=head1 INN Perl Filtering and Authentication Support

This is $Revision: 1.3 $ dated $Date: 2000/03/26 08:13:21 $.

This file documents INN's built-in support for Perl filtering and reader
authentication.  The code is based very heavily on work by Christophe
Wolfhugel <wolf@pasteur.fr>, and his work was in turn inspired by the
existing TCL support.  Please send any bug reports to inn-bugs@isc.org,
not to Christophe, as the code has been modified heavily since he
originally wrote it.

The Perl filtering support is described in more detail below.  Basically,
it allows you to supply a Perl function that is invoked on every article
received by innd from a peer (the innd filter) or by nnrpd from a reader
(the nnrpd filter).  This function can decide whether to accept or reject
the article, and can optionally do other, more complicated processing
(such as add history entries, cancel articles, spool local posts into a
holding area, or even modify the headers of locally submitted posts).

For Perl filtering support, you need to have Perl version 5.004 or newer.
Earlier versions of Perl will fail with a link error at compilation time.
http://language.perl.com/info/software.html should have the latest Perl
version.

To enable Perl support, you have to specify B<--with-perl> when you run
configure.  See F<INSTALL> for more information.

=head1 The innd Perl Filter

When innd starts, it first loads the file _PATH_PERL_STARTUP_INND (defined
in F<include/paths.h>, by default F<startup_innd.pl>) and then loads the
file _PATH_PERL_FILTER_INND (also defined in F<include/paths.h>, by
default F<filter_innd.pl>).  Both of these files must be located in the
directory specified by pathfilter in F<inn.conf>
(F</usr/local/news/bin/filter> by default).  The default directory for
filter code can be specified at configure time by giving the flag
B<--with-filter-dir> to configure.

INN doesn't care what Perl functions you define in what files.  The only
thing that's different about the two files is when they're loaded.
F<startup_innd.pl> is loaded only once, when innd first starts, and is
never reloaded as long as innd is running.  Any modifications to that file
won't be noticed by innd; only stopping and restarting innd can cause it
to be reloaded.

F<filter_innd.pl>, on the other hand, can be reloaded on command (with
C<ctlinnd reload filter.perl>).  Whenever F<filter_innd.pl> is loaded,
including the first time at innd startup, the Perl function
filter_before_reload() is called before it's reloaded and the function
filter_after_reload() is called after it's reloaded (if the functions
exist).  Additionally, any code in either F<startup_innd.pl> or
F<filter_innd.pl> at the top level (in other words, not inside a sub { })
is automatically executed by Perl when the files are loaded.

This allows one to do things like write out filter statistics whenever the
filter is reloaded, load a cache into memory, flush cached data to disk,
or other similar operations that should only happen at particular times or
with manual intervention.  Remember, any code not inside functions in
F<startup_innd.pl> is executed when that file is loaded, and it's loaded
only once when innd first starts.  That makes it the ideal place to put
initialization code that should only run once, or code to load data that
was preserved on disk across a stop and restart of innd (perhaps using
filter_mode() -- see below).

As mentioned above, C<ctlinnd reload filter.perl> (or C<ctlinnd reload
all>) will cause F<filter_innd.pl> to be reloaded.  If the function
filter_art() is defined after the file has been reloaded, filtering is
turned on.  Otherwise, filtering is turned off.  (Note that due to the way
Perl stores functions, once you've defined filter_art(), you can't
undefine it just by deleting it from the file and reloading the filter.
You'll need to replace it with an empty sub.)

The Perl function filter_art() is the heart of a Perl filter.  Whenever an
article is received from a peer, via either IHAVE or TAKETHIS,
filter_art() is called if Perl filtering is turned on.  It receives no
arguments, and should return a single scalar value.  That value should be
the empty string to indicate that INN should accept the article, or some
rejection message to indicate that the article should be rejected.

filter_art() has access to a global hash named %hdr, which contains all of
the standard headers present in the article and their values.  The
standard headers are:

        Approved, Control, Date, Distribution, Expires, From, Lines,
        Message-ID, Newsgroups, Path, Reply-To, Sender, Subject,
        Supersedes, Bytes, Also-Control, References, Keywords, X-Trace,
        NNTP-Posting-Host, Followup-To, Organization, Content-Type,
        Content-Base, Content-Disposition, X-Newsreader, X-Mailer,
        X-Cancelled-By, X-Canceled-By, Cancel-Key

(so, for example, the Newsgroups header of the article is accessible
inside the Perl filter as C<$hdr{Newsgroups}>).  In addition,
C<$hdr{__BODY__}> will contain the full body of the article and
C<$hdr{__LINES__}> will contain the number of lines in the body of the
article.

