INN Perl Filtering and Authentication Support

    This is $Revision: 1.16.2.1 $ dated $Date: 2000/09/18 22:38:39 $.

    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 --with-perl when you run
    configure.  See INSTALL for more information.

The innd Perl Filter

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

    filter_innd.pl, on the other hand, can be reloaded on command (with
    `ctlinnd reload filter.perl').  Whenever 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 startup_innd.pl or
    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 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, `ctlinnd reload filter.perl' (or `ctlinnd reload
    all') will cause 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 `$hdr{Newsgroups}').  In addition,
    `$hdr{__BODY__}' will contain the full body of the article and
    `$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 `$hdr{Lines}' is the contents of the Lines:
    header of the article and may bear no resemblence to the actual length
    of the article.  `$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, `$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
    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 `ctlinnd perl n' and back on again
    with `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).

Supported innd Callbacks

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

    INN::addhist(*messageid*, *arrival*, *articledate*, *expire*, *paths*)
        Adds *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
        *arrival* is normally the time at which the server accepts the
        article, the *articledate* is from the Date header of the article,
        the *expire* is from the Expires header of the article, and the
        *paths* field is the storage API token.)  Returns true on success,
        false otherwise.

    INN::article(*messageid*)
        Returns the full article (as a simple string) identified by
        *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.

    INN::cancel(*messageid*)
        Cancels *messageid*.  (This is equivalent to `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.

    INN::filesfor(*messageid*)
        Returns the *paths* field of the history entry for the given
        *messageid*.  This will be the storage API token for the message. 
        If *messageid* isn't found in the history database, returns undef.

    INN::havehist(*messageid*)
        Looks up *messageid* in the history database and returns true if
        it's found, false otherwise.

    INN::head(*messageid*)
        Returns the header (as a simple string) of the article identified by
        *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).

    INN::newsgroup(*newsgroup*)
        Returns the status of *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 *newsgroup*
        isn't in the active file, returns undef.

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

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):

    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.

The nnrpd Posting Filter

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

    If 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,
    *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 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 inn.conf, where they can be posted by running `rnews -U'.  If you use
    this functionality, make sure those directories exist.

Perl Authentication Support for nnrpd

    The functionality described in this section is likely to be merged into
    the new 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 inn.conf is set to true, nnrpd will authenticate
    readers by calling a Perl function rather than reading readers.conf and
    using the normal authentication mechanism.  If it is set, nnrpd loads
    _PATH_PERL_AUTH (defined in include/paths.h, by default nnrpd_auth.pl). 
    This file must be located in the directory specified by pathfilter in
    inn.conf (/usr/local/news/bin/filter by default).  The default directory
    for filter code can be specified at configure time by giving the flag
    --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:
    `$attributes{type}' will contain either "connect", indicating a new
    connection is in progress, or "authenticate", indicating that a client
    has sent an AUTHINFO command.  `$attributes{hostname}' will contain the
    hostname (or the IP address if it doesn't resolve) of the client machine
    and `$attributes{ipaddress}' will contain its IP address (as a string). 
    If type was "authenticate", `$attributes{username}' will contain the
    provided username and `$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 five-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.

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 startup_innd.pl, filter_innd.pl,
    filter_nnrpd.pl, and nnrpd_auth.pl files that contain some simplistic
    examples.  Look them over as a starting point when writing your own.

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.
