Compiling and Installing Your Filter

To compile a filter, modify the Makefile provided with the sample program, or:

  • Put the include and Sendmail directories in your include path (e.g. -I/path/to/include -I/path/to/sendmail).
  • Make sure libmilter.a is in your library path, and link your application with it (e.g. "-lmilter").
  • Compile with pthreads, either by using -pthread for gcc, or linking with a pthreads support library (-lpthread).

Your compile command line will look like

cc -I/path/to/include -I/path/to/sendmail -c myfile.c

and your linking command line will look something like

cc -o myfilter [object-files] -L[library-location] -lmilter -pthread

Configuring Sendmail

If you use a sendmail version older than 8.13 please see the instructions for your version. The default compilation options for sendmail enable support for milters since 8.13.

Next, you must add the desired filters to your sendmail configuration (.mc) file. Mail filters have three equates: The required S= equate specifies the socket where sendmail should look for the filter; the optional F= and T= equates specify flags and timeouts, respectively. All equates names, equate field names, and flag values are case sensitive.

The current flags (F=) are:

Flag Meaning
R Reject connection if filter unavailable
T Temporary fail connection if filter unavailable

If a filter is unavailable or unresponsive and no flags have been specified, the MTA will continue normal handling of the current connection. The MTA will try to contact the filter again on each new connection.

There are three fields inside of the T= equate: S, R, and E. Note the separator between each is a ";" (semicolon), as "," (comma) already separates equates.

The value of each field is a decimal number followed by a single letter designating the units ("s" for seconds, "m" for minutes). The fields have the following meanings:

Flag Meaning
C Timeout for connecting to a filter. If set to 0, the system's connect(2) timeout will be used. Default: 5m
S Timeout for sending information from the MTA to a filter. Default: 10s
R Timeout for reading reply from the filter. Default: 10s
E Overall timeout between sending end-of-message to filter and waiting for the final acknowledgment. Default: 5m

The following example specifies three filters. The first two rendezvous on Unix-domain sockets in the /var/run directory; the third uses an IP socket on port 999.

INPUT_MAIL_FILTER(`filter1', `S=unix:/var/run/f1.sock, F=R')
INPUT_MAIL_FILTER(`filter2', `S=unix:/var/run/f2.sock, F=T, T=S:1s;R:1s;E:5m')
INPUT_MAIL_FILTER(`filter3', `S=inet:999@localhost, T=C:2m')

define(`confINPUT_MAIL_FILTERS', `filter2,filter1,filter3')

m4 ../m4/cf.m4 >

By default, the filters would be run in the order declared, i.e. "filter1, filter2, filter3"; however, since confINPUT_MAIL_FILTERS is defined, the filters will be run "filter2, filter1, filter3". 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().

The above macros will result in the following lines being added to your .cf file:

Xfilter1, S=unix:/var/run/f1.sock, F=R
Xfilter2, S=unix:/var/run/f2.sock, F=T, T=S:1s;R:1s;E:5m
Xfilter3, S=inet:999@localhost, T=C:2m

O InputMailFilters=filter2,filter1,filter3

Finally, the sendmail macros accessible via smfi_getsymval can be configured by defining the following m4 variables (or cf options):

In .mc file In .cf file Default Value
confMILTER_MACROS_CONNECT Milter.macros.connect j, _, {daemon_name}, {if_name}, {if_addr}
confMILTER_MACROS_HELO Milter.macros.helo {tls_version}, {cipher}, {cipher_bits}, {cert_subject}, {cert_issuer}
confMILTER_MACROS_ENVFROMMilter.macros.envfrom i, {auth_type}, {auth_authen}, {auth_ssf}, {auth_author},{mail_mailer}, {mail_host}, {mail_addr}
confMILTER_MACROS_ENVRCPT Milter.macros.envrcpt {rcpt_mailer}, {rcpt_host}, {rcpt_addr}

For information about available macros and their meanings, please consult the sendmail documentation.