INN Python Filtering and Authentication Support

    This is $Revision: 1.8 $, dated $Date: 2003/04/04 16:47:21 $.

    This file documents INN's built-in optional support for Python article
    filtering.  It is patterned after the TCL and Perl hooks previously
    added by Bob Heiney and Christophe Wolfhugel.

    For this filter to work successfully, you will need to have Python 1.5.2
    (the latest at this writing) installed.  You can obtain it from
    <http://www.python.org>.

    The innd Python interface and the original Python filtering
    documentation was written by Greg Andruk (nee Fluffy)
    <gerglery@usa.net>.  The Python authentication and authorization support
    for nnrpd and the original documentation for it was written by Ilya
    Etingof <ilya@glas.net>, 12/1999.

NOTE TO RED HAT LINUX USERS

    Python will be preinstalled, but it may not include all the headers and
    libraries required for embedding into INN.  You will need to add the
    development package.  Better yet, get the source kit from the above URL
    and build it yourself.  Be sure when installing Python on Red Hat, to
    run configure with "--prefix=/usr" so that there are no version
    conflicts with the "factory" installation.  You can also find a
    selection of well made RPMs at
    <ftp://starship.python.net/pub/crew/andrich/>.

INSTALLATION

    Once you have built and installed Python, you can cause INN to use it by
    adding the "--with-python" switch to your configure command.

    See the ctlinnd(8) manual page to learn how to enable, disable and
    reload Python filters on a running server ("ctlinnd mode", "ctlinnd
    python y|n", "ctlinnd reload filter.python").

    Also, see the example filter_innd.py script in your filters directory
    for a demonstration of how to get all this working.

WRITING AN INND FILTER

    You need to create a filter_innd.py module in INN's filter directory
    (see the pathfilter setting in inn.conf).  A heavily-commented sample is
    provided that you can use as a template for your own filter.  There is
    also an INN.py module there which is not actually used by INN; it is
    there so you can test your module interactively.

    First, define a class containing the methods you want to provide to
    innd.  Methods innd will use if present are:

    __init__(self)
        Not explicitly called by innd, but will run whenever the filter
        module is (re)loaded.  This is a good place to initialize constants
        or pick up where filter_before_reload or filter_close left off.

    filter_before_reload(self)
        This will execute any time a "ctlinnd reload all" or "ctlinnd reload
        filter.python" command is issued.  You can use it to save statistics
        or reports for use after reloading.

    filter_close(self)
        This will run when a "ctlinnd shutdown" command is received.

    filter_art(self, art)
        art is a dictionary containing an article's headers and body.  This
        method is called every time innd receives an article.  The following
        can be defined:

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

        All the above values will be buffer objects holding the contents of
        the same named article headers, except for the special __BODY__ and
        __LINES__ items.  Items not present in the article will contain
        None.

        __BODY__ is a buffer object containing the article's entire body,
        and __LINES__ is an int holding innd's reckoning of the number of
        lines in the article.  All the other elements will be buffers with
        the contents of the same-named article headers.

        If you want to accept an article, return None or an empty string. 
        To reject, return a non-empty string.  The rejection strings will be
        shown to local clients and your peers, so keep that in mind when
        phrasing your rejection responses.

    filter_messageid(self, msgid)
        msgid is a buffer object containing the ID of an article being
        offered by IHAVE or CHECK.  Like with filter_art(), the message will
        be refused if you return a non-empty string.  If you use this
        feature, keep it light because it is called at a rather busy place
        in innd's main loop.  Also, do not rely on this function alone to
        reject by ID; you should repeat the tests in filter_art() to catch
        articles sent with TAKETHIS but no CHECK.

    filter_mode(self, oldmode, newmode, reason)
        When the operator issues a ctlinnd pause, throttle or go command,
        this function can be used to do something sensible in accordance
        with the state change.  Stamp a log file, save your state on
        throttle, etc.  oldmode and newmode will be strings containing one
        of the values in ('running', 'throttled', 'paused', 'unknown') --
        oldmode is the state innd was in before ctlinnd was run, newmode is
        the state innd will be in after the command finishes.  reason is the
        comment string provided on the ctlinnd command line.

