transport medium.
Security considerations: News articles may constitute "control
messages", which can have effects on a
host's news system beyond just addition
of information. Since control messages
may occur in normal news flow, most hosts
are suitably defended against undesired
effects already, but transmission of news
articles via mail may bypass
firewall-type defenses.
Published specification: RFC 5537
Body part: A complete proto-article ready for
injection into Netnews or an article
being relayed to another agent.
Applications that use this media type:
Injecting agents, Netnews moderators.
Intended usage: COMMON
Change controller: IETF
usage=moderate indicates the article is intended for a moderator,
usage=inject for an injecting agent, and usage=relay for a relaying
agent. The entity receiving the article may only implement one type
of agent, in which case the parameter MAY be omitted.
Contrary to the prior registration of this media type, article
batches are not permitted as a body part. Multiple messages or a
message with multiple application/news-transmission parts may be used
instead.
4.2. application/news-groupinfo
The application/news-groupinfo media type is used in conjunction with
the newgroup control message (see Section 5.2.1). Its body part
contains brief information about a newsgroup: the newsgroup name, its
description, and its moderation status.
The MIME media type definition of application/news-groupinfo is:
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RFC 5537 Netnews Architecture and Protocols November 2009
MIME type name: application
MIME subtype name: news-groupinfo
Required parameters: none
Optional parameters: charset, which MUST be a charset
registered for use with MIME text types.
It has the same syntax as the parameter
defined for text/plain [RFC 2046].
Specifies the charset of the body part.
If not given, the charset defaults to
US-ASCII [ASCII].
Encoding considerations: 7bit or 8bit encoding MUST be used to
maintain compatibility.
Security considerations: None.
Interoperability considerations:
Disposition should by default be inline.
Applications that use this media type:
Control message issuers, relaying
agents, serving agents.
Published specification: RFC 5537
Intended usage: COMMON
Change controller: IETF
The content of the application/news-groupinfo body part is defined
as:
groupinfo-body = [ newsgroups-tag CRLF ]
newsgroups-line CRLF
newsgroups-tag = %x46.6F.72 SP %x79.6F.75.72 SP
%x6E.65.77.73.67.72.6F.75.70.73 SP
%x66.69.6C.65.3A
; case sensitive
; "For your newsgroups file:"
newsgroups-line = newsgroup-name
[ 1*HTAB newsgroup-description ]
[ *WSP moderation-flag ]
newsgroup-description
= eightbit-utext *( *WSP eightbit-utext )
moderation-flag = SP "(" %x4D.6F.64.65.72.61.74.65.64 ")"
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RFC 5537 Netnews Architecture and Protocols November 2009
; SPACE + case sensitive "(Moderated)"
eightbit-utext = VCHAR / %d127-255
This unusual format is backward-compatible with the scanning of the
body of newgroup control messages for descriptions done by Netnews
implementations that predate this specification. Although optional,
the <newsgroups-tag> SHOULD be included for backward compatibility.
The <newsgroup-description> MUST NOT contain any occurrence of the
string "(Moderated)" within it. Moderated newsgroups MUST be marked
by appending the case-sensitive text " (Moderated)" at the end.
While a charset parameter is defined for this media type, most
existing software does not understand MIME header fields or correctly
handle descriptions in a variety of charsets. Using a charset of US-
ASCII where possible is therefore RECOMMENDED; if not possible, UTF-8
[RFC 3629] SHOULD be used. Regardless of the charset used, the
constraints of the above grammar MUST be met and the <newsgroup-name>
MUST be represented in that charset using the same octets as would be
used with a charset of US-ASCII.
4.3. application/news-checkgroups
The application/news-checkgroups media type contains a list of
newsgroups within a hierarchy or hierarchies, including their
descriptions and moderation status. It is primarily for use with the
checkgroups control message (see Section 5.2.3).
The MIME media type definition of application/news-checkgroups is:
MIME type name: application
MIME subtype name: news-checkgroups
Required parameters: none
Optional parameters: charset, which MUST be a charset
registered for use with MIME text types.
It has the same syntax as the parameter
defined for text/plain [RFC 2046].
Specifies the charset of the body part.
If not given, the charset defaults to
US-ASCII [ASCII].
Encoding considerations: 7bit or 8bit encoding MUST be used to
maintain compatibility.
