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Man Pages

Manual Reference Pages  -  MIMEDEFANG (8)


mimedefang - Sendmail MIME mail filter


See Also


mimedefang prcap

mimedefang -p connection -m mx_socket_name -U user [options]


mimedefang is a filter built around Sendmail 8.11’s milter API for mail filters. It collects each incoming message and runs a filter on the message. This is useful for deleting attachments which may be a security risk on poorly-designed systems like Microsoft Windows.

mimedefang does not actually run the Perl filter; instead, it communicates with mimedefang-multiplexor(8), which manages a pool of persistent Perl processes. See the mimedefang-multiplexor man page for additional information.


If you invoke mimedefang with the single argument prcap, it prints information about the version of Milter it is linked against and exits. Otherwise, you should invoke mimedefang as shown in the second line of the SYNOPSIS.

-U user Runs mimedefang as user rather than root. The user argument must match the argument to mimedefang-multiplexor’s -U option as well.

-y If the -y command-line option is given, MIMEDefang will call smfi_setsymlist to set the list of macros it wants. This function leaked memory in versions of Sendmail prior to 8.14.4 so by default we do not call it. If you are running an older version of sendmail, you should explicitly set the list of macros you want in the Sendmail configuration file.

-z spooldir
  Set the spool directory to spooldir. If this option is omitted, the spool directory defaults to /var/spool/MIMEDefang.

-p connection
  The -p switch is required and specifies the milter connection type. Typically, you should run mimedefang on the same computer as sendmail. Therefore, you should use a UNIX-domain socket for the connection type. The suggested value for the -p switch is mimedefang.sock under the spool directory.

-m mx_socket_name
  Specifies the socket for communicating with mimedefang-multiplexor(8). The mx_socket_name specifies the path of the UNIX-domain socket. See mimedefang-multiplexor(8) for details.

-b backlog
  Sets the "backlog" argument to the listen(2) system call to backlog. If this option is omitted, then the operating-system default backlog is used.

-G Normally, mimedefang uses a umask of 077 when creating the milter socket and files. If you would like the socket to be readable and writeable by the group and files to be group-readable, supply the -G option. This causes the umask to be 007 whenever UNIX-domain sockets are created and 027 whenever files are created. Note: if your milter library is too old to have the smfi_opensocket() function, the -G option causes mimedefang to use a umask of 007 throughout its execution.

Note that by default, /var/spool/MIMEDefang is created with mode 0700. If you use the -G option, you probably should change the mode to 0750.

-d The -d switch causes mimedefang not to delete the temporary spool files it creates for incoming messages. This is for debugging purposes only and should never be used on a production mail server.

-r Causes mimedefang to perform a relay check before processing any messages. It calls into a user-supplied Perl function called filter_relay with the IP address and host name of the sending relay. (See mimedefang-filter(5) for details.)

-H Causes mimedefang to perform a HELO check before processing any messages. It calls into a user-supplied Perl function called filter_helo with the IP address and host name of the sending relay, and the HELO argument. (See mimedefang-filter(5) for details.)

-s Causes mimedefang to perform a sender check before processing the message body. It calls into a user-supplied Perl function called filter_sender with the envelope address of the sender. (See mimedefang-filter(5) for details.)

-t Causes mimedefang to perform recipient checks before processing the message body. It calls into a user-supplied Perl function called filter_recipient with the envelope address of each recipient. (See mimedefang-filter(5) for details.)

-q Permits the multiplexor to queue new connections. See the section QUEUEING REQUESTS in the mimedefang-multiplexor man page. Note that this option and the -R option are mutually-exclusive. If you supply -q, then -R is ignored.

-k Causes mimedefang not to delete working directories if a filter fails. This lets you obtain the message which caused the filter to fail and determine what went wrong. mimedefang logs the directory containing the failed message using syslog.

-P fileName
  Causes mimedefang to write its process-ID (after becoming a daemon) to the specified file.

-R num Normally, mimedefang tempfails a new SMTP connection if there are no free slaves. Supplying the -R num option makes mimedefang tempfail new connections if there are fewer than num free slaves, unless the connection is from the local host. This allows you to favour connections from localhost so your clientmqueue doesn’t build up. Note that supplying -R 0 is subtly different from omitting the option; in this case, mimedefang permits new connections from localhost to queue, but not connections from other hosts (unless you also supply the -q option.)

The purpose of the -R option is to reserve resources for clientmqueue runs. Otherwise, on a very busy mail server, clientmqueue runs can starve for a long time, leading to delays for locally-generated or streamed mail. We recommend using a small number for num; probably no more than 3 or 10% of the total number of slaves (whichever is smaller.)

Note that this option and the -q option are mutually-exclusive. If you supply -q, then -R is ignored.

-C Conserve file descriptors by opening and closing disk files more often. (Disk files are never held open across Milter callbacks.) While this shortens the length of time a file descriptor is open, it also leaves more opportunities for the open to fail. We do not recommend the use of this flag except on very busy systems that exhibit failures due to a shortage of file descriptors.

-T Causes mimedefang to log the run-time of the Perl filter using syslog.

-x string
  Add string as the content of the X-Scanned-By: header. If you set string to the empty string (i.e. -x ""), then no X-Scanned-By: header will be added.

-X Do not add an X-Scanned-By: header. Specifying -X is equivalent to specifying -x "".

-D Do not fork into the background and become a daemon. Instead, stay in the foreground. Useful mainly for debugging or if you have a supervisory process managing mimedefang.

-M This option is obsolete; it is accepted for backward-compatibility, but is ignored.

-N Normally, mimedefang sees all envelope recipients, even ones that Sendmail knows to be invalid. If you don’t want Sendmail to perform a milter callback for recipients it knows to be invalid, invoke mimedefang with the -N flag. Please note that this flag only works with Sendmail and Milter 8.14.0 and newer. It has no effect if you’re running an older version of Sendmail or Milter.

-S facility
  Specifies the syslog facility for log messages. The default is mail. See openlog(3) for a list of valid facilities. You can use either the short name ("mail") or long name ("LOG_MAIL") for the facility name.

-a macro Pass the value of the specified Sendmail macro through to the Perl filter. You can repeat the -a option to write more macros than the built-in defaults. Note that in addition to asking mimedefang to pass the macro value to the filter, you must configure Sendmail to pass the macro through to mimedefang using the confMILTER_MACROS_ENVFROM definition in Sendmail’s m4 configuration file.

-c Strip "bare" carriage-returns (CR) characters from the message body. A bare CR should never appear in an e-mail message. Older versions of mimedefang used to strip them out automatically, but now they are left in by default. The -c option enables the older behavior.
-h Print usage information and exit.


When mimedefang starts, it connects to sendmail using the milter API. (See the Sendmail 8.11 documentation.) For each incoming message, mimedefang creates a temporary directory and saves information in the directory. At various phases during the SMTP conversation, mimedefang communicates with mimedefang-multiplexor to perform various operations. mimedefang-multiplexor manages a pool of persistent Perl processes that actually perform the mail scanning operations.

When a Perl process scans an e-mail, the temporary spool directory contains certain files; details of the communication protocol between mimedefang and the Perl script are in mimedefang-protocol(7).


mimedefang does violence to the flow of e-mail. The Perl filter is quite picky and assumes that MIME e-mail messages are well-formed. While I have tried to make the script safe, I take no responsibility for lost or mangled e-mail messages or any security holes this script may introduce.


mimedefang was written by Dianne Skoll <>. The mimedefang home page is

SEE ALSO, mimedefang-filter(5), mimedefang-multiplexor(8), mimedefang-protocol(7)

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