    To register your methods with innd, you need to create an instance of
    your class, import the built-in INN module, and pass the instance to
    INN.set_filter_hook().  For example:

        class Filter:
            def filter_art(self, art):
                ...
                blah blah
                ...

            def filter_messageid(self, id):
                ...
                yadda yadda
                ...

        import INN
        myfilter = Filter()
        INN.set_filter_hook(myfilter)

    When writing and testing your Python filter, don't be afraid to make use
    of try:/except: and the provided INN.syslog() function.  stdout and
    stderr will be disabled, so your filter will die silently otherwise.

    Also, remember to try importing your module interactively before loading
    it, to ensure there are no obvious errors.  One typo can ruin your whole
    filter.  A dummy INND.py module is provided to facilitate testing
    outside the server.  To test, change into your filter directory and use
    a command like:

        python -ic 'import INN, filter_innd'

    You can define as many or few of the methods listed above as you want in
    your filter class (it's fine to define more methods for your own use;
    innd won't use them but your filter can).  If you *do* define the above
    methods, GET THE PARAMETER COUNTS RIGHT.  There are checks in innd to
    see if the methods exist and are callable, but if you define one and get
    the parameter counts wrong, INND WILL DIE.  You have been warned.  Be
    careful with your return values, too.  The filter_art() and
    filter_messageid() methods have to return strings, or None.  If you
    return something like an int, innd will not be happy.

WHAT'S THE DEAL WITH THESE BUFFER OBJECTS?

    Buffer objects are cousins of strings, new in Python 1.5.2.  They are
    supported, but at this writing you won't yet find much about them in the
    Python documentation.  Using buffer objects may take some getting used
    to, but we can create buffers much faster and with less memory than
    strings.

    For most of the operations you will perform in filters (like re.search,
    string.find, md5.digest) you can treat buffers just like strings, but
    there are a few important differences you should know about:

        # Make a string and a two buffers.
        s = "abc"
        b = buffer("def")
        bs = buffer("abc")

        s == bs          # - This is false because the types differ...
        buffer(s) == bs  # - ...but this is true, the types now agree.
        s == str(bs)     # - This is also true, but buffer() is faster.
        s[:2] == bs[:2]  # - True.  Buffer slices are strings.

        # While most string methods will take either a buffer or string,
        # string.join insists on using only strings.
        string.join([str(b), s], '.')   # returns 'def.abc'

        e = s + b        # This raises a TypeError, but...

        # ...these two both return the string 'abcdef'. The first one
        # is faster -- choose buffer() over str() whenever you can.
        e = buffer(s) + b
        f = s + str(b)

        g = b + '>'      # This is legal, returns the string 'def>'.

FUNCTIONS SUPPLIED BY THE BUILT-IN INND MODULE

    Not only can innd use Python, but your filter can use some of innd's
    features too.  Here is some sample Python code to show what you get:

        import INN

        # Python's native syslog module isn't compiled in by default,
        # so the INN module provides a replacement.  The first parameter
        # tells the Unix syslogger what severity to use; you can
        # abbreviate down to one letter and it's case insensitive.
        # Available levels are (in increasing levels of seriousness)
        # Debug, Info, Notice, Warning, Err, Crit, and Alert. (If you
        # provide any other string, it will be defaulted to Notice.)  The
        # second parameter is the message text.  The syslog entries will
        # go to the same log files innd itself uses, with a 'python:'
        # prefix.
        syslog('warning', 'I will not buy this record.  It is scratched.')
        animals = 'eels'
        vehicle = 'hovercraft'
        syslog('N', 'My %s is full of %s.' % (vehicle, animals))

        # Let's cancel an article!  This only deletes the message on the
        # local server; it doesn't send out a control message or anything
        # scary like that.  Returns 1 if successful, else 0.
        if INN.cancel('<meow$123.456@solvangpastries.edu>'):
            canceled = "yup"
        else:
            canceled = "nope"

        # Check if a given message is in history. This doesn't
        # necessarily mean the article is on your spool; canceled and
        # expired articles hang around in history for a while, and
        # rejected articles will be in there if you have enabled
        # remember_trash in inn.conf. Returns 1 if found, else 0.
        if INN.havehist('<z456$789.abc@isc.org>'):
            comment = "*yawn* I've already seen this article."
        else:
            comment = 'Mmm, fresh news.'

