3 <TITLE>Architecture</TITLE>
7 $Id: design.html,v 1.13 2013-11-22 20:51:39 ca Exp $
16 <LI>Implementing Filtering Policies
17 <LI>MTA - Filter Communication
22 The Sendmail Content Management API (Milter) provides an interface for
23 third-party software to validate and modify messages as they pass
24 through the mail transport system. Filters can process messages'
25 connection (IP) information, envelope protocol elements, message
26 headers, and/or message body contents, and modify a message's
27 recipients, headers, and body. The MTA configuration file specifies
28 which filters are to be applied, and in what order, allowing an
29 administrator to combine multiple independently-developed filters.
32 We expect to see both vendor-supplied, configurable mail filtering
33 applications and a multiplicity of script-like filters designed by and
34 for MTA administrators.
35 A certain degree of coding sophistication and
36 domain knowledge on the part of the filter provider is assumed.
37 This allows filters to exercise fine-grained control at the SMTP level.
38 However, as will be seen in the example, many filtering applications
39 can be written with relatively little protocol knowledge,
40 but a basic understanding (e.g., as documented in RFC 5321:
41 <EM>The dialog is purposely lock-step, one-at-a-time</EM>)
45 Given these expectations, the API is designed to achieve the following
50 Filter processes should not need to run as root
51 (of course, they can if required, but that is a local issue);
52 this will simplify coding
53 and limit the impact of security flaws in the filter program.
56 Coding failures in a Milter process that cause that process
58 should not stop mail delivery.
59 Faced with such a failure,
60 sendmail should use a default mechanism,
61 either behaving as if the filter were not present
62 or as if a required resource were unavailable.
63 The latter failure mode will generally have sendmail return
64 a 4xx SMTP code (although in later phases of the SMTP protocol
65 it may cause the mail to be queued for later processing).
68 The API should make implementation of a new filter
69 no more difficult than absolutely necessary.
72 <LI>Encourage good thread practice
73 by defining thread-clean interfaces including local data hooks.
74 <LI>Provide all interfaces required
75 while avoiding unnecessary pedanticism.
79 Simple filters should not seriously impact overall MTA performance.
82 <H2>Implementing Filtering Policies</H2>
84 Milter is designed to allow a server administrator to combine
85 third-party filters to implement a desired mail filtering policy. For
86 example, if a site wished to scan incoming mail for viruses on several
87 platforms, eliminate unsolicited commercial email, and append a mandated
88 footer to selected incoming messages, the administrator could configure
89 the MTA to filter messages first through a server based anti-virus
90 engine, then via a large-scale spam-catching service, and finally
91 append the desired footer if the message still met requisite criteria.
92 Any of these filters could be added or changed independently.
95 Thus the site administrator, not the filter writer, controls the
96 overall mail filtering environment. In particular, he/she must decide
97 which filters are run, in what order they are run, and how they
98 communicate with the MTA. These parameters, as well as the
99 actions to be taken if a filter becomes unavailable, are selectable
100 during MTA configuration. <A href="installation.html">Further
101 details</A> are available later in this document.
103 <H2>MTA - Filter communication</H2>
105 Filters run as separate processes, outside of the sendmail address
106 space. The benefits of this are threefold:
109 <LI>The filter need not run with "root" permissions, thereby
110 avoiding a large family of potential security problems.</LI>
112 <LI>Failures in a particular filter will not affect the MTA or
115 <LI>The filter can potentially have higher performance because of
116 the parallelism inherent in multiple processes.</LI>
120 Each filter may communicate with multiple MTAs at the same time over
121 local or remote connections, using multiple threads of execution.
122 <A HREF="#figure-1">Figure 1</A> illustrates a possible network of
123 communication channels between a site's filters, its MTAs, and other
127 <A name="figure-1"><IMG src="figure1.jpg" ALT=""></A><BR>
128 <B>Figure 1: A set of MTA's interacting with a set of filters.</B>
131 The Milter library (libmilter) implements the communication protocol.
132 It accepts connections from various MTAs, passes the relevant data to
133 the filter through callbacks, then makes appropriate responses based
134 on return codes. A filter may also send data to the MTA as a result
135 of library calls. <A href="#figure-2">Figure 2</A> shows a single
136 filter process processing messages from two MTAs:
139 <IMG src="figure2.jpg" ALT=""><BR>
140 <B>Figure 2: A filter handling simultaneous requests from two MTA's.</B>
144 Copyright (c) 2000, 2003 Proofpoint, Inc. and its suppliers.
147 By using this file, you agree to the terms and conditions set
148 forth in the LICENSE.