The contents of the %hdr hash for a typical article may therefore look
something like this:

        %hdr = (Subject      => 'MAKE MONEY FAST!!', 
                From         => 'Joe Spamer <him@example.com>',
                Date         => '10 Sep 1996 15:32:28 UTC',
                Newsgroups   => 'alt.test',
                Path         => 'news.example.com!not-for-mail',
                Organization => 'Spammers Anonymous',
                Lines        => '5',
                Distribution => 'usa',
                'Message-ID' => '<6.20232.842369548@example.com>',
                __BODY__     => 'Send five dollars to the ISC, c/o ...',
                __LINES__    => 5
        );

Note that the value of C<$hdr{Lines}> is the contents of the Lines: header
of the article and may bear no resemblence to the actual length of the
article.  C<$hdr{__LINES__}> is the line count calculated by INN, and is
guaranteed to be accurate.

The %hdr hash should not be modified inside filter_art().  Instead, if any
of the contents need to be modified temporarily during filtering (smashing
case, for example), copy them into a seperate variable first and perform
the modifications on the copy.  Currently, C<$hdr{__BODY__}> is the only
data that will cause your filter to die if you modify it, but in the
future other keys may also contain live data.  Modifying live INN data in
Perl will hopefully only cause a fatal exception in your Perl code that
disables Perl filtering until you fix it, but it's possible for it to
cause article munging or even core dumps in INN.  So always, always make a
copy first.

As mentioned above, if filter_art() returns the empty string (''), the
article is accepted.  Note that this must be the empty string, not 0 or
undef.  Otherwise, the article is rejected, and whatever scalar
filter_art() returns (typically a string) will be taken as the reason why
the article was rejected.  This reason will be returned to the remote peer
as well as logged to the news logs.  (innreport, in its nightly report,
will summarize the number of articles rejected by the Perl filter and
include a count of how many articles were rejected with each reason
string.)

One other type of filtering is also supported.  If Perl filtering is
turned on and the Perl function filter_messageid() is defined, that
function will be called for each message ID received from a peer (via
either CHECK or IHAVE).  The function receives a single argument, the
message ID, and like filter_art() should return an empty string to accept
the article or an error string to refuse the article.  This function is
called before any history lookups and for every article offered to innd
with CHECK or IHAVE (before the actual article is sent).  Accordingly, the
message ID is the only information it has about the article (the %hdr hash
will be empty).  This code would sit in a performance-critical hot path in
a typical server, and therefore should be as fast as possible, but it can
do things like refuse articles from certain hosts or cancels for already
rejected articles (if they follow the $alz convention) without having to
take the network bandwidth hit of accepting the entire article first.

Note that you cannot rely on filter_messageid() being called for every
incoming article; articles sent via TAKETHIS without an earlier CHECK will
never pass through filter_messageid() and will only go through
filter_art().

Finally, whenever ctlinnd throttle, ctlinnd pause, or ctlinnd go is run,
the Perl function filter_mode() is called if it exists.  It receives no
arguments and returns no value, but it has access to a global hash %mode
that contains three values:

        Mode       The current server mode (throttled, paused, or running)
        NewMode    The new mode the server is going to
        reason     The reason that was given to ctlinnd

One possible use for this function is to save filter state across a
restart of innd.  There isn't any Perl function which is called when INN
shuts down, but using filter_mode() the Perl filter can dump it's state to
disk whenever INN is throttled.  Then, if the news administrator follows
the strongly recommended shutdown procedure of throttling the server
before shutting it down, the filter state will be safely saved to disk and
can be reloaded when innd restarts (possibly by F<startup_innd.pl>).

The state of the Perl interpretor in which all of these Perl functions run
is preserved over the lifetime of innd.  In other words, it's allowed for
the Perl code to create its own global Perl variables, data structures,
saved state, and the like, and all of that will be available to
filter_art() and filter_messageid() each time they're called.  The only
variable INN fiddles with (or pays any attention to at all) is %hdr, which
is cleared after each call to filter_art().

