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RFC 5537


Netnews Architecture and Protocols

Part 2 of 2, p. 24 to 48
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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

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

   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

   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" ":" SP
                             = 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

   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

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

         groupinfo-body      = [ newsgroups-tag CRLF ]
                                  newsgroups-line CRLF
         newsgroups-tag      = %x46.6F.72 SP %x79.6F.75.72 SP
                                  %x6E. SP
                                  ; case sensitive
                                  ; "For your newsgroups file:"
         newsgroups-line     = newsgroup-name
                                  [ 1*HTAB newsgroup-description ]
                                  [ *WSP moderation-flag ]
                             = eightbit-utext *( *WSP eightbit-utext )
         moderation-flag     = SP "(" %x4D.6F. ")"

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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

         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/

   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

   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

   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.  newgroup Control Message Example

   A newgroup control message requesting creation of the moderated

         From: "example.* Administrator" <admin@noc.example>
         Date: 27 Feb 2002 12:50:22 +0200
         Subject: cmsg newgroup moderated
         Approved: admin@noc.example
         Control: newgroup moderated
         Message-ID: <>
         MIME-Version: 1.0
         Content-Type: multipart/mixed; boundary="nxtprt"
         Content-Transfer-Encoding: 8bit

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         This is a MIME control message.
         Content-Type: application/news-groupinfo; charset=us-ascii

         For your newsgroups file:      About the example.* groups (Moderated)

         Content-Type: text/plain; charset=us-ascii

         A moderated newsgroup for announcements about new newsgroups in
         the example.* hierarchy.


   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:


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,

   [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

   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


   Charles H. Lindsey
   5 Clerewood Avenue
   Heald Green
   Cheshire  SK8 3JU
   United Kingdom

   Phone: +44 161 436 6131