ispMailGate - a general purpose filtering MDA for sendmail


This is an alpha release! What you are using now is tested by using a comparatively small test suite in our local environment. You are perhaps planning to include this software in a production environment. We don't discourage to do so, but we strongly advise you to be extremely careful. In particular, start by filtering only mails for a very small number of email adresses, perhaps your own and something similar. That is, be extremely cautios when modifying your sendmail configuration.

See INSTALLATION and SENDMAIL CONFIGURATION below for a detailed description of your sendmail setup.


For running standalone:

    ispMailGateD -f <sender> <recipient1> [... <recipientN>]

For running as a daemon:

    ispMailGateD -s [-d] [-t <tmpdir>] [-a <facility>] [-p <pidfile>]
        [-u <unixsock>]


IspMailGate is a general purpose email filtering system. The program gets included into a sendmail configuration as a delivery agent (MDA) and the usual sendmail rules can be applied for deciding which emails to feed into ispMailGate. The true filters are implemented as modules, so its easy to extend the possibilities of ispMailGate. Current modules offer automatic compression and decompression, encryption, decryption and certification with PGP or virus scanning.

The program can run in a usual standalone mode, but that's not recommended, except for debugging and similar tasks. The recommended mode will be running the program as a server, completely independent from sendmail. A small C program (called a wrapper) will instead be configured as sendmails MDA. This wrapper connects to the server via a well known Unix socket (by default ~unix_sock~), passes its command line arguments and standard input to the server and disconnects. Obviously this second solution has much better performance as you load the Perl interpreter only once.

Command Line Interface

The following options affect ispMailGate's behaviour:


Advices the ispMailGate to use syslog facility <fac>. By default syslog entries are written as facility ~facility~.


The program runs in debugging mode, logging information into the syslog. Perhaps more information than you like ... :-)


Sets a mails sender.


Tells the program not to run in standalone mode and instead detach from the shell to enter server mode. This mode is currently not usable, as the wrapper is not yet available.


Sets the programs directory for temporary files to <dir>. When unpacking a complex and big multipart mail, the ispMailGate may need surprisingly much space. By default ~tmp_dir~ is used.


Tells the server to listen on file <sock> for unix socket connections. By default the server uses ~unix_sock~.



To start with the requirements: You need

1.) A running sendmail (recommended: 8.9.3 or later); if you don't have sendmail or an older version, you find the current release at

2.) A late version of Perl (5.005 or later); if you don't have Perl, get it from any CPAN mirror, for example

3.) The MIME-tools module (version 4.116 or later), its prerequired modules (MailTools, MIME-Base64 and IO-Stringy) and the IO::Tee module (version 0.61 or later). All these modules are available from any CPAN mirror, for example

Installing a Perl module is quite easy, btw. Either you use the automatic CPAN interface (requires an Internet connection or something similar) by executing

    perl -MCPAN -e shell

or you fetch the modules with FTP, extract the tar.gz files, go into the distribution directory (for example MIME-tools-4.116) and do a

    perl Makefile.PL
    make test
    make install

You'll like it! :-)

System preparation

Although ispMailGate is usually started as root, because certain initialization settings need root permissions, it must not continue running as root. Instead it impersonates itself to the same UID and GID that sendmail uses for delivering mails. In what follows, I assume UID daemon and GID <mail>, as used on a Red Hat Linux box.

IspMailGate needs its own directory for creating temporary files. Usually this could be /var/spool/ispmailgate or something similar. Make sure that the daemon user from above, (but noone else) has access to this directory:

    mkdir /var/spool/ispmailgate
    chown daemon /var/spool/ispmailgate
    chgrp mail /var/spool/ispmailgate
    chmod 700 /var/spool/ispmailgate

Program installation

The program is installable like any other Perl module. Indeed, you can even use the automatic CPAN installation. If you are not used to CPAN installation or cannot use the CPAN shell, you need to perform a manual installation: First, fetch the archive from any CPAN mirror, for example

and extract the archive with

    gzip -cd Mail-IspMailGate-<version>.tar.gz | tar xf -

After that, do a

    cd Mail-IspMailGate-<version>

and do a

    perl Makefile.PL
    make test           # You need to be root here!
    make install        # You need to be root here!

While running "perl Makefile.PL", you'll be prompted a lot of questions. In general you can answer them by simply hitting return, the defaults should be fine. The questions are explained in the section CONFIGURATION FILE below, because your answers are used for creating this config file. "CONFIGURATION FILE".

If "make test" reports any errors, let me know.


To understand the required sendmail configuration, let's first take a look at the following diagram:

        | Incoming       |
        | mail for       |
        | |

            | Port 25
            | (SMTP)

        +----------------------+        +---------------------+
        |                      |        |                     |
        |  Sendmail, with      |        |  IspMailGate        |
        |  IspMailGate support |  ----> |  receives and       |
        |  integrated          |        |  processes the mail |
        |                      |        |                     |
        +----------------------+        +---------------------+

           V                                    |
        +---------------------------+           | Port 26
        | mailertable:              |           |
        | => ispmailgate |           |
        +---------------------------+           |

        +-------------------------+   |                       |
        | Other mailertable:      |   |  Second sendmail,     |
        | => mailhost  | < |  without IspMailGate, |
        +-------------------------+   |  other mailertable    |
                                      |                       |

The idea is that we have two sendmail instances: The first one is accepting mails from the outside world (it could well be smapd or another SMTP wrapper).