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RFC 5537 Netnews Architecture and Protocols November 2009
Security considerations: This media type provides only a means
for conveying a list of newsgroups and
does not provide any information
indicating whether the sender is
authorized to state which newsgroups
should exist within a hierarchy. Such
authorization must be accomplished by
other means.
Interoperability considerations:
Disposition should by default be inline.
Applications that use this media type:
Control message issuers, relaying
agents, serving agents.
Published specification: RFC 5537
Intended usage: COMMON
Change controller: IETF
The content of the application/news-checkgroups body part is defined
as:
checkgroups-body = *( valid-group CRLF )
valid-group = newsgroups-line ; see Section 4.2
The same restrictions on charset, <newsgroup-name>, and <newsgroup-
description> apply for this media type as for application/
news-groupinfo.
One application/news-checkgroups message may contain information for
one or more hierarchies and is considered complete for any hierarchy
for which it contains a <valid-group> unless accompanied by external
information limiting its scope (such as a <chkscope> parameter to a
checkgroups control message, as described in Section 5.2.3). In
other words, an application/news-checkgroups body part consisting of
example.moderated A moderated newsgroup (Moderated)
example.test An unmoderated test group
is a statement that the example.* hierarchy contains two newsgroups,
example.moderated and example.test, and no others. This media type
therefore MUST NOT be used for conveying partial information about a
hierarchy; if a group from a given hierarchy is present, all groups
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RFC 5537 Netnews Architecture and Protocols November 2009
that exist in that hierarchy MUST be listed unless its scope is
limited by external information, in which case all groups SHOULD be
listed.
Spaces are used in this example for formatting reasons. In an actual
message, the newsgroup name and description MUST be separated by one
or more tabs (HTAB, ASCII %d09), not spaces.
5. Control Messages
A control message is an article that contains a Control header field
and thereby indicates that some action should be taken by an agent
other than distribution and display. Any article containing a
Control header field (defined in Section 3.2.3 of [RFC 5536]) is a
control message. Additionally, the action of an article containing a
Supersedes header field is described here; while such an article is
not a control message, it specifies an action similar to the cancel
control message.
The <control-command> of a Control header field comprises a <verb>,
which indicates the action to be taken, and one or more <argument>
values, which supply the details. For some control messages, the
body of the article is also significant. Each recognized <verb> (the
control message type) is described in a separate section below.
Agents MAY accept other control message types than those specified
below, and MUST either ignore or reject control messages with
unrecognized types. Syntactic definitions of valid <argument> values
and restrictions on control message bodies are given in the section
for each control message type.
Contrary to [RFC 1036], the presence of a Subject header field
starting with the string "cmsg " MUST NOT cause an article to be
interpreted as a control message. Agents MAY reject an article that
has such a Subject header field and no Control header field as
ambiguous. Likewise, the presence of a <newsgroup-name> ending in
".ctl" in the Newsgroups header field or the presence of an Also-
Control header field MUST NOT cause the article to be interpreted as
a control message.
5.1. Authentication and Authorization
Control messages specify actions above and beyond the normal
processing of an article and are therefore potential vectors of abuse
and unauthorized action. There is, at present, no standardized means
of authenticating the sender of a control message or verifying that
the contents of a control message were sent by the claimed sender.
There are, however, some unstandardized authentication mechanisms in
common use, such as [PGPVERIFY].
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RFC 5537 Netnews Architecture and Protocols November 2009
Agents acting on control messages SHOULD take steps to authenticate
control messages before acting on them, as determined by local
authorization policy. Whether this is done via the use of an
unstandardized authentication protocol, by comparison with
information obtained through another protocol, by human review, or by
some other means is left unspecified by this document. Further
extensions or revisions of this protocol are expected to standardize
a digital signature mechanism.
Agents are expected to have their own local authorization policies
for which control messages will be honored. No Netnews agent is ever
required to act on any control message. The following descriptions
specify the actions that a control message requests, but an agent MAY
always decline to act on any given control message.
5.2. Group Control Messages
A group control message is any control message type that requests
some update to the list of newsgroups known to a news server. The
standard group control message types are "newgroup", "rmgroup", and
"checkgroups".
Before honoring any group control message, an agent MUST check the
newsgroup or newsgroups affected by that control message and decline
to create any newsgroups not in conformance with the restrictions in
Section 3.1.4 of [RFC 5536].
