3 <TITLE>Technical Overview</TITLE>
7 $Id: overview.html,v 1.22 2013-11-22 20:51:39 ca Exp $
10 <H1>Technical Overview</H1>
15 <LI><A HREF="#Initialization">Initialization</A>
16 <LI><A HREF="#ControlFlow">Control Flow</A>
17 <LI><A HREF="#Multithreading">Multithreading</A>
18 <LI><A HREF="#ResourceManagement">Resource Management</A>
19 <LI><A HREF="#SignalHandling">Signal Handling</A>
22 <H2><A NAME="Initialization">Initialization</A></H2>
24 In addition to its own initialization,
25 libmilter expects a filter to initialize several parameters
26 before calling <A HREF="smfi_main.html">smfi_main</A>:
28 <LI>The callbacks the filter wishes to be called, and the types of
29 message modification it intends to perform (required, see
30 <A HREF="smfi_register.html">smfi_register</A>).
32 <LI>The socket address to be used when communicating with the MTA
33 (required, see <A HREF="smfi_setconn.html">smfi_setconn</A>).
35 <LI>The number of seconds to wait for MTA connections before
36 timing out (optional, see
37 <A HREF="smfi_settimeout.html">smfi_settimeout</A>).
40 If the filter fails to initialize libmilter,
41 or if one or more of the parameters it has passed are invalid,
42 a subsequent call to smfi_main will fail.
44 <H2><A NAME="ControlFlow">Control Flow</A></H2>
47 The following pseudocode describes the filtering process from the
48 perspective of a set of <CODE>N</CODE> MTA's,
49 each corresponding to a connection.
50 Callbacks are shown beside the processing stages in which they are invoked;
51 if no callbacks are defined for a particular stage,
52 that stage may be bypassed.
53 Though it is not shown,
54 processing may be aborted at any time during a message,
56 <A HREF="xxfi_abort.html">xxfi_abort</A> callback is invoked and control
57 returns to <CODE>MESSAGE</CODE>.
60 For each of N connections
63 egotiate MTA/milter capabilities/requirements (<A HREF="xxfi_negotiate.html">xxfi_negotiate</A>)
65 process connection (<A HREF="xxfi_connect.html">xxfi_connect</A>)
67 process helo/ehlo (<A HREF="xxfi_helo.html">xxfi_helo</A>)
68 MESSAGE:For each message in this connection (sequentially)
71 process sender (<A HREF="xxfi_envfrom.html">xxfi_envfrom</A>)
75 process recipient (<A HREF="xxfi_envrcpt.html">xxfi_envrcpt</A>)
79 process DATA (<A HREF="xxfi_data.html">xxfi_data</A>)
81 process header (<A HREF="xxfi_header.html">xxfi_header</A>)
82 process end of headers (<A HREF="xxfi_eoh.html">xxfi_eoh</A>)
84 process this body block (<A HREF="xxfi_body.html">xxfi_body</A>)
85 process end of message (<A HREF="xxfi_eom.html">xxfi_eom</A>)
89 process end of connection (<A HREF="xxfi_close.html">xxfi_close</A>)
93 <P>Note: Filters are contacted in order defined in config file.</P>
96 To write a filter, a vendor supplies callbacks to process relevant
97 parts of a message transaction.
98 The library then controls all sequencing, threading,
99 and protocol exchange with the MTA.
100 <A HREF="#figure-3">Figure 3</A> outlines control flow for a filter
101 process, showing where different callbacks are invoked.
104 <DIV ALIGN="center"><A NAME="figure-3"></A>
105 <TABLE border=1 cellspacing=0 cellpadding=2 width="70%">
106 <TR bgcolor="#dddddd"><TH>SMTP Commands</TH><TH>Milter Callbacks</TH></TR>
107 <TR><TD>(open SMTP connection)</TD><TD>xxfi_connect</TD></TR>
108 <TR><TD>HELO ...</TD><TD>xxfi_helo</TD></TR>
109 <TR><TD>MAIL From: ...</TD><TD>xxfi_envfrom</TD></TR>
110 <TR><TD>RCPT To: ...</TD><TD>xxfi_envrcpt</TD></TR>
111 <TR><TD>[more RCPTs]</TD><TD>[xxfi_envrcpt]</TD></TR>
112 <TR><TD>DATA</TD><TD>xxfi_data</TD></TR>
113 <TR><TD>Header: ...</TD><TD>xxfi_header</TD></TR>
114 <TR><TD>[more headers]</TD><TD>[xxfi_header]</TD></TR>
115 <TR><TD> </TD><TD>xxfi_eoh</TD></TR>
116 <TR><TD>body... </TD><TD>xxfi_body</TD></TR>
117 <TR><TD>[more body...]</TD><TD>[xxfi_body]</TD></TR>
118 <TR><TD>.</TD><TD>xxfi_eom</TD></TR>
119 <TR><TD>QUIT</TD><TD>xxfi_close</TD></TR>
120 <TR><TD>(close SMTP connection)</TD><TD> </TD></TR>
122 <B>Figure 3: Milter callbacks related to an SMTP transaction.</B>
126 Note that although only a single message is shown above, multiple
127 messages may be sent in a single connection.
