1<HTML> 2<HEAD> 3<TITLE>Technical Overview</TITLE> 4</HEAD> 5<BODY> 6<!-- 7$Id: overview.html,v 1.19 2006/12/21 18:23:47 ca Exp $ 8--> 9 10<H1>Technical Overview</H1> 11 12<H2>Contents</H2> 13 14<UL> 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> 20</UL> 21 22<H2><A NAME="Initialization">Initialization</A></H2> 23 24In addition to its own initialization, 25libmilter expects a filter to initialize several parameters 26before calling <A HREF="smfi_main.html">smfi_main</A>: 27<UL> 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>). 31 32 <LI>The socket address to be used when communicating with the MTA 33 (required, see <A HREF="smfi_setconn.html">smfi_setconn</A>). 34 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>). 38</UL> 39<P> 40If the filter fails to initialize libmilter, 41or if one or more of the parameters it has passed are invalid, 42a subsequent call to smfi_main will fail. 43 44<H2><A NAME="ControlFlow">Control Flow</A></H2> 45 46<P> 47The following pseudocode describes the filtering process from the 48perspective of a set of <CODE>N</CODE> MTA's, 49each corresponding to a connection. 50Callbacks are shown beside the processing stages in which they are invoked; 51if no callbacks are defined for a particular stage, 52that stage may be bypassed. 53Though it is not shown, 54processing may be aborted at any time during a message, 55in which case the 56<A HREF="xxfi_abort.html">xxfi_abort</A> callback is invoked and control 57returns to <CODE>MESSAGE</CODE>. 58<P> 59<PRE> 60For each of N connections 61{ 62 For each filter 63 process connection/helo (<A HREF="xxfi_connect.html">xxfi_connect</A>, <A HREF="xxfi_helo.html">xxfi_helo</A>) 64MESSAGE:For each message in this connection (sequentially) 65 { 66 For each filter 67 process sender (<A HREF="xxfi_envfrom.html">xxfi_envfrom</A>) 68 For each recipient 69 { 70 For each filter 71 process recipient (<A HREF="xxfi_envrcpt.html">xxfi_envrcpt</A>) 72 } 73 For each filter 74 { 75 process DATA (<A HREF="xxfi_data.html">xxfi_data</A>) 76 For each header 77 process header (<A HREF="xxfi_header.html">xxfi_header</A>) 78 process end of headers (<A HREF="xxfi_eoh.html">xxfi_eoh</A>) 79 For each body block 80 process this body block (<A HREF="xxfi_body.html">xxfi_body</A>) 81 process end of message (<A HREF="xxfi_eom.html">xxfi_eom</A>) 82 } 83 } 84 For each filter 85 process end of connection (<A HREF="xxfi_close.html">xxfi_close</A>) 86} 87</PRE> 88 89<P>Note: Filters are contacted in order defined in config file.</P> 90 91<P> 92To write a filter, a vendor supplies callbacks to process relevant 93parts of a message transaction. 94The library then controls all sequencing, threading, 95and protocol exchange with the MTA. 96<A HREF="#figure-3">Figure 3</A> outlines control flow for a filter 97process, showing where different callbacks are invoked. 98</P> 99 100<DIV ALIGN="center"><A NAME="figure-3"></A> 101<TABLE border=1 cellspacing=0 cellpadding=2 width="70%"> 102<TR bgcolor="#dddddd"><TH>SMTP Commands</TH><TH>Milter Callbacks</TH></TR> 103<TR><TD>(open SMTP connection)</TD><TD>xxfi_connect</TD></TR> 104<TR><TD>HELO ...</TD><TD>xxfi_helo</TD></TR> 105<TR><TD>MAIL From: ...</TD><TD>xxfi_envfrom</TD></TR> 106<TR><TD>RCPT To: ...</TD><TD>xxfi_envrcpt</TD></TR> 107<TR><TD>[more RCPTs]</TD><TD>[xxfi_envrcpt]</TD></TR> 108<TR><TD>DATA</TD><TD>xxfi_data</TD></TR> 109<TR><TD>Header: ...</TD><TD>xxfi_header</TD></TR> 110<TR><TD>[more headers]</TD><TD>[xxfi_header]</TD></TR> 111<TR><TD> </TD><TD>xxfi_eoh</TD></TR> 112<TR><TD>body... </TD><TD>xxfi_body</TD></TR> 113<TR><TD>[more body...]</TD><TD>[xxfi_body]</TD></TR> 114<TR><TD>.</TD><TD>xxfi_eom</TD></TR> 115<TR><TD>QUIT</TD><TD>xxfi_close</TD></TR> 116<TR><TD>(close SMTP connection)</TD><TD> </TD></TR> 117</TABLE> 118<B>Figure 3: Milter callbacks related to an SMTP transaction.