        # Here we are running a local spam filter, so why eat all those
        # cancels?  We can add fake entries to history so they'll get
        # refused.  Returns 1 on success, 0 on failure.
        canceled_id = buffer('<meow$123.456@isc.org>')
        if INN.addhist("<cancel." + canceled_id[1:]):
            thought = "Eat my dust, roadkill!"
        else:
            thought = "Darn, someone beat me to it."

        # We can look at the header or all of an article already on spool,
        # too.  Might be useful for long-memory despamming or
        # authentication things.  Each is returned (if present) as a
        # string object; otherwise you'll end up with an empty string.
        artbody = INN.article('<foo$bar.baz@bungmunch.edu>')
        artheader = INN.head('<foo$bar.baz@bungmunch.edu>')

        # Finally, do you want to see if a given newsgroup is moderated or
        # whatever?  INN.newsgroup returns the last field of a group's
        # entry in active as a string.
        froupflag = INN.newsgroup('alt.fan.karl-malden.nose')
        if froupflag == '':
            moderated = 'no such newsgroup'
        elif froupflag == 'y':
            moderated = "nope"
        elif froupflag == 'm':
            moderated = "yep"
        else:
            moderated = "something else"

CHANGES TO PYTHON AUTHENTICATION AND ACCESS CONTROL SUPPORT FOR
NNRPD

    The old authentication and access control functionality has been
    combined with the new readers.conf mechanism by Erik Klavon
    <erik@eriq.org>; bug reports should however go to inn-bugs@isc.org, not
    Erik.

    The remainder of this section is an introduction to the new mechanism
    (which uses the python_auth, python_access, and python_dynamic
    readers.conf parameters) with porting/migration suggestions for people
    familiar with the old mechanism (identifiable by the now deprecated
    nnrpperlauth: parameter in inn.conf).

    Other people should skip this section.

    The python_auth parameter allows the use of Python to authenticate a
    user. Authentication scripts (like those from the old mechanism) are
    listed in readers.conf using python_auth in the same manner other
    authenticators are using auth:

            python_auth: "auth1.py"

    Scripts should be placed as before in the filter directory (see the
    *pathfilter* setting in inn.conf). The new hook method authen_init takes
    no arguments and its return value is ignored; its purpose is to provide
    a means for authentication specific initialization. The hook method
    authen_close is the more specific analogue to the old close method.
    These method hooks are not required.

    The argument dictionary passed to authenticate remains the same, except
    for the removal of the "type" entry which is no longer needed in this
    modification and the addition of several new entries (port, intipaddr,
    intport) described below. The return tuple now only contains either two
    or three elements, the first of which is the NNTP response code. The
    second is an error string which is passed to the client if the response
    code indicates that the authentication attempt has failed. This allows a
    specific error message to be generated by the Python script in place of
    the generic message "Authentication failed". An optional third return
    element, if present, will be used to match the connection with the user:
    parameter in access groups and will also be the username logged. If this
    element is absent, the username supplied by the client during
    authentication will be used, as was the previous behavior.

    The python_access parameter (described below) is new; it allows the
    dynamic generation of an access group of an incoming connection using a
    Python script. If a connection matches an auth group which has a
    python_access parameter, all access groups in readers.conf are ignored;
    instead the procedure described below is used to generate an access
    group. This concept is due to Jeffery M. Vinocur.

