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			    A Tutorial on
		       Gatewaying Mail and News
				 with
		    Sendmail, C-News and newsgate
				   
				  by
				   
			   George M. Jones
		      george@cis.ohio-state.edu

			    August 7, 1991


This document describes how to set up gateways between mailing lists
and newsgroups.  Some gateways are different.  Your mileage may vary.

Introduction

  Mailing lists and newsgroups are similar.  Both provide one-to-many
  communication on selected topics.  There are differences in the delivery,
  storage and display mechanisms.  Some people prefer to receive
  information via news reading mechanisms some people prefer to receive
  information via mail.  News readers tend to have the advantage of being
  able to categorize information better (by delivering it to different
  newsgroups) and thus to enabling the user to deal with a higher volume of
  information.  News also has the advantage of reducing storage
  requirements by allowing all users to read the same copy of a stored
  message.

  This document describes how to set up mail/news gateways on UNIX systems
  running Sendmail 5.61 or later, Cnews and Rich Salz's mail2news and
  news2mail programs.  It is assumed that the local sendmail.cf
  configuration supports mailing to newsgroups (e.g. mail to
  "comp.org.usenix" would land in the newsgroup).

Theory of operation

  The basic job that has to be accomplished in mailing lists and newsgroups
  is to assure that everything that appears in the newsgroup is sent to the
  mailing list and that everything that is sent to the mailing list appears
  in the newsgroup, and duplication must be avoided (all messages should
  appear once, and only once in both the newsgroup and on the mailing
  list).

Files, Programs, and Flags

  The following files and programs are important in the process of
  gatewaying mailing lists and newsgroups

  The sys file 

    Usually /var/lib/news/sys.  This file determines what news groups are
    sent to your "neighbor" sites and what program is called to deliver it
    to them.  Newsgroups can be gatewayed to mailing lists by putting an
    entry in the sys file specifying which groups are to be sent to the
    list and specifying the news2mail program (with proper arguments) as
    the means of getting it there.

    A typical line sys file entry is

      internet:gnu.emacs.gnus,!gnu.emacs.gnus.all/all::\
	/usr/bin/news2mail info-gnus-english info-gnus-english-mail \
  	info-gnus-english-request cis.ohio-state.edu

    See news(5) for more information on the sys file.

  news2mail

    News2mail is a program that is responsible for taking a news message,
    hacking the headers to make it compliant with RFC 822, and sending it
    out to the list.  See news2mail(1) for more information.

  The aliases file

    Usually /etc/aliases.  This contains various aliases pertaining to
    the mailing list.  Typically, there are aliases of the form

	list-name:		post-list-name 

    if the list is locally maintained, or

	list-name-local:	post-list-name,some,user,names

    if the list is a local exploder of a list maintained elsewhere.  The
    post-* alias is Typically something like

	post-list-name: "|/usr/bin/mail2news -n some.group \
		-d dist -o '.Gateway .Org .Name' -x internet"

    With the result that mail sent to "list-name" is posted to the list, or
    mail sent to "list-name-local" is both posted to the list and sent to
    several local users.  If the mailing list is maintained locally, there
    will be an alias of the form

	list-name-request:	address-of-list-maintainer

    to which requests about the list are to be sent.  If there are problems
    in delivering mail to "list-name", and there is an alias of the form

	owner-list-name:	who-gets-the-bad-news

    then bounces and errors will be sent to the owner of the list, not the
    sender of the message.  Often the addresses of locally maintained
    mailing lists are maintained in a file, rather than individually in the
    aliases file.  This allows maintenance of mailing list membership by
    people who do not have the necessary permissions to update the aliases
    file.  These sorts of aliases take the form

	list-name-mail:	:include:/news/mailing-lists/list-name.list

    Sometimes, the an entry in the included name-list sends messages back
    to the post-list-name address.

    So, putting it all together, a real life example is

	info-gnus-english: 	post-info-gnus-english@news
	post-info-gnus-english: "|/usr/bin/mail2news -n gnu.emacs.gnus \
		-d gnu -o '.GNUs .Not .Usenet' -x internet"
	info-gnus-english-mail:	:include:/usr/lib/news/info-gnus-english/Aliases
	info-gnus-english-request:	romig
	owner-info-gnus-english:	info-gnus-english-request

  mail2news

    mail2news is responsible for taking an RFC822 mail message, hacking on
    the headers sufficiently to make it acceptable as a news article and
    handing it off to inews(1).  See mail2news(1) for a complete
    description of the options and header hacking.

    Problems with NNTP inews

    Note that mail2news calls inews directly and passes certain flags to
    it.  In the examples above, the -d and -x flags are passed to inews.
    The use of the -x flag is crucial to avoiding duplication (see below).
    Also note that NNTP version of inews ("mini-inews") accepts only the -h
    flag and ignores all others.  These two points become extremely
    relevant if you have news configured in an environment with one machine
    acting as the central news machine (say, with everyone else NFS
    mounting the same spool directory) and all other machines normally
    using NNTP to post.  In order for things to work, mail2news has to be
    able to execute the "real" inews.

