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This directory contains the source files for libmilter. The sendmail Mail Filter API (Milter) is designed to allow third-party programs access to mail messages as they are being processed in order to filter meta-information and content. This README file describes the steps needed to compile and run a filter, through reference to a sample filter which is attached at the end of this file. It is necessary to first build libmilter.a, which can be done by issuing the './Build' command in SRCDIR/libmilter . Starting with 8.13 sendmail is compiled by default with support for the milter API. Note: if you want to write a milter in Java, then see http://sendmail-jilter.sourceforge.net/ +----------------+ | SECURITY HINTS | +----------------+ Note: we strongly recommend not to run any milter as root. Libmilter does not need root access to communicate with sendmail. It is a good security practice to run a program only with root privileges if really necessary. A milter should probably check first whether it runs as root and refuse to start in that case. libmilter will not unlink a socket when running as root. +----------------------+ | CONFIGURATION MACROS | +----------------------+ Libmilter uses a set of C preprocessor macros to specify platform specific features of the C compiler and standard C libraries. SM_CONF_POLL Set to 1 if poll(2) should be used instead of select(2). +-------------------+ | BUILDING A FILTER | +-------------------+ The following command presumes that the sample code from the end of this README is saved to a file named 'sample.c' and built in the local platform- specific build subdirectory (SRCDIR/obj.*/libmilter). cc -I../../include -o sample sample.c libmilter.a ../libsm/libsm.a -pthread It is recommended that you build your filters in a location outside of the sendmail source tree. Modify the compiler include references (-I) and the library locations accordingly. Also, some operating systems may require additional libraries. For example, SunOS 5.X requires '-lresolv -lsocket -lnsl'. Depending on your operating system you may need a library instead of the option -pthread, e.g., -lpthread. Filters must be thread-safe! Many operating systems now provide support for POSIX threads in the standard C libraries. The compiler flag to link with threading support differs according to the compiler and linker used. Check the Makefile in your appropriate obj.*/libmilter build subdirectory if you are unsure of the local flag used. Note that since filters use threads, it may be necessary to alter per process limits in your filter. For example, you might look at using setrlimit() to increase the number of open file descriptors if your filter is going to be busy. +----------------------------------------+ | SPECIFYING FILTERS IN SENDMAIL CONFIGS | +----------------------------------------+ Filters are specified with a key letter ``X'' (for ``eXternal''). For example: Xfilter1, S=local:/var/run/f1.sock, F=R Xfilter2, S=inet6:999@localhost, F=T, T=C:10m;S:1s;R:1s;E:5m Xfilter3, S=inet:3333@localhost specifies three filters. Filters can be specified in your .mc file using the following: INPUT_MAIL_FILTER(`filter1', `S=local:/var/run/f1.sock, F=R') INPUT_MAIL_FILTER(`filter2', `S=inet6:999@localhost, F=T, T=C:10m;S:1s;R:1s;E:5m') INPUT_MAIL_FILTER(`filter3', `S=inet:3333@localhost') The first attaches to a Unix-domain socket in the /var/run directory; the second uses an IPv6 socket on port 999 of localhost, and the third uses an IPv4 socket on port 3333 of localhost. The current flags (F=) are: R Reject connection if filter unavailable T Temporary fail connection if filter unavailable 4 Shut down connection if filter unavailable (with a 421 temporary error). If none of these is specified, the message is passed through sendmail in case of filter errors as if the failing filters were not present. Finally, you can override the default timeouts used by sendmail when talking to the filters using the T= equate. There are four fields inside of the T= equate: Letter Meaning C Timeout for connecting to a filter (if 0, use system timeout) S Timeout for sending information from the MTA to a filter R Timeout for reading reply from the filter E Overall timeout between sending end-of-message to filter and waiting for the final acknowledgment Note the separator between each is a ';' as a ',' already separates equates and therefore can't separate timeouts. The default values (if not