128 Note also that a message or connection may be aborted by
129 either the remote host or the MTA
130 at any point during the SMTP transaction.
131 If this occurs during a message (between the MAIL command and the final "."),
133 <A HREF="xxfi_abort.html">xxfi_abort</A> routine will be called.
134 <A HREF="xxfi_close.html">xxfi_close</A> is called any time the
137 <H2><A NAME="Multithreading">Multithreading</A></H2>
140 A single filter process may handle any number of connections
142 All filtering callbacks must therefore be reentrant,
143 and use some appropriate external synchronization methods to access
145 Furthermore, since there is not a one-to-one correspondence
146 between threads and connections
147 (N connections mapped onto M threads, M <= N),
148 connection-specific data must be accessed
149 through the handles provided by the Milter library.
150 The programmer cannot rely on library-supplied thread-specific data blocks
151 (e.g., <CODE>pthread_getspecific(3)</CODE>) to store connection-specific data.
152 See the API documentation for
153 <A HREF="smfi_setpriv.html">smfi_setpriv</A> and
154 <A HREF="smfi_getpriv.html">smfi_getpriv</A> for details.
156 <H2><A NAME="ResourceManagement">Resource Management</A></H2>
158 Since filters are likely to be long-lived,
159 and to handle many connections,
160 proper deallocation of per-connection resources is important.
161 The lifetime of a connection is bracketed by calls to the
162 callbacks <A HREF="xxfi_connect.html">xxfi_connect</A> and
163 <A HREF="xxfi_close.html">xxfi_close</A>.
164 Therefore connection-specific
165 resources (accessed via <A HREF="smfi_getpriv.html">smfi_getpriv</A>
166 and <A HREF="smfi_setpriv.html">smfi_setpriv</A>) may be allocated in
167 <A HREF="xxfi_connect.html">xxfi_connect</A>,
168 and should be freed in
169 <A HREF="xxfi_close.html">xxfi_close</A>.
170 For further information see
171 the <A HREF="api.html#conn-msg">discussion</A> of message- versus
172 connection-oriented routines.
174 note that there is only one connection-specific data pointer per connection.
177 Each message is bracketed by calls to
178 <A HREF="xxfi_envfrom.html">xxfi_envfrom</A> and
179 <A HREF="xxfi_eom.html">xxfi_eom</A> (or
180 <A HREF="xxfi_abort.html">xxfi_abort</A>),
181 implying that message-specific resources can be allocated
182 and reclaimed in these routines.
183 Since the messages in a connection are processed sequentially by each filter,
184 there will be only one active message associated with a given
185 connection and filter (and connection-private data block).
186 These resources must still be accessed through
187 <A HREF="smfi_getpriv.html">smfi_getpriv</A> and
188 <A HREF="smfi_setpriv.html">smfi_setpriv</A>,
189 and must be reclaimed in
190 <A HREF="xxfi_abort.html">xxfi_abort</A>.
192 <H2><A NAME="SignalHandling">Signal Handling</A></H2>
194 libmilter takes care of signal handling,
195 the filters are not influenced directly by signals.
196 There are basically two types of signal handlers:
199 <LI><TT>Stop</TT>: no new connections from the MTA will be accepted,
200 but existing connections are allowed to continue.
201 <LI><TT>Abort</TT>: all filters will be stopped as soon as the next
202 communication with the MTA happens.
205 Filters are not terminated asynchronously
206 (except by signals that can't be caught).
207 In the case of <TT>Abort</TT> the
208 <A HREF="xxfi_abort.html">xxfi_abort</A> callback is usually invoked
209 if there is an active transaction.
210 However, if an invoked callback takes too long to execute
211 (the maximum time <TT>Abort</TT> waits is currently 5s)
212 <!-- XREF: MI_CHK_TIME -->
213 then the filter is simply terminated, i.e.,
215 <A HREF="xxfi_abort.html">xxfi_abort</A> callback
217 <A HREF="xxfi_close.html">xxfi_close</A> callback
222 Copyright (c) 2000, 2001, 2003, 2006, 2018 Proofpoint, Inc. and its suppliers.
225 By using this file, you agree to the terms and conditions set
226 forth in the LICENSE.