Perl filtering can be turned off with C<ctlinnd perl n> and back on again
with C<ctlinnd perl y>.  Perl filtering is turned off automatically if
loading of the filter fails or if the filter code returns any sort of a
fatal error (either due to Perl itself or due to a die in the Perl code).

=head1 Supported innd Callbacks

innd makes seven functions available to any of its embedded Perl code.
Those are:

=over 4

=item INN::addhist(I<messageid>, I<arrival>, I<articledate>, I<expire>, I<paths>)

Adds I<messageid> to the history database.  All of the arguments except
the first one are optional; the times default to the current time and the
paths field defaults to the empty string.  (For those unfamiliar with the
fields of a history database entry, the I<arrival> is normally the time at
which the server accepts the article, the I<articledate> is from the Date
header of the article, the I<expire> is from the Expires header of the
article, and the I<paths> field is the storage API token.)  Returns true
on success, false otherwise.

=item INN::article(I<messageid>)

Returns the full article (as a simple string) identified by I<messageid>,
or undef if it isn't found.  Each line will end with a simple \n, but
leading periods may still be doubled if the article is stored in wire
format.

=item INN::cancel(I<messageid>)

Cancels I<messageid>.  (This is equivalent to C<ctlinnd cancel>; it
cancels the message on the local server, but doesn't post a cancel message
or do anything else that affects anything other than the local server.)
Returns true on success, false otherwise.

=item INN::filesfor(I<messageid>)

Returns the I<paths> field of the history entry for the given
I<messageid>.  This will be the storage API token for the message.  If
I<messageid> isn't found in the history database, returns undef.

=item INN::havehist(I<messageid>)

Looks up I<messageid> in the history database and returns true if it's
found, false otherwise.

=item INN::head(I<messageid>)

Returns the header (as a simple string) of the article identified by
I<messageid>, or undef if it isn't found.  Each line will end with a
simple \n (in other words, regardless of the format of article storage,
the returned string won't be in wire format).

=item INN::newsgroup(I<newsgroup>)

Returns the status of I<newsgroup> (the last field of the active file
entry for that newsgroup).  See active(5) for a description of the
possible values and their meanings (the most common are "y" for an
unmoderated group and "m" for a moderated group).  If I<newsgroup> isn't
in the active file, returns undef.

=back

These functions can only be used from inside the innd Perl filter; they're
not available in the nnrpd filter.

=head1 Common Callbacks

The following additional function is available from inside filters
embedded in innd, and is also available from filters embedded in nnrpd
(see below):

=over 4

=item INN::syslog(level, message)

Logs a message via syslog(2).  This is quite a bit more reliable and
portable than trying to use Sys::Syslog from inside the Perl filter.  Only
the first character of the level argument matters; the valid letters are
the first letters of ALERT, CRIT, ERR, WARNING, NOTICE, INFO, and DEBUG
(case-insensitive) and specify the priority at which the message is
logged.  If a level that doesn't match any of those levels is given, the
default priority level is LOG_NOTICE.  The second argument is the message
to log; it will be prefixed by "filter: " and logged to syslog with
facility LOG_NEWS.

=back

=head1 The nnrpd Posting Filter

When nnrpd starts, it first loads the file _PATH_PERL_FILTER_NNRPD
(defined in F<include/paths.h>, by default F<filter_nnrpd.pl>).  This file
must be located in the directory specified by pathfilter in F<inn.conf>
(F</usr/local/news/bin/filter> by default).  The default directory for
filter code can be specified at configure time by giving the flag
B<--with-filter-dir> to configure.

If F<filter_nnrpd.pl> loads successfully and defines the Perl function
filter_post(), Perl filtering is turned on.  Otherwise, it's turned off.
If filter_post() ever returns a fatal error (either from Perl or from a
die in the Perl code), Perl filtering is turned off for the life of that
nnrpd process and any further posts made during that session won't go
through the filter.