set in the config) are: T=C:5m;S:10s;R:10s;E:5m where 's' is seconds and 'm' is minutes. Which filters are invoked and their sequencing is handled by the InputMailFilters option. Note: if InputMailFilters is not defined no filters will be used. O InputMailFilters=filter1, filter2, filter3 This is is set automatically according to the order of the INPUT_MAIL_FILTER commands in your .mc file. Alternatively, you can reset its value by setting confINPUT_MAIL_FILTERS in your .mc file. This options causes the three filters to be called in the same order they were specified. It allows for possible future filtering on output (although this is not intended for this release). Also note that a filter can be defined without adding it to the input filter list by using MAIL_FILTER() instead of INPUT_MAIL_FILTER() in your .mc file. To test sendmail with the sample filter, the following might be added (in the appropriate locations) to your .mc file: INPUT_MAIL_FILTER(`sample', `S=local:/var/run/f1.sock') +------------------+ | TESTING A FILTER | +------------------+ Once you have compiled a filter, modified your .mc file and restarted the sendmail process, you will want to test that the filter performs as intended. The sample filter takes one argument -p, which indicates the local port on which to create a listening socket for the filter. Maintaining consistency with the suggested options for sendmail.cf, this would be the UNIX domain socket located in /var/run/f1.sock. % ./sample -p local:/var/run/f1.sock If the sample filter returns immediately to a command line, there was either an error with your command or a problem creating the specified socket. Further logging can be captured through the syslogd daemon. Using the 'netstat -a' command can ensure that your filter process is listening on the appropriate local socket. Email messages must be injected via SMTP to be filtered. There are two simple means of doing this; either using the 'sendmail -bs' command, or by telnetting to port 25 of the machine configured for milter. Once connected via one of these options, the session can be continued through the use of standard SMTP commands. % sendmail -bs 220 test.sendmail.com ESMTP Sendmail 8.14.0/8.14.0; Thu, 22 Jun 2006 13:05:23 -0500 (EST) HELO localhost 250 test.sendmail.com Hello testy@localhost, pleased to meet you MAIL From:<testy> 250 2.1.0 <testy>... Sender ok RCPT To:<root> 250 2.1.5 <root>... Recipient ok DATA 354 Enter mail, end with "." on a line by itself From: firstname.lastname@example.org To: email@example.com Subject: testing sample filter Sample body . 250 2.0.0 dB73Zxi25236 Message accepted for delivery QUIT 221 2.0.0 test.sendmail.com closing connection In the above example, the lines beginning with numbers are output by the mail server, and those without are your input. If everything is working properly, you will find a file in /tmp by the name of msg.XXXXXXXX (where the Xs represent any combination of letters and numbers). This file should contain the message body and headers from the test email entered above. If the sample filter did not log your test email, there are a number of methods to narrow down the source of the problem. Check your system logs written by syslogd and see if there are any pertinent lines. You may need to reconfigure syslogd to capture all relevant data. Additionally, the logging level of sendmail can be raised with the LogLevel option. See the sendmail(8) manual page for more information. +--------------+ | REQUIREMENTS | +--------------+ libmilter requires pthread support in the operating system. Moreover, it requires that the library functions it uses are thread safe; which is true for the operating systems libmilter has been developed and tested on. On some operating systems this requires special compile time options (e.g., not just -pthread). libmilter is currently known to work on (modulo problems in the pthread support of some specific versions): FreeBSD 3.x, 4.x SunOS 5.x (x >= 5) AIX 4.3.x HP UX 11.x Linux (recent versions/distributions) libmilter is currently not supported on: IRIX 6.x Ultrix Feedback about problems (and possible fixes) is welcome. +--------------------------+ | SOURCE FOR SAMPLE FILTER | +--------------------------+ Note that the filter example.c may not be thread safe on some operating systems. You should check your system man pages for the functions used below to verify the functions are thread safe. $Revision: 8.42 $, Last updated $Date: 2006-06-29 17:10:16 $