    In the old implementation, the authorization method allowed for access
    control on a per-group basis. That functionality is preserved in the new
    implementation by the inclusion of the python_dynamic parameter in
    readers.conf. The only change is the corresponding method name of
    "dynamic" as opposed to "authorize"; "domain" and "range" are the same
    as before except for the addition of several new entries to the argument
    dictionary (port, intipaddr, intport) described below. Additionally, the
    associated optional housekeeping methods dynamic_init and dynamic_close
    may be implemented if needed.

    This new implementation should provide all of the previous capabilities
    of the Python hooks, in combination with the flexibility of readers.conf
    and the use of other authentication and resolving programs (including
    the Perl hooks!). To use Python code that predates the new mechanism,
    you would need to modify the code slightly (see below for the new
    specification) and supply a simple readers.conf file. If you don't want
    to modify your code, the sample directory has nnrpd_auth_wrapper.py,
    nnrpd_access_wrapper.py, and nnrpd_dynamic_wrapper.py which should allow
    you to use your old code without needing to change it.

    However, before trying to use your old Python code, you may want to
    consider replacing it entirely with non-Python authentication. (With
    readers.conf and the regular authenticator and resolver programs, much
    of what once required Python can be done directly.)  Even if the
    functionality is not available directly, you may wish to write a new
    authenticator or resolver (which can be done in whatever language you
    prefer).

PYTHON AUTHENTICATION SUPPORT FOR NNRPD

    Python authentication, dynamic access group generation and dynamic
    access control support in nnrpd along with filtering support in innd may
    be compiled in by passing "--with-python" "configure".

    Support for authentication via Python is provided in nnrpd by the
    inclusion of a python_auth: parameter in a readers.conf auth group.
    python_auth: works exactly like the auth: parameter in readers.conf,
    except that it calls the script given as argument using the Python hook
    rather then treating it as an external program. Multiple, mixed use of
    python_auth: with other auth: statements including perl_auth: is
    permitted. Each auth: statement will be tried in the order they appear
    in the auth group until either one succeeds or all are exhausted.

    If the processing of readers.conf requires that a python_auth: statement
    be used for authentication, Python is loaded (if it has yet to be) and
    the file given as argument to the python_auth: parameter is loaded as
    well. If a Python object with a method authen_init is hooked in during
    the loading of that file, then that method is called immediately after
    the file is loaded. If no errors have occurred, the method authenticate
    is called. Depending on the NNTP response code returned by authenticate,
    the authentication hook either succeeds or fails, after which the
    processing of the auth group continues as usual. When the connection
    with the client is closed, the method authen_close is called if it
    exists.

DYNAMIC GENERATION OF ACCESS GROUPS

    A Python script may be used to dynamically generate an access group
    which is then used to determine the access rights of the client. This
    occurs whenever the python_access: parameter is specified in an auth
    group which has successfully matched the client. Only one python_access:
    statement is allowed in an auth group. This parameter should not be
    mixed with a perl_access: statement in the same auth group.

    When a python_access: parameter is encountered, Python is loaded (if it
    has yet to be) and the file given as argument is loaded as well. If a
    Python object with a method "access_init" is hooked in during the
    loading of that file, then that method is called immediately after the
    file is loaded. If no errors have occurred, the method "access" is
    called. The dictionary returned by "access" is used to generate an
    access group that is then used to determine the access rights of the
    client. When the connection with the client is closed, the method
    "access_close" is called, if it exists.

    While you may include the users: parameter in a dynamically generated
    access group, some care should be taken (unless your pattern is just *
    which is equivalent to leaving the parameter out). The group created
    with the values returned from the Python script is the only one
    considered when nnrpd attempts to find an access group matching the
    connection. If a users: parameter is included and it doesn't match the
    connection, then the client will be denied access since there are no
    other access groups which could match the connection.

DYNAMIC ACCESS CONTROL

    If you need to have access control rules applied immediately without
    having to restart all the nnrpd processes, you may apply access control
    on a per newsgroup basis using the Python dynamic hooks (as opposed to
    readers.conf, which does the same on per user basis). These hooks are
    activated through the inclusion of the python_dynamic: parameter in a
    readers.conf auth group. Only one python_dynamic: statement is allowed
    in an auth group.