While Perl filtering is on, every article received by nnrpd via the POST
command is passed to the filter_post() Perl function before it is passed
to INN (or mailed to the moderator of a moderated newsgroup).  If
filter_post() returns an empty string (''), the article is accepted and
normal processing of it continues.  Otherwise, the article is rejected and
the string returned by filter_post() is returned to the client as the
error message (with some exceptions; see below).

filter_post() has access to a global hash %hdr, which contains all of the
headers of the article.  (Unlike the innd Perl filter, %hdr for the nnrpd
Perl filter contains *all* of the headers, not just the standard ones.  If
any of the headers are duplicated, though, %hdr will contain only the
value of the second occurance of the header.  nnrpd will reject the
article before the filter runs if any of the standard headers are
duplicated.)  It also has access to the full body of the article in the
variable $body, and if the poster authenticated via AUTHINFO (or if either
Perl authentication or a readers.conf authentication method is used and
produces user information), it has access to the authenticated username of
the poster in the variable $user.

Unlike the innd Perl filter, the nnrpd Perl filter can modify the %hdr
hash.  In fact, if the Perl variable $modify_headers is set to true after
filter_post() returns, the contents of the %hdr hash will be written back
to the article replacing the original headers.  filter_post() can
therefore make any modifications it wishes to the headers and those
modifications will be reflected in the article as it's finally posted.
The article body cannot be modified in this way; any changes to $body will
just be ignored.

Be careful when using the ability to modify headers.  filter_post() runs
after all the normal consistency checks on the headers and after server
supplied headers (like Message-ID: and Date:) are filled in.  Deleting
required headers or modifying headers that need to follow a strict format
can result in nnrpd trying to post nonsense articles (which will probably
then be rejected by innd).  If $modify_headers is set, I<everything> in
the %hdr hash is taken to be article headers and added to the article.

If filter_post() returns something other than the empty string, this
message is normally returned to the client as an error.  There are two
exceptions:  If the string returned begins with "DROP", the post will be
silently discarded and success returned to the client.  If the string
begins with "SPOOL", success is returned to the client, but the post is
saved in a directory named "spam" under the directory specified by
pathincoming in F<inn.conf> (in a directory named "spam/mod" if the post
is to a moderated group).  This is intended to allow manual inspection of
the suspect messages; if they should be posted, they can be manually moved
out of the subdirectory to the directory specified by pathincoming in
F<inn.conf>, where they can be posted by running C<rnews -U>.  If you use
this functionality, make sure those directories exist.

=head1 Perl Authentication Support for nnrpd

The functionality described in this section is likely to be merged into
the new F<readers.conf> method of specifying reader authentication,
probably as an authentication type of Perl.  The details are likely to be
substantially the same, but there may be some minor changes.  The
following documentation describes the current method.

If nnrpperlauth in F<inn.conf> is set to true, nnrpd will authenticate
readers by calling a Perl function rather than reading F<readers.conf> and
using the normal authentication mechanism.  If it is set, nnrpd loads
_PATH_PERL_AUTH (defined in F<include/paths.h>, by default
F<nnrpd_auth.pl>).  This file must be located in the directory specified
by pathfilter in F<inn.conf> (F</usr/local/news/bin/filter> by default).
The default directory for filter code can be specified at configure time
by giving the flag B<--with-filter-dir> to configure.

If a Perl function auth_init() is defined by that file, it is called
immediately after the file is loaded.  It takes no arguments and returns
nothing.

Provided nnrpperlauth is true, the file loads without errors, auth_init()
(if present) runs without fatal errors, and a Perl function authenticate()
is defined, authenticate() will be called during the processing of a
connection, authentication request, or a disconnect.  authenticate() takes
no arguments, but it has access to a global hash %attributes which
contains information about the connection as follows: C<$attributes{type}>
will contain either "connect", indicating a new connection is in progress,
or "authenticate", indicating that a client has sent an AUTHINFO command.
C<$attributes{hostname}> will contain the hostname (or the IP address if
it doesn't resolve) of the client machine and C<$attributes{ipaddress}>
will contain its IP address (as a string).  If type was "authenticate",
C<$attributes{username}> will contain the provided username and
C<$attributes{password}> the password.

authenticate() should return a five-element array.  The first element is
the NNTP response code to return to the client, the second element is a
boolean value indicating whether the client is allowed to read, the third
element is a boolean value indicating whether the client is allowed to
post, the fourth element is a wildmat(3) expression that says what groups
the client is allowed to read, and the fifth element is the maximum bytes
per second a client is permitted to use for retrieving articles.