</B> 119</DIV> 120 121<P> 122Note that although only a single message is shown above, multiple 123messages may be sent in a single connection. 124Note also that a message or connection may be aborted by 125either the remote host or the MTA 126at any point during the SMTP transaction. 127f this occurs during a message (between the MAIL command and the final "."), 128the filter's 129<A HREF="xxfi_abort.html">xxfi_abort</A> routine will be called. 130<A HREF="xxfi_close.html">xxfi_close</A> is called any time the 131connection closes. 132 133<H2><A NAME="Multithreading">Multithreading</A></H2> 134 135<P> 136A single filter process may handle any number of connections 137simultaneously. 138All filtering callbacks must therefore be reentrant, 139and use some appropriate external synchronization methods to access 140global data. 141Furthermore, since there is not a one-to-one correspondence 142between threads and connections 143(N connections mapped onto M threads, M <= N), 144connection-specific data must be accessed 145through the handles provided by the Milter library. 146The programmer cannot rely on library-supplied thread-specific data blocks 147(e.g., <CODE>pthread_getspecific(3)</CODE>) to store connection-specific data. 148See the API documentation for 149<A HREF="smfi_setpriv.html">smfi_setpriv</A> and 150<A HREF="smfi_getpriv.html">smfi_getpriv</A> for details. 151 152<H2><A NAME="ResourceManagement">Resource Management</A></H2> 153 154Since filters are likely to be long-lived, 155and to handle many connections, 156proper deallocation of per-connection resources is important. 157The lifetime of a connection is bracketed by calls to the 158callbacks <A HREF="xxfi_connect.html">xxfi_connect</A> and 159<A HREF="xxfi_close.html">xxfi_close</A>. 160Therefore connection-specific 161resources (accessed via <A HREF="smfi_getpriv.html">smfi_getpriv</A> 162and <A HREF="smfi_setpriv.html">smfi_setpriv</A>) may be allocated in 163<A HREF="xxfi_connect.html">xxfi_connect</A>, 164and should be freed in 165<A HREF="xxfi_close.html">xxfi_close</A>. 166For further information see 167the <A HREF="api.html#conn-msg">discussion</A> of message- versus 168connection-oriented routines. 169In particular, 170note that there is only one connection-specific data pointer per connection. 171<P> 172 173Each message is bracketed by calls to 174<A HREF="xxfi_envfrom.html">xxfi_envfrom</A> and 175<A HREF="xxfi_eom.html">xxfi_eom</A> (or 176<A HREF="xxfi_abort.html">xxfi_abort</A>), 177implying that message-specific resources can be allocated 178and reclaimed in these routines. 179Since the messages in a connection are processed sequentially by each filter, 180there will be only one active message associated with a given 181connection and filter (and connection-private data block). 182These resources must still be accessed through 183<A HREF="smfi_getpriv.html">smfi_getpriv</A> and 184<A HREF="smfi_setpriv.html">smfi_setpriv</A>, 185and must be reclaimed in 186<A HREF="xxfi_abort.html">xxfi_abort</A>. 187 188<H2><A NAME="SignalHandling">Signal Handling</A></H2> 189 190libmilter takes care of signal handling, 191the filters are not influenced directly by signals. 192There are basically two types of signal handlers: 193 194<OL> 195<LI><TT>Stop</TT>: no new connections from the MTA will be accepted, 196but existing connections are allowed to continue. 197<LI><TT>Abort</TT>: all filters will be stopped as soon as the next 198communication with the MTA happens. 199</OL> 200 201Filters are not terminated asynchronously 202(except by signals that can't be caught). 203In the case of <TT>Abort</TT> the 204<A HREF="xxfi_abort.html">xxfi_abort</A> callback is invoked. 205 206<HR size="1"> 207<FONT size="-1"> 208Copyright (c) 2000, 2001, 2003, 2006 Sendmail, Inc. and its suppliers. 209All rights reserved. 210<BR> 211By using this file, you agree to the terms and conditions set 212forth in the LICENSE. 213</FONT> 214</BODY> 215</HTML> 216