    Solutions

    There are two solutions.  One, hack your sendmail.cf and aliases file
    such that all mail to mailing lists is delivered to the machine acting
    as the central news repository and having a copy of the "real" inews,
    not the NNTP version of inews.  This is the solution used in the
    example above. Sendmail.cf hacking is left as an exercise for the
    reader.

    Two, put a shell wrapper around mail2news (or inews) and use rsh(1) to
    force mail2news (or inews) to execute on the central news machine.
    This second option can be problematic if you don't have rsh, if not all
    users are allowed to log in to the central news machine, or if you are
    not well versed in shell quoting and rsh argument passing mechanisms.

 
  Avoiding duplication

    One of the major issues in mail/news gatwaying is how to avoid
    duplication.  The news software solves the problem of avoiding
    duplicates in the newsgroups by maintaining a list of the message-ids
    (the history file) that have been seen and rejecting messages with the
    same id.  Avoiding duplication on mailing lists is somewhat more
    difficult.  You want mail sent to the list to be posted.  You want
    everything posted to be sent back to the list, but you don't want mail
    that goes through the gateway and is posted to be sent back to the
    list.

    The solution as implemented in the examples above is as follows: mail
    that comes in to the mailing list address is sent through mail2news
    with the inews -x flag to exclude distribution of the article to a
    pseudo news neighbor called "internet".  The sys file has an entry that
    sends all articles posted to the group to the pseudo news neighbor
    "internet" via the news2mail program...soooo...things coming from the
    list will be posted, but not sent back to the list because of the "-x
    internet" flag and things posted locally to the group (or transferred
    to us as news by other neighbors) will be sent to the list.  Digital
    watches are pretty neat !
    
    Another possible solution is to use the distribution header to avoid
    duplication.  The basic idea is to force messages coming in from the
    mailing list to be posted with a specific distribution (say, by using
    the "-d fromlist" flag to mail2news in the aliases file entry for the
    list), and then having the sys file send everything to the list EXCEPT
    messages with a distribution of "fromlist" (see news(5) for details).
    The biggest problem with this is that you would then have to pass the
    "fromlist" distribution to neighbors interested in receiving the
    gatewayed mailing list as news.

Examples

  The following two examples illustrate how to set up two of the most
  common types of gateways, a bi-directional gateway for a locally
  maintained mailing list, and a bi-directional gateway for a mailing list
  maintained elsewhere.  Issues not addressed are one-way gateways, since
  they are simplifications of the examples below, and propagation of
  newsgroups to neighbors (you may be gatewaying a popular mailing list for
  the convenience of local users who prefer it in news, or you may be
  acting as "the" gateway for a newsgroup that is propagated world wide).
  These examples can be used as models for setting up new gateways

  Locally maintained mailing list

    Aliases file entries

	info-gnus-english: 		post-info-gnus-english@news
	post-info-gnus-english: 	"|/usr/bin/mail2news -n gnu.emacs.gnus -d gnu -o '.GNUs .Not .Usenet' -x internet"
	info-gnus-english-mail:		:include:/news/mailing-lists/info-gnus-english.list
	info-gnus-english-request:	william
	owner-info-gnus-english:	info-gnus-english-request

    where the .list file contains a list of email addresses, one per line.

    sys file entries

	internet:gnu.emacs.gnus,!gnu.emacs.gnus.all/all::\
		/usr/bin/news2mail info-gnus-english info-gnus-english-mail info-gnus-english-request cis.ohio-state.edu

    
  Externally maintained mailing list

    Aliases file entries

	sun-nets: 		sun-nets@umiacs.umd.edu
	sun-nets-local:		post-sun-nets@news,
				tom,
				richard,
				harry
	post-sun-nets: 		"|/usr/bin/mail2news -n osu.sys.sun.nets -d osu -o '.Sun .Nets .Mailing .List' -x internet"
	owner-sun-nets-local:	sally
	owner-sun-nets: 	usenet
	owner-post-sun-nets: 	usenet


    sys file entries

	internet:osu.sys.sun.nets,!osu.sys.sun.nets.all/all::\
	  /usr/bin/news2mail sun-nets sun-nets sun-nets-request umiacs.umd.edu

    It is highly recommended that you check with the maintainer of a
    mailing list before gatewaying the mailing list into a newsgroup and
    propagating the newsgroup beyond the local organization.

Acknowledgments

  The setup described here is largely a description of how mail/news
  gatewaying is done at The Ohio State University Department of Computer
  and Information Science.  Many of the ideas were originaly implenmented
  by Karl Kleinpaste and Bob Sutterfield using B-News and Eric Fair's
  nrecnews and gateway scripts.