If type is connect, the NNTP response code should probably be chosen from
one of the following values:  200 (reading and posting allowd), 201 (no
posting allowed), 480 (authentication required), or 502 (permission
denied).  If the code returned is 502, nnrpd will print a permission
refused message, drop the connection, and exit.

If the type is authentication, the NNTP response code should probably be
either 281 (authentication successful) or 502 (authentication
unsuccessful).  If the code returned is anything other than 281, nnrpd
will print an authentication error message and drop the connection and
exit.

If authenticate() dies (either due to a Perl error or due to calling die),
or if it returns anything other than the four-element array described
above, an internal error will be reported to the client, the exact error
will be logged to syslog, and nnrpd will drop the connection and exit.

=head1 Notes on Writing Embedded Perl

All Perl evaluation is done inside an implicit eval block, so calling die
in Perl code will not kill the innd or nnrpd process.  Neither will Perl
errors (such as syntax errors).  However, such errors will have negative
effects (fatal errors in the innd or nnrpd filter will cause filtering to
be disabled, and fatal errors in the nnrpd authentication code will cause
the client connection to be terminated).

Calling exit directly, however, *will* kill the innd or nnrpd process, so
don't do that.  Similarly, you probably don't want to call fork (or any
other function that results in a fork such as system, IPC::Open3::open3(),
or any use of backticks) since there are possibly unflushed buffers that
could get flushed twice, lots of open state that may not get closed
properly, and innumerable other potential problems.  In general, be aware
that all Perl code is running inside a large and complicated C program,
and Perl code that impacts the process as a whole is best avoided.

You can use print and warn inside Perl code to send output to STDOUT or
STDERR, but you probably shouldn't.  Instead, open a log file and print to
it instead (or, in the innd filter, use INN::syslog() to write messages
via syslog like the rest of INN).  If you write to STDOUT or STDERR, where
that data will go depends on where the filter is running; inside innd, it
will go to the news log or the errlog, and inside nnrpd it will probably
go nowhere but could go to the client.  The nnrpd filter takes some steps
to try to keep output from going across the network connection to the
client (which would probably result in a very confused client), but best
not to take the chance.

For similar reasons, try to make your Perl code -w clean, since Perl
warnings are written to STDERR.  (INN won't run your code under -w, but
better safe than sorry, and some versions of Perl have some mandatory
warnings you can't turn off.)

You *can* use modules in your Perl code, just like you would in an
ordinary Perl script.  You can even use modules that dynamically load C
code.  Just make sure that none of the modules you use go off behind your
back to do any of the things above that are best avoided.

Whenever you make any modifications to the Perl code, and particularly
before starting INN or reloading filter.perl with new code, you should run
perl -wc on the file.  This will at least make sure you don't have any
glaring syntax errors.  Remember, if there are errors in your code,
filtering will be disabled, which could mean that posts you really wanted
to reject will leak through and authentication of readers may be totally
broken.

The samples directory has example F<startup_innd.pl>, F<filter_innd.pl>,
F<filter_nnrpd.pl>, and F<nnrpd_auth.pl> files that contain some
simplistic examples.  Look them over as a starting point when writing your
own.

=head1 Available Packages

This is an unofficial list of known filtering packages at the time of
publication.  This is not an endorsement of these filters by the ISC or
the INN developers, but is included as assistance in locating packages
which make use of this filter mechanism.

  CleanFeed               Jeremy Nixon <jeremy@exit109.com>
  <URL:http://www.exit109.com/~jeremy/news/cleanfeed.html>
        A spam filter catching excessive multi-posting and a host of
        other things.  Uses filter_innd.pl exclusively, requires the MD5
        Perl module.  Probably the most popular and widely-used Perl
        filter around.

  Usenet II Filter        Edward S. Marshall <emarshal@xnet.com>
  <URL:http://www.xnet.com/~emarshal/inn/filter_nnrpd.pl>
        Checks for "soundness" according to Usenet II guidelines in the
        net.* hierarchy.  Designed to use filter_nnrpd.pl.

  News Gizmo              Aidan Cully <aidan@panix.com>
  <URL:http://www.panix.com/gizmo/>
        A posting filter for helping a site enforce Usenet-II soundness,
        and for quotaing the number of messages any user can post to
        Usenet daily.
