Sendmail 8.13.3 Programmer's Guide

Table Of Contents

Introduction

Communication Between an MTA and Milter

Chapter 1 7

Communication Between an MTA and Milter

Filter applications run as separate processes outside the Sendmail

address space. The beneﬁts of ﬁlter applications running as separate

processes are as follows:

• Filter applications do not need to run with "root" permissions,

thereby, avoiding a large family of potential security problems.

• Failures in a particular ﬁlter do not affect Sendmail or other ﬁlters.

• Filter applications can have high performance because of the

parallelism inherent in multiple processes.

Each ﬁlter can communicate with multiple MTAs at the same time over

local or remote connections, using multiple threads of execution.

Figure 1-1 illustrates a network of communication channels between the

ﬁlters, its MTAs, and other MTAs on the network.

Figure 1-1 Communication Channel Between MTAs and Filters

The Milter library (libmilter.a) implements the communication

protocol. It accepts connections from various MTAs and passes the

relevant data to the ﬁlter through callbacks.

MTA

Filter 1

Filter 2

Filter 3

Internet