qpsmtpd/docs/plugins.pod

#
# This file is best read with ``perldoc plugins.pod''
#

### 
# Conventions:
#  plugin names:  F<myplugin>
#  constants:     I<LOGDEBUG>
#  smtp commands, answers: B<HELO>, B<250 Queued!>
#
# Notes: 
#  * due to restrictions of some POD parsers, no C<<$object->method()>>
#    are allowed, use C<$object-E<gt>method()> 
# 

=head1 Introduction

Plugins are the heart of qpsmtpd. The core implements only basic SMTP protocol
functionality. No useful function can be done by qpsmtpd without loading 
plugins.

Plugins are loaded on startup where each of them register their interest in 
various I<hooks> provided by the qpsmtpd core engine.

At least one plugin B<must> allow or deny the B<RCPT> command to enable 
receiving mail. The F<check_relay> plugin is the standard plugin for this.
Other plugins provide extra functionality related to this; for example the
F<resolvable_fromhost> plugin.

=head2 Loading Plugins

The list of plugins to load are configured in the I<config/plugins>
configuration file. One plugin per line, empty lines and lines starting
with I<#> are ignored. The order they are loaded is the same as given 
in this config file. This is also the order the registered I<hooks>
are run. The plugins are loaded from the F<plugins/> directory or 
from a subdirectory of it. If a plugin should be loaded from such a 
subdirectory, the directory must also be given, like the 
F<virus/clamdscan> in the example below. Alternate plugin directories
may be given in the F<config/plugin_dirs> config file, one directory
per line, these will be searched first before using the builtin fallback
of F<plugins/> relative to the qpsmtpd root directory. It may be 
necessary, that the F<config/plugin_dirs> must be used (if you're using 
F<Apache::Qpsmtpd>, for example).

Some plugins may be configured by passing arguments in the F<plugins>
config file.

A plugin can be loaded two or more times with different arguments by adding
I<:N> to the plugin filename, with I<N> being a number, usually starting at 
I<0>. 

Another method to load a plugin is to create a valid perl module, drop this
module in perl's C<@INC> path and give the name of this module as
plugin name. The only restriction to this is, that the module name B<must>
contain I<::>, e.g. C<My::Plugin> would be ok, C<MyPlugin> not. Appending of
I<:0>, I<:1>, ... does not work with module plugins.

 check_relay
 virus/clamdscan
 spamassassin reject_threshold 7
 my_rcpt_check example.com
 my_rcpt_check:0 example.org
 My::Plugin

=head1 Anatomy of a plugin

A plugin has at least one method, which inherits from the 
C<Qpsmtpd::Plugin> object. The first argument for this method is always the 
plugin object itself (and usually called C<$self>). The most simple plugin 
has one method with a predefined name which just returns one constant.

 # plugin temp_disable_connection
 sub hook_connect {
    return(DENYSOFT, "Sorry, server is temporarily unavailable.");
 }

While this is a valid plugin, it is not very useful except for rare 
circumstances. So let us see what happens when a plugin is loaded.

=head2 Initialisation

After the plugin is loaded the C<init()> method of the plugin is called, 
if present. The arguments passed to C<init()> are 

=over 4

=item $self 

the current plugin object, usually called C<$self>

=item $qp

the Qpsmtpd object, usually called C<$qp>.

=item @args

the values following the plugin name in the F<plugins> config, split by 
white space. These arguments can be used to configure the plugin with 
default and/or static config settings, like database paths, 
timeouts, ...

=back

This is mainly used for inheriting from other plugins, but may be used to do
the same as in C<register()>.

The next step is to register the hooks the plugin provides. Any method which 
is named C<hook_$hookname> is automagically added.

Plugins should be written using standard named hook subroutines. This 
allows them to be overloaded and extended easily. Because some of the 
callback names have characters invalid in subroutine names , they must be 
translated. The current translation routine is C<s/\W/_/g;>, see 
L</Hook - Subroutine translations> for more info. If you choose 
not to use the default naming convention, you need to register the hooks in 
your plugin in the C<register()> method (see below) with the 
C<register_hook()> call on the plugin object.

  sub register {
    my ($self, $qp, @args) = @_;
    $self->register_hook("mail", "mail_handler");
    $self->register_hook("rcpt", "rcpt_handler");
  }
  sub mail_handler { ... }
  sub rcpt_handler { ... }

The C<register()> method is called last. It receives the same arguments as 
C<init()>. There is no restriction, what you can do in C<register()>, but 
creating database connections and reuse them later in the process may not be
a good idea. This initialisation happens before any C<fork()> is done. 
Therefore the file handle will be shared by all qpsmtpd processes and the 
database will probably be confused if several different queries arrive on 
the same file handle at the same time (and you may get the wrong answer, if
any). This is also true for the pperl flavor but 
not for F<qpsmtpd> started by (x)inetd or tcpserver. 

In short: don't do it if you want to write portable plugins.

=head2 Hook - Subroutine translations

As mentioned above, the hook name needs to be translated to a valid perl
C<sub> name. This is done like

 ($sub = $hook) =~ s/\W/_/g;
 $sub = "hook_$sub";

Some examples follow, for a complete list of available (documented ;-))
hooks (method names), use something like

 $ perl -lne 'print if s/^=head2\s+(hook_\S+)/$1/' docs/plugins.pod

All valid hooks are defined in F<lib/Qpsmtpd/Plugins.pm>, C<our @hooks>.