All of the group control messages MUST have an Approved header field
(Section 3.2.1 of [RFC 5536]). Group control messages without an
Approved header field SHOULD NOT be honored.
Group control messages affecting specific groups (newgroup and
rmgroup control messages, for example) SHOULD include the <newsgroup-
name> for the group or groups affected in their Newsgroups header
field. Other newsgroups MAY be included in the Newsgroups header
field so that the control message will reach more news servers, but
due to the special relaying rules for group control messages (see
Section 3.6) this is normally unnecessary and may be excessive.
5.2.1. The newgroup Control Message
The newgroup control message requests that the specified group be
created or, if already existing, that its moderation status or
description be changed. The syntax of its Control header field is:
control-command =/ Newgroup-command
Newgroup-command = "newgroup" Newgroup-arguments
Newgroup-arguments = 1*WSP newsgroup-name
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RFC 5537 Netnews Architecture and Protocols November 2009
[ 1*WSP newgroup-flag ]
newgroup-flag = "moderated"
If the request is honored, the moderation status of the group SHOULD
be set in accordance with the presence or absence of the <newgroup-
flag> "moderated". "moderated" is the only flag defined by this
protocol. Other flags MAY be defined by extensions to this protocol
and accepted by agents. If an agent does not recognize the
<newgroup-flag> of a newgroup control message, it SHOULD ignore that
control message.
The body of a newgroup message SHOULD contain an entity of type
application/news-groupinfo specifying the description of the
newsgroup, either as the entire body or as an entity within a
multipart/mixed object [RFC 2046]. If such an entity is present, the
moderation status specified therein MUST match the moderation status
specified by the <newgroup-flag>. The body of a newgroup message MAY
contain other entities (encapsulated in multipart/mixed) that provide
additional information about the newsgroup or the circumstances of
the control message.
In the absence of an application/news-groupinfo entity, a news server
MAY search the body of the message for the line "For your newsgroups
file:" and take the following line as a <newsgroups-line>. Prior to
the standardization of application/news-groupinfo, this was the
convention for providing a newsgroup description.
If the request is honored and contains a newsgroup description, and
if the news server honoring it stores newsgroup descriptions, the
stored newsgroup description SHOULD be updated to the description
specified in the control message, even if no other property of the
group has changed.
5.2.1.1. newgroup Control Message Example
A newgroup control message requesting creation of the moderated
newsgroup example.admin.info.
From: "example.* Administrator" <admin@noc.example>
Newsgroups: example.admin.info
Date: 27 Feb 2002 12:50:22 +0200
Subject: cmsg newgroup example.admin.info moderated
Approved: admin@noc.example
Control: newgroup example.admin.info moderated
Message-ID: <ng-example.admin.info-20020227@noc.example>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="nxtprt"
Content-Transfer-Encoding: 8bit
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RFC 5537 Netnews Architecture and Protocols November 2009
This is a MIME control message.
--nxtprt
Content-Type: application/news-groupinfo; charset=us-ascii
For your newsgroups file:
example.admin.info About the example.* groups (Moderated)
--nxtprt
Content-Type: text/plain; charset=us-ascii
A moderated newsgroup for announcements about new newsgroups in
the example.* hierarchy.
--nxtprt--
Spaces are used in this example for formatting reasons. In an actual
message, the newsgroup name and description MUST be separated by one
or more tabs (HTAB, ASCII %d09), not spaces.
5.2.2. The rmgroup Control Message
The rmgroup control message requests that the specified group be
removed from a news server's list of valid groups. The syntax of its
Control header field is:
control-command =/ Rmgroup-command
Rmgroup-command = "rmgroup" Rmgroup-arguments
Rmgroup-arguments = 1*WSP newsgroup-name
The body of the control message MAY contain anything, usually an
explanatory text.
