path: root/doc/rfc/rfc5537.txt

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



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   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




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1.  Introduction

1.1.  Basic Concepts

   "Netnews" is a set of protocols for generating, storing, and
   retrieving news "articles" whose format is defined in [RFC5536], 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)
   [RFC3977], 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 [RFC5536],
   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 [RFC2119].

1.4.  Syntax Notation

   Syntax defined in this document uses the Augmented Backus-Naur Form
   (ABNF) notation (including the Core Rules) defined in [RFC5234] and
   constructs defined in [RFC5536] and [RFC5322].





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   The ABNF rules defined elsewhere and used in this document are:

         CRLF                = <see [RFC5234] Appendix B.1>
         DIGIT               = <see [RFC5234] Appendix B.1>
         HTAB                = <see [RFC5234] Appendix B.1>
         SP                  = <see [RFC5234] Appendix B.1>
         WSP                 = <see [RFC5234] Appendix B.1>
         VCHAR               = <see [RFC5234] Appendix B.1>

         argument            = <see [RFC5536] Section 3.2.3>
         article-locator     = <see [RFC5536] Section 3.2.14>
         component           = <see [RFC5536] Section 3.1.4>
         control-command     = <see [RFC5536] Section 3.2.3>
         diag-keyword        = <see [RFC5536] Section 3.1.5>
         diag-match          = <see [RFC5536] Section 3.1.5>
         diag-other          = <see [RFC5536] Section 3.1.5>
         dist-name           = <see [RFC5536] Section 3.2.4>
         msg-id              = <see [RFC5536] Section 3.1.3>
         newsgroup-name      = <see [RFC5536] Section 3.1.4>
         path-diagnostic     = <see [RFC5536] Section 3.1.5>
         path-identity       = <see [RFC5536] Section 3.1.5>
         path-nodot          = <see [RFC5536] Section 3.1.5>
         tail-entry          = <see [RFC5536] Section 3.1.5>
         verb                = <see [RFC5536] Section 3.2.3>

         display-name        = <see [RFC5322] Section 3.4>
         local-part          = <see [RFC5322] Section 3.4.1>
         mailbox             = <see [RFC5322] Section 3.4>

1.5.  Definitions

   Any term used in this document that is defined in Section 1.5 of
   [RFC5536] 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 [RFC5536]).  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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   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 [RFC5322] 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 [RFC3977] 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 [RFC2045].  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/rfc822 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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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 [RFC5536], and reading
   agents SHOULD be robust in the presence of violations of the Netnews
   article format where possible.




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   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
   [RFC5536]) 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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       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.



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       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 [RFC1036] 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 [RFC5536]
   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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   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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   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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   Posting agents SHOULD ensure that proto-articles they create are
   valid according to [RFC5536] 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
   [RFC5322]), 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 [RFC5322], 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"
   [RFC2606].

   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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   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



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      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 [RFC5322].  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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   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
   [RFC5536] 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



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   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 [RFC5536].  It SHOULD reject
        any proto-article that contains a header field deprecated for
        Netnews (see, for example, [RFC3798]).  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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        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 [RFC5536] 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 [RFC5536].

   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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        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 [RFC5322], 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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   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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   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
       [RFC5536] 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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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 [RFC5536]), 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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   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 [RFC5536] 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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   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 [RFC5536]) 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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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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   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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   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 [RFC5536].  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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      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 [RFC5536]
   and [RFC5322].





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         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.






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   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 [RFC4288].

   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/rfc822
   (defined in Section 5.2.1 of [RFC2046]) 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/rfc822

     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



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                               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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     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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      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 [RFC2046].
                                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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                                  ; 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
   [RFC3629] 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 [RFC2046].
                                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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      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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   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 [RFC5536]) 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 [RFC1036], 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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   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 [RFC5536].

   All of the group control messages MUST have an Approved header field
   (Section 3.2.1 of [RFC5536]).  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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                                  [ 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 [RFC2046].  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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         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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   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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   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 [RFC1036], 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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   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 [RFC3977] 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 [RFC5536]
         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 [RFC1036], the relayer-name MUST be given as the last
   argument in the Control header field.




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   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 [RFC5536] 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





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   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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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.



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   The flooding algorithm used by Netnews transports such as NNTP
   [RFC3977] 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 [RFC4288].

         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/rfc822 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.

   [RFC2046]      Freed, N. and N. Borenstein, "Multipurpose Internet
                  Mail Extensions (MIME) Part Two: Media Types",
                  RFC 2046, November 1996.

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




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

   [RFC4288]      Freed, N. and J. Klensin, "Media Type Specifications
                  and Registration Procedures", BCP 13, RFC 4288,
                  December 2005.

   [RFC5234]      Crocker, D. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5322]      Resnick, P., Ed., "Internet Message Format", RFC 5322,
                  October 2008.

   [RFC5536]      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>.

   [RFC1036]      Horton, M. and R. Adams, "Standard for interchange of
                  USENET messages", RFC 1036, December 1987.

   [RFC2045]      Freed, N. and N. Borenstein, "Multipurpose Internet
                  Mail Extensions (MIME) Part One: Format of Internet
                  Message Bodies", RFC 2045, November 1996.

   [RFC2606]      Eastlake, D. and A. Panitz, "Reserved Top Level DNS
                  Names", BCP 32, RFC 2606, June 1999.

   [RFC3798]      Hansen, T. and G. Vaudreuil, "Message Disposition
                  Notification", RFC 3798, May 2004.

   [RFC3977]      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.




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Appendix A.  Changes to the Existing Protocols

   This document prescribes many changes, clarifications, and new
   features since the protocol described in [RFC1036].  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 [RFC1036] but
   are widely implemented by Netnews servers have been standardized and
   specified in detail.



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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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