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 19The Sendmail Content Management API (Milter) provides an interface for 20third-party software to validate and modify messages as they pass 21through the mail transport system. Filters can process messages' 22connection (IP) information, envelope protocol elements, message 23headers, and/or message body contents, and modify a message's 24recipients, headers, and body. The MTA configuration file specifies 25which filters are to be applied, and in what order, allowing an 26administrator to combine multiple independently-developed filters. 27 28<p> 29We expect to see both vendor-supplied, configurable mail filtering 30applications and a multiplicity of script-like filters designed by and 31for MTA administrators. A certain degree of coding sophistication and 32domain knowledge on the part of the filter provider is assumed. This 33allows filters to exercise fine-grained control at the SMTP level. 34However, as will be seen in the example, many filtering applications 35can be written with relatively little protocol knowledge. 36 37<p> 38Given these expectations, the API is designed to achieve the following 39goals: 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 77Milter is designed to allow a server administrator to combine 78third-party filters to implement a desired mail filtering policy. For 79example, if a site wished to scan incoming mail for viruses on several 80platforms, eliminate unsolicited commercial email, and append a mandated 81footer to selected incoming messages, the administrator could configure 82the MTA to filter messages first through a server based anti-virus 83engine, then via a large-scale spam-catching service, and finally 84append the desired footer if the message still met requisite criteria. 85Any of these filters could be added or changed independently. 86 87<p> 88Thus the site administrator, not the filter writer, controls the 89overall mail filtering environment. In particular, he/she must decide 90which filters are run, in what order they are run, and how they 91communicate with the MTA. These parameters, as well as the 92actions to be taken if a filter becomes unavailable, are selectable 93during MTA configuration. <a href="installation.html">Further 94details</a> are available later in this document. 95 96<h2>MTA - Filter communication</h2> 97 98Filters run as separate processes, outside of the sendmail address 99space. 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> 113Each filter may communicate with multiple MTAs at the same time over 114local or remote connections, using multiple threads of execution. <a 115href="#figure-1">Figure 1</a> illustrates a possible network of 116communication channels between a site's filters, its MTAs, and other 117MTAs 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> 124The Milter library (libmilter) implements the communication protocol. 125It accepts connections from various MTAs, passes the relevant data to 126the filter through callbacks, then makes appropriate responses based 127on return codes. A filter may also send data to the MTA as a result 128of library calls. <a href="#figure-2">Figure 2</a> shows a single 129filter 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"> 137Copyright (c) 2000, 2003 Sendmail, Inc. and its suppliers. 138All rights reserved. 139<br> 140By using this file, you agree to the terms and conditions set 141forth in the LICENSE. 142</font> 143</body> 144</html> 145