5.2.3. The checkgroups Control Message
The checkgroups control message contains a list of all the valid
groups in a hierarchy with descriptions and moderation status. It
requests that a news server update its valid newsgroup list for that
hierarchy to include the groups specified, remove any groups not
specified, and update group descriptions and moderation status to
match those given in the checkgroups control message. The syntax of
its Control header field is:
control-command =/ Checkgroup-command
Checkgroup-command = "checkgroups" Checkgroup-arguments
Checkgroup-arguments= [ chkscope ] [ chksernr ]
chkscope = 1*( 1*WSP ["!"] newsgroup-name )
chksernr = 1*WSP "#" 1*DIGIT
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RFC 5537 Netnews Architecture and Protocols November 2009
A checkgroups message is interpreted as an exhaustive list of the
valid groups in all hierarchies or sub-hierarchies with a prefix
listed in the <chkscope> argument, excluding any sub-hierarchy where
the <chkscope> argument is prefixed by "!". For complex cases with
multiple <chkscope> arguments, start from an empty list of groups,
include all groups in the checkgroups control message matching
<chkscope> arguments without a "!" prefix, and then exclude all
groups matching <chkscope> arguments with a "!" prefix. Follow this
method regardless of the order of the <chkscope> arguments in the
Control header field.
If no <chkscope> argument is given, it applies to all hierarchies for
which group statements appear in the body of the message.
Since much existing software does not honor the <chkscope> argument,
the body of the checkgroups control message MUST NOT contain group
statements for newsgroups outside the intended scope and SHOULD
contain a correct newsgroup list even for sub-hierarchies excluded
with "!" <chkscope> terms. News servers, however, MUST honor
<chkscope> as specified here.
The <chksernr> argument may be any positive integer. If present, it
MUST increase with every change to the newsgroup list, MUST NOT ever
decrease, and MUST be included in all subsequent checkgroups control
messages with the same scope. If provided, news servers SHOULD
remember the <chksernr> value of the previous checkgroups control
message honored for a particular hierarchy or sub-hierarchy and
decline to honor any subsequent checkgroups control message for the
same hierarchy or sub-hierarchy with a smaller <chksernr> value or
with no <chksernr> value.
There is no upper limit on the length of <chksernr>, other than the
limitation on the length of header fields. Implementations may
therefore want to do comparisons by zero-padding the shorter of two
<chksernr> values on the left and then doing a string comparison,
rather than assuming <chksernr> can be manipulated as a number.
For example, the following Control header field
Control: checkgroups de !de.alt #2009021301
indicates that the body of the message will list every newsgroup in
the de.* hierarchy, excepting the de.alt.* sub-hierarchy, and should
not be honored if a checkgroups control message with a serial number
greater than 2009021301 was previously honored. The serial number in
this example was formed from the date in YYYYMMDD format, followed by
a two-digit sequence number within that date.
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RFC 5537 Netnews Architecture and Protocols November 2009
The body of the message is an entity of type application/
news-checkgroups. It SHOULD be declared as such with appropriate
MIME headers, but news servers SHOULD interpret checkgroups messages
that lack the appropriate MIME headers as if the body were of type
application/news-checkgroups for backward compatibility.
5.3. The cancel Control Message
The cancel control message requests that a target article be
withdrawn from circulation and access. The syntax of its Control
header field is:
control-command =/ Cancel-command
Cancel-command = "cancel" Cancel-arguments
Cancel-arguments = 1*WSP msg-id
The argument identifies the article to be cancelled by its message
identifier. The body of the control message MAY contain anything,
usually an explanatory text.
A serving agent that elects to honor a cancel message SHOULD make the
article unavailable to reading agents (perhaps by deleting it
completely). If the cancel control message arrives before the
article it targets, news servers choosing to honor it SHOULD remember
the message identifier that was cancelled and reject the cancelled
article when it arrives.
To best ensure that it will be relayed to the same news servers as
the original message, a cancel control message SHOULD have the same
Newsgroups header field as the message it is cancelling.
Cancel control messages listing moderated newsgroups in their
Newsgroups header field MUST contain an Approved header field like
any other article in a moderated newsgroup. This means that cancels
posted to a moderated newsgroup will normally be sent to the
moderator first for approval. Outside of moderated newsgroups,
cancel messages are not required to contain an Approved header field.
Contrary to [RFC 1036], cancel control messages are not required to
contain From and Sender header fields matching the target message.
This requirement only encouraged cancel issuers to conceal their
identity and provided no security.
5.4. The Supersedes Header Field
The presence of a Supersedes header field in an article requests that
the message identifier given in that header field be withdrawn in
exactly the same manner as if it were the target of a cancel control
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RFC 5537 Netnews Architecture and Protocols November 2009
message. Accordingly, news servers SHOULD apply to a Supersedes
header field the same authentication and authorization checks as they
would apply to cancel control messages. If the Supersedes header
field is honored, the news server SHOULD take the same actions as it
would take when honoring a cancel control message for the given
target article.