=head3 Translation table

 hook                          method
 ----------                    ------------
 config                        hook_config
 queue                         hook_queue
 data                          hook_data
 data_post                     hook_data_post
 quit                          hook_quit
 rcpt                          hook_rcpt
 mail                          hook_mail
 ehlo                          hook_ehlo
 helo                          hook_helo
 auth                          hook_auth
 auth-plain                    hook_auth_plain
 auth-login                    hook_auth_login
 auth-cram-md5                 hook_auth_cram_md5
 connect                       hook_connect
 reset_transaction             hook_reset_transaction
 unrecognized_command          hook_unrecognized_command

=head2 Inheritance

Inheriting methods from other plugins is an advanced topic. You can alter 
arguments for the underlying plugin, prepare something for the I<real> 
plugin or skip a hook with this. Instead of modifying C<@ISA>
directly in your plugin, use the C<isa_plugin()> method from the 
C<init()> subroutine.

  # rcpt_ok_child
  sub init {
    my ($self, $qp, @args) = @_;
    $self->isa_plugin("rcpt_ok");
  }

  sub hook_rcpt {
    my ($self, $transaction, $recipient) = @_;
    # do something special here...
    $self->SUPER::hook_rcpt($transaction, $recipient);
  }

See also chapter C<Changing return values> and 
F<contrib/vetinari/rcpt_ok_maxrelay> in SVN.

=head2 Config files

Most of the existing plugins fetch their configuration data from files in the
F<config/> sub directory. This data is read at runtime and may be changed
without restarting qpsmtpd. 
B<(FIXME: caching?!)>
The contents of the files can be fetched via

  @lines = $self->qp->config("my_config");

All empty lines and lines starting with C<#> are ignored.

If you don't want to read your data from files, but from a database you can 
still use this syntax and write another plugin hooking the C<config>
hook. 

=head2 Logging

Log messages can be written to the log file (or STDERR if you use the 
F<logging/warn> plugin) with 

  $self->log($loglevel, $logmessage);

The log level is one of (from low to high priority)

=over 4

=item * 

LOGDEBUG

=item *

LOGINFO

=item *

LOGNOTICE

=item *

LOGWARN

=item *

LOGERROR

=item *

LOGCRIT

=item *

LOGALERT

=item *

LOGEMERG

=back

While debugging your plugins, set your plugins loglevel to LOGDEBUG. This
will log every logging statement within your plugin.

For more information about logging, see F<docs/logging.pod>.

=head2 Information about the current plugin

Each plugin inherits the public methods from C<Qpsmtpd::Plugin>. 

=over 4

=item plugin_name()

Returns the name of the currently running plugin

=item hook_name()

Returns the name of the running hook

=item auth_user()

Returns the name of the user the client is authed as (if authentication is 
used, of course)

=item auth_mechanism()

Returns the auth mechanism if authentication is used

=item connection()

Returns the C<Qpsmtpd::Connection> object associated with the current 
connection

=item transaction()

Returns the C<Qpsmtpd::Transaction> object associated with the current 
transaction

=back

=head2 Temporary Files

The temporary file and directory functions can be used for plugin specific 
workfiles and will automatically be deleted at the end of the current 
transaction.

=over 4

=item temp_file()

Returns a unique name of a file located in the default spool directory, 
but does not open that file (i.e. it is the name not a file handle).

=item temp_dir()

Returns the name of a unique directory located in the default spool 
directory, after creating the directory with 0700 rights. If you need a 
directory with different rights (say for an antivirus daemon), you will
need to use the base function C<$self-E<gt>qp-E<gt>temp_dir()>, which takes a 
single parameter for the permissions requested (see L<mkdir> for details). 
A directory created like this will not be deleted when the transaction 
is ended.

=item spool_dir()

Returns the configured system-wide spool directory.

=back


=head2 Connection and Transaction Notes

Both may be used to share notes across plugins and/or hooks. The only real
difference is their life time. The connection notes start when a new 
connection is made and end, when the connection ends. This can, for example,
be used to count the number of none SMTP commands. The plugin which uses 
this is the F<count_unrecognized_commands> plugin from the qpsmtpd core
distribution.

The transaction note starts after the B<MAIL FROM: > command and are just 
valid for the current transaction, see below in the I<reset_transaction>
hook when the transaction ends.


=head1 Return codes

Each plugin must return an allowed constant for the hook and (usually)
optionally a ``message'' for the client.
Generally all plugins for a hook are processed until one returns 
something other than I<DECLINED>.

Plugins are run in the order they are listed in the F<plugins> 
configuration file.

The return constants are defined in C<Qpsmtpd::Constants> and have
the following meanings:

=over 4

=item DECLINED

Plugin declined work; proceed as usual. This return code is I<always allowed>
unless noted otherwise.

=item OK

Action allowed.

=item DENY

Action denied.

=item DENYSOFT

Action denied; return a temporary rejection code (say B<450> instead 
of B<550>).

=item DENY_DISCONNECT

Action denied; return a permanent rejection code and disconnect the client. 
Use this for "rude" clients. Note that you're not supposed to do this 
according to the SMTP specs, but bad clients don't listen sometimes.

=item DENYSOFT_DISCONNECT

Action denied; return a temporary rejection code and disconnect the client. 
See note above about SMTP specs.

=item DONE

Finishing processing of the request. Usually used when the plugin sent the 
response to the client.

=back

=cut