contrib/sendmail/libmilter/docs/design.html

   1 <html>
   2 <head>
   3 <title>Architecture</title>
   4 </head>
   5 <body>
   6
   7 <h1>Architecture</h1>
   8
   9 <h2>Contents</h2>
  10
  11 <ul>
  12     <li>Design Goals
  13     <li>Implementing Filtering Policies
  14     <li>MTA - Filter Communication
  15 </ul>
  16
  17 <h2>Goals</h2>
  18
  19 The Sendmail Content Management API (Milter) provides an interface for
  20 third-party software to validate and modify messages as they pass
  21 through the mail transport system.  Filters can process messages'
  22 connection (IP) information, envelope protocol elements, message
  23 headers, and/or message body contents, and modify a message's
  24 recipients, headers, and body.  The MTA configuration file specifies
  25 which filters are to be applied, and in what order, allowing an
  26 administrator to combine multiple independently-developed filters.
  27
  28 <p>
  29 We expect to see both vendor-supplied, configurable mail filtering
  30 applications and a multiplicity of script-like filters designed by and
  31 for MTA administrators.  A certain degree of coding sophistication and
  32 domain knowledge on the part of the filter provider is assumed.  This
  33 allows filters to exercise fine-grained control at the SMTP level.
  34 However, as will be seen in the example, many filtering applications
  35 can be written with relatively little protocol knowledge.
  36
  37 <p>
  38 Given these expectations, the API is designed to achieve the following
  39 goals:
  40
  41 <OL>
  42   <LI>Safety/security.
  43         Filter processes should not need to run as root
  44         (of course, they can if required, but that is a local issue);
  45         this will simplify coding
  46         and limit the impact of security flaws in the filter program.
  47 <p>
  48   <LI>Reliability.
  49         Coding failures in a Milter process that cause that process
  50         to hang or core-dump
  51         should not stop mail delivery.
  52         Faced with such a failure,
  53         sendmail should use a default mechanism,
  54         either behaving as if the filter were not present
  55         or as if a required resource were unavailable.
  56         The latter failure mode will generally have sendmail return
  57         a 4xx SMTP code (although in later phases of the SMTP protocol
  58         it may cause the mail to be queued for later processing).
  59 <p>
  60   <LI>Simplicity.
  61         The API should make implementation of a new filter
  62         no more difficult than absolutely necessary.
  63         Subgoals include:
  64         <UL>
  65           <LI>Encourage good thread practice
  66               by defining thread-clean interfaces including local data hooks.
  67           <LI>Provide all interfaces required
  68               while avoiding unnecessary pedanticism.
  69         </UL>
  70 <p>
  71   <LI>Performance.
  72         Simple filters should not seriously impact overall MTA performance.
  73 </OL>
  74
  75 <h2>Implementing Filtering Policies</h2>
  76
  77 Milter is designed to allow a server administrator to combine
  78 third-party filters to implement a desired mail filtering policy.  For
  79 example, if a site wished to scan incoming mail for viruses on several
  80 platforms, eliminate unsolicited commercial email, and append a mandated
  81 footer to selected incoming messages, the administrator could configure
  82 the MTA to filter messages first through a server based anti-virus
  83 engine, then via a large-scale spam-catching service, and finally
  84 append the desired footer if the message still met requisite criteria.
  85 Any of these filters could be added or changed independently.
  86
  87 <p>
  88 Thus the site administrator, not the filter writer, controls the
  89 overall mail filtering environment.  In particular, he/she must decide
  90 which filters are run, in what order they are run, and how they
  91 communicate with the MTA.  These parameters, as well as the
  92 actions to be taken if a filter becomes unavailable, are selectable
  93 during MTA configuration.  <a href="installation.html">Further
  94 details</a> are available later in this document.
  95
  96 <h2>MTA - Filter communication</h2>
  97
  98 Filters run as separate processes, outside of the sendmail address
  99 space.  The benefits of this are threefold:
 100
 101 <OL>
 102   <LI>The filter need not run with "root" permissions, thereby
 103         avoiding a large family of potential security problems.</LI>
 104
 105   <LI>Failures in a particular filter will not affect the MTA or
 106         other filters.</LI>
 107
 108   <LI>The filter can potentially have higher performance because of
 109         the parallelism inherent in multiple processes.</LI>
 110 </OL>
 111
 112 <p>
 113 Each filter may communicate with multiple MTAs at the same time over
 114 local or remote connections, using multiple threads of execution.  <a
 115 href="#figure-1">Figure 1</a> illustrates a possible network of
 116 communication channels between a site's filters, its MTAs, and other
 117 MTAs on the network:
 118 </p>
 119 <div align="center">
 120 <a name="figure-1"><img src="figure1.jpg" ALT=""></A><br>
 121 <b>Figure 1: A set of MTA's interacting with a set of filters.</b>
 122 </div>
 123 <p>
 124 The Milter library (libmilter) implements the communication protocol.
 125 It accepts connections from various MTAs, passes the relevant data to
 126 the filter through callbacks, then makes appropriate responses based
 127 on return codes.  A filter may also send data to the MTA as a result
 128 of library calls.  <a href="#figure-2">Figure 2</a> shows a single
 129 filter process processing messages from two MTAs:
 130 </p>
 131 <div align="center">
 132 <img src="figure2.jpg" ALT=""><br>
 133 <b>Figure 2: A filter handling simultaneous requests from two MTA's.</b>
 134 </div>
 135 <hr size="1">
 136 <font size="-1">
 137 Copyright (c) 2000, 2003 Sendmail, Inc. and its suppliers.
 138 All rights reserved.
 139 <br>
 140 By using this file, you agree to the terms and conditions set
 141 forth in the LICENSE.
 142 </font>
 143 </body>
 144 </html>