The article containing the Supersedes header field, whether or not
the Supersedes header field is honored, SHOULD be handled as a normal
article and SHOULD NOT receive the special treatment of control
messages described in Section 3.7.
5.5. The ihave and sendme Control Messages
The ihave and sendme control messages implement a predecessor of the
NNTP [RFC 3977] protocol. They are largely obsolete on the Internet
but still see use in conjunction with some transport protocols such
as UUCP [UUCP]. News servers are not required to support them.
ihave and sendme control messages share similar syntax for their
Control header fields and bodies:
control-command =/ Ihave-command
Ihave-command = "ihave" Ihave-arguments
Ihave-arguments = 1*WSP *( msg-id 1*WSP ) relayer-name
control-command =/ Sendme-command
Sendme-command = "sendme" Sendme-arguments
Sendme-arguments = Ihave-arguments
relayer-name = path-identity ; see 3.1.5 of [RFC 5536]
ihave-body = *( msg-id CRLF )
sendme-body = ihave-body
The body of the article consists of a list of <msg-id>s, one per
line. The message identifiers SHOULD be put in the body of the
article, not in the Control header field, but news servers MAY
recognize and process message identifiers in the Control header field
for backward compatibility. Message identifiers MUST NOT be put in
the Control header field if they are present in the body of the
control message.
The ihave message states that the named relaying agent has received
articles with the specified message identifiers, which may be of
interest to the relaying agents receiving the ihave message. The
sendme message requests that the agent receiving it send the articles
having the specified message identifiers to the named relaying agent.
Contrary to [RFC 1036], the relayer-name MUST be given as the last
argument in the Control header field.
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RFC 5537 Netnews Architecture and Protocols November 2009
Upon receipt of the sendme message (and a decision to honor it), the
receiving agent sends the article or articles requested. The
mechanism for this transmission is unspecified by this document and
is arranged between the sites involved.
These control messages are normally sent as point-to-point articles
between two sites and not then sent on to other sites. Newsgroups
beginning with "to." are reserved for such point-to-point
communications and are formed by prepending "to." to a <relayer-name>
to form a <newsgroup-name>. Articles with such a group in their
Newsgroups header fields SHOULD NOT be sent to any news server other
than the one identified by <relayer-name>.
5.6. Obsolete Control Messages
The following control message types are declared obsolete by this
document and SHOULD NOT be sent or honored:
sendsys
version
whogets
senduuname
6. Security Considerations
Netnews is designed for broad dissemination of public messages and
offers little in the way of security. What protection Netnews has
against abuse and impersonation is provided primarily by the
underlying transport layer. In large Netnews networks where news
servers cannot be relied upon to enforce authentication and
authorization requirements at the transport layer, articles may be
trivially forged and widely read, and the identities of article
senders and the privacy of articles cannot be assured.
See Section 5 of [RFC 5536] for further security considerations
related to the format of articles.
6.1. Compromise of System Integrity
Control messages pose a particular security concern since acting on
unauthorized control messages may cause newsgroups to be removed,
articles to be deleted, and unwanted newsgroups to be created.
Administrators of news servers SHOULD therefore take steps to verify
the authenticity of control messages as discussed in Section 5.1.
Articles containing Supersedes header fields are effectively cancel
control messages and SHOULD be subject to the same checks as
Allbery & Lindsey Standards Track PAGE 42
RFC 5537 Netnews Architecture and Protocols November 2009
discussed in Section 5.4. Currently, many sites are ignoring all
cancel control messages and Supersedes header fields due to the
difficulty of authenticating them and their widespread abuse.
Cancel control messages are not required to have the same Newsgroups
header field as the messages they are cancelling. Since they are
sometimes processed before the original message is received, it may
not be possible to check that the Newsgroup header fields match.
This allows a malicious poster to inject a cancel control message for
an article in a moderated newsgroup without adding an Approved header
field to the control message, and to hide malicious cancel control
messages from some reading agents by using a different Newsgroups
header field so that the cancel control message is not accepted by
all news servers that accepted the original message.
All agents should be aware that all article content, most notably
including the content of the Control header field, is potentially
untrustworthy and malicious. Articles may be constructed in
syntactically invalid ways to attempt to overflow internal buffers,
violate hidden assumptions, or exploit implementation weaknesses.