    When a python_dynamic: parameter is encountered, Python is loaded (if it
    has yet to be) and the file given as argument is loaded as well. If a
    Python object with a method "dynamic_init" is hooked in during the
    loading of that file, then that method is called immediately after the
    file is loaded. Every time a reader asks nnrpd to read or post an
    article, the Python method "dynamic" is invoked before proceeding with
    the requested operation. Based on the value returned by "dynamic", the
    operation is either permitted or denied. When the connection with the
    client is closed, the method "access_close" is called if it exists.

WRITING A PYTHON NNRPD AUTHENTICATION MODULE

    You need to create a nnrpd_auth.py module in INN's filter directory (see
    the *pathfilter* setting in inn.conf) where you should define a class
    holding certain methods depending on which hooks you want to use.

    The following methods are known to nnrpd:

    __init__(self)
        Not explicitly called by nnrpd, but will run whenever the auth
        module is loaded. Use this method to initialize any general
        variables or open a common database connection. This method may be
        omitted.

    authen_init(self)
        Initialization function specific to authentication. This method may
        be omitted.

    authenticate(self, attributes)
        Called when a python_auth statement is reached in the processing of
        readers.conf. Connection attributes are passed in the "attributes"
        dictionary. Returns a response code, an error string, and an
        optional string to be used in place of the client-supplied username
        (both for logging and for matching the connection with an access
        group).

    authen_close(self)
        This method is invoked on nnrpd termination.  You can use it to save
        state information or close a database connection.

    access_init(self)
        Initialization function specific to generation of an access group.
        This method may be omitted.

    access(self, attributes)
        Called when a python_access statement is reached in the processing
        of readers.conf. Connection attributes are passed in the
        "attributes" dictionary. Returns a dictionary of values representing
        statements to be included in an access group.

    access_close(self)
        This method is invoked on nnrpd termination. You can use it to save
        state information or close a database connection.

    dynamic_init(self)
        Initialization function specific to dynamic access control. This
        method may be omitted.

    dynamic(self, attributes)
        Called when a client requests a newsgroup, an article or attempts to
        post. Connection attributes are passed in the "attributes"
        dictionary.  Returns "None" to grant access, or a non-empty string
        (which will be reported back to the client) otherwise.

    dynamic_close(self)
        This method is invoked on nnrpd termination.  You can use it to save
        state information or close a database connection.

    attributes dictionary
        The keys and associated values of the "attributes" dictionary are
        described below.

        type
            "read" or "post" values specify the authentication type; only
            valid for the "dynamic" method

        hostname
            resolved hostname (or IP address if resolution fails) of
            connected reader

        ipaddress
            IP address of connected reader

        port
            port of connected reader

        interface
            hostname of the local endpoint of the NNTP connection

        intipaddr
            IP address of the local endpoint of the NNTP connection

        intport
            port of the local endpoint of the NNTP connection

        user
            username as reader passed with AUTHINFO command, or None if not
            applicable

        pass
            password as reader passed with AUTHINFO command, or None if not
            applicible

        newsgroup
            name of the newsgroup reader requests read or post access to;
            only valid for the "dynamic" method

        All the above values are buffer objects (see the notes above on what
        buffer objects are).

    To register your methods with nnrpd, you need to create an instance of
    your class, import the built-in nnrpd module, and pass the instance to
    nnrpd.set_auth_hook().  For example:

        class AUTH:
            def authenticate(self, attributes):
                ...

            def authorize(self, attributes):
                ...

        import nnrpd
        myauth = AUTH()
        nnrpd.set_auth_hook(myauth)

    There is also an nnrpd.py module there that is not actually used by
    nnrpd but provides the same set of functions as built-in nnrpd module. 
    This stub module may be used when debugging your own module.

FUNCTIONS SUPPLIED BY THE BUILT-IN NNRPD MODULE

    As of this writing, nnrpd built-in module exports the following
    functions:

    set_auth_hook()
        used to pass a reference to the instance of authentication and
        authorization class to nnrpd

    syslog()
        intended to be a replacement for a Python native syslog