For example, some news server implementations have been successfully
attacked via inclusion of Unix shell code in the Control header
field. All article contents, and particularly control message
contents, SHOULD be handled with care and rigorously verified before
any action is taken on the basis of the contents of the article.
A malicious poster may add an Approved header field to bypass the
moderation process of a moderated newsgroup. Injecting agents SHOULD
verify that messages approved for a moderated newsgroup are being
injected by the moderator using authentication information from the
underlying transport or some other authentication mechanism arranged
with the moderator. There is, at present, no standardized method of
authenticating approval of messages to moderated groups, although
some unstandardized authentication methods such as [PGPMOOSE] are in
common use.
A malicious news server participating in a Netnews network may bypass
checks performed by injecting agents, forge Path header fields and
other trace information (such as Injection-Info header fields), and
otherwise compromise the authorization requirements of a Netnews
network. News servers SHOULD use the facilities of the underlying
transport to authenticate their peers and reject articles from
injecting and relaying agents that do not follow the requirements of
this protocol or the Netnews network.
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RFC 5537 Netnews Architecture and Protocols November 2009
6.2. Denial of Service
The proper functioning of individual newsgroups can be disrupted by
the excessive posting of unwanted articles; by the repeated posting
of identical or near identical articles; by posting followups that
either are unrelated to their precursors or that quote their
precursors in full with the addition of minimal extra material
(especially if this process is iterated); by crossposting to, or
requesting followups to, totally unrelated newsgroups; and by abusing
control messages and the Supersedes header field to delete articles
or newsgroups.
Such articles intended to deny service, or other articles of an
inflammatory nature, may also have their From or Reply-To addresses
set to valid but incorrect email addresses, thus causing large
volumes of email to descend on the true owners of those addresses.
Users and agents should always be aware that identifying information
in articles may be forged.
A malicious poster may prevent an article from being seen at a
particular site by including in the Path header field of the proto-
article the <path-identity> of that site. Use of the <diag-keyword>
"POSTED" by injecting agents to mark the point of injection can
prevent this attack.
Primary responsibility for preventing such attacks lies with
injecting agents, which can apply authentication and authorization
checks via the underlying transport and prevent those attempting
denial-of-service attacks from posting messages. If specific
injecting agents fail to live up to their responsibilities, they may
be excluded from the Netnews network by configuring relaying agents
to reject articles originating from them.
A malicious complainer may submit a modified copy of an article (with
an altered Injection-Info header field, for instance) to the
administrator of an injecting agent in an attempt to discredit the
author of that article and even to have his posting privileges
removed. Administrators SHOULD therefore obtain a genuine copy of
the article from their own serving agent before taking action in
response to such a complaint.
6.3. Leakage
Articles that are intended to have restricted distribution are
dependent on the goodwill of every site receiving them. Restrictions
on dissemination and retention of articles may be requested via the
Archive and Distribution header fields, but such requests cannot be
enforced by the protocol.
Allbery & Lindsey Standards Track PAGE 44
RFC 5537 Netnews Architecture and Protocols November 2009
The flooding algorithm used by Netnews transports such as NNTP
[RFC 3977] is extremely good at finding any path by which articles can
leave a subnet with supposedly restrictive boundaries, and
substantial administrative effort is required to avoid this.
Organizations wishing to control such leakage are advised to
designate a small number of gateways to handle all news exchanges
with the outside world.
The sendme control message (Section 5.5), insofar as it is still
used, can be used to request articles that the requester should not
have access to.
7. IANA Considerations
IANA has registered the following media types, described elsewhere in
this document, for use with the Content-Type header field, in the
IETF tree in accordance with the procedures set out in [RFC 4288].
application/news-transmission (4.1)
application/news-groupinfo (4.2)
application/news-checkgroups (4.3)
application/news-transmission is a change to a previous registration.
IANA has registered the new header field, Original-Sender, in the
Permanent Message Header Field Repository, using the template in
Section 3.10.3.
IANA has changed the status of the message/news media type to
"OBSOLETE". message/RFC 822 should be used instead. An updated
template is included in Section 4.
8. References
8.1. Normative References
[ASCII] American National Standard for Information Systems,
"Coded Character Sets - 7-Bit American National
Standard Code for Information Interchange (7-Bit
ASCII)", ANSI X3.4, 1986.
[RFC 2046] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types",
RFC 2046, November 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Allbery & Lindsey Standards Track PAGE 45
RFC 5537 Netnews Architecture and Protocols November 2009
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC 4288] Freed, N. and J. Klensin, "Media Type Specifications
and Registration Procedures", BCP 13, RFC 4288,
December 2005.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC 5536] Murchison, K., Ed., Lindsey, C., and D. Kohn, "Netnews
Article Format", RFC 5536, November 2009.
8.2. Informative References
[PGPMOOSE] Rose, G., "PGP Moose", November 1998.
[PGPVERIFY] Lawrence, D., "Signing Control Messages", August 2001,
<ftp://ftp.isc.org/pub/pgpcontrol/FORMAT>.
[RFC 1036] Horton, M. and R. Adams, "Standard for interchange of
USENET messages", RFC 1036, December 1987.
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[RFC 2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC 3798] Hansen, T. and G. Vaudreuil, "Message Disposition
Notification", RFC 3798, May 2004.
[RFC 3977] Feather, C., "Network News Transfer Protocol (NNTP)",
RFC 3977, October 2006.
[SON-OF-1036] Spencer, H., "News Article Format and Transmission",
Work in Progress, May 2009.
[USEAGE] Lindsey, C., "Usenet Best Practice", Work in Progress,
March 2005.
[UUCP] O'Reilly, T. and G. Todino, "Managing UUCP and
Usenet", O'Reilly & Associates Ltd., January 1992.
Allbery & Lindsey Standards Track PAGE 46
RFC 5537 Netnews Architecture and Protocols November 2009
Appendix A. Changes to the Existing Protocols
This document prescribes many changes, clarifications, and new
features since the protocol described in [RFC 1036]. Most notably:
o A new, backward-compatible Path header field format that permits
standardized embedding of additional trace and authentication
information is now RECOMMENDED. See Section 3.2. Folding of the
Path header is permitted.
o Trimming of the References header field is REQUIRED, and a
mechanism for doing so is defined.
o Addition of the new Injection-Date header field is required in
some circumstances for posting agents (Section 3.4.2) and
injecting agents (Section 3.5), and MUST be used by news servers
for date checks (Section 3.6). Injecting agents are also strongly
encouraged to put all local trace information in the new
Injection-Info header field.
o A new media type is defined for transmitting Netnews articles
through other media (Section 4.1), and moderators SHOULD prepare
to receive submissions in that format (Section 3.5.1).
o Certain control messages (Section 5.6) are declared obsolete, and
the special significance of "cmsg" at the start of a Subject
header field is removed.
o Additional media types are defined for improved structuring,
specification, and automated processing of control messages
(Sections 4.2 and 4.3).
o Two new optional parameters are added to the checkgroups control
message.
o The message/news media type is declared obsolete.
o Cancel control messages are no longer required to have From and
Sender header fields matching those of the target message, as this
requirement added no real security.
o The relayer-name parameter in the Control header field of ihave
and sendme control messages is now required.
In addition, many protocol steps and article verification
requirements that are unmentioned or left ambiguous by [RFC 1036] but
are widely implemented by Netnews servers have been standardized and
specified in detail.
Allbery & Lindsey Standards Track PAGE 47
RFC 5537 Netnews Architecture and Protocols November 2009
Appendix B. Acknowledgements
This document is the result of a twelve-year effort and the number of
people that have contributed to its content are too numerous to
mention individually. Many thanks go out to all past and present
members of the USEFOR Working Group of the Internet Engineering Task
Force (IETF) and the accompanying mailing list.
Special thanks are due to Henry Spencer, whose [SON-OF-1036] draft
served as the initial basis for this document.
Authors' Addresses
Russ Allbery (editor)
Stanford University
P.O. Box 20066
Stanford, CA 94309
US
EMail: rra@stanford.edu
URI: http://www.eyrie.org/~eagle/
Charles H. Lindsey
5 Clerewood Avenue
Heald Green
Cheadle
Cheshire SK8 3JU
United Kingdom
Phone: +44 161 436 6131
EMail: chl@clerew.man.ac.uk
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RFC TOTAL SIZE: 120366 bytes
PUBLICATION DATE: Sunday, November 29th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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