Mails accepted by the first instance will optionally be piped into IspMailGate, depending on your mailertable or whatever method you choose for selecting a delivery agent.

As soon as IspMailGate has processed your mails, they cannot be delivered to the same sendmail: Sendmail would detect the same recipients as before, and feed the mail back into IspMailGate. The immediate result would be an endless loop.

Instead we configure another instance of sendmail, running for example on port 26, which accepts mails by IspMailGate. This second sendmail will typically be protected to the outside world, for example by using a packet filter.

Of course it is possible to replace the second sendmail with a mail server running on another machine. However, I discourage you to do so, because IspMailGate has no integrated spooling system: If the foreign host would be down, you mail would be lost forever! However, you can easily configure the second sendmail for delivery to the same host, this time using a safe spooling system.


Before reading on, you should have some basic knowledge on sendmail configuration. In particular you should be used to work with

the m4 macro file that is used to create (If you had the impression that hacking is the method of choice for configuring sendmail, your knowledge of sendmail is a little bit too basic :-)


the file that is used for selecting delivery agents, based on a domain name

the file that is used for selecting "local" domains.

All these files are described in the file cf/README from the sendmail distribution. The same file is available online as sendmail documentation on I strongly encourage you to read about it.

In what follows I assume that we have two hosts: is a firewall running IspMailGate and two instances of sendmail. is the real mail server. Mail for the domain shall be checked by IspMailGate, passed to the second instance of sendmail and finally delivered to the real mail server.

Furthermore I assume that the cf files from the sendmail distribution are in /usr/lib/sendmail-cf. This is the case on a Red Hat Linux box, if you have installed the RPM package sendmail-cf. It may be /usr/share/sendmail with SuSE Linux or anything else on another system.

First of all we copy the file examples/ispmailgate.m4 from the IspMailGate distribution to /usr/lib/sendmail-cf/mailer. Then we create the following files:

  define(`confCW_FILE', `/etc/mail/')
  define(`ALIAS_FILE', `/etc/mail/aliases')
  define(`QUEUE_DIR', `/var/spool/mqueue')
  FEATURE(mailertable, `hash -o /etc/mail/mailertable')
  FEATURE(virtusertable, `hash -o /etc/mail/virtusertable')
  define(`confCW_FILE', `/etc/mail/')
  define(`ALIAS_FILE', `/etc/mail/aliases')
  define(`QUEUE_DIR', `/var/spool/mqueue2')
  define(`confDAEMON_OPTIONS', `port=26')
  FEATURE(mailertable, `hash -o /etc/mail2/mailertable')
  FEATURE(virtusertable, `hash -o /etc/mail/virtusertable')

You see, this is almost the same file, with three exceptions:


The first sendmail is running on the default port 25, because it has no special port settings. The second sendmail is running on port 26.


The first sendmail is using /etc/mail/mailertable, the second has /etc/mail2/mailertable.


The second sendmail doesn't have a mailer ispmailgate.


The first sendmail is using another spool directory, /var/spool/mqueue2. (I am not sure, whether this is required, however it doesn't harm, so let's be on the safe side.)


You see, the only difference between these two instances of mailertable is that the domain is treated different.

Finally we have to execute some commands to get this configuration running:

        mkdir /var/spool/mqueue2
        # The following should match the settings of
        # /var/spool/mqueue exactly.
        chown root /var/spool/mqueue2
        chgrp root /var/spool/mqueue2
        chmod 755 /var/spool/mqueue

        cd /etc/mail
        m4 >
        makemap hash mailertable <mailertable
        cd /etc/mail2
        m4 >
        makemap hash mailertable <mailertable

The above is fine for Linux. On other operating systems you might need to use GNU m4 rather than the builtin m4 and replace the map type hash with dbm. In particular this applies to Solaris.

Finally, kill any running sendmail and start the new versions with

        chmod 755 /etc/mail /etc/mail2
        chmod 644 /etc/mail/ /etc/mail2/
        /usr/sbin/sendmail -C /etc/mail/ -bd -q1h
        /usr/sbin/sendmail -C /etc/mail2/ -bd -q1h

(The chmod commands because sendmail is really picky about group writable files and directories.)


The program depends on a local configuration file, read as the Mail::IspMailGate::Config module. In other words, this configuration file is pure Perl code defining certain variables under the name space Mail::IspMailGate::Config. The module is read from the file ~installsitelib~/Mail/IspMailGate/

The following variables are meaningful to the program:


The programs version; do not modify without a good reason.


Set's the default directory for creating temporary files, currently ~tmp_dir~. You can modify this with the --tmpdir directive, see above.


The unix socket that the client connects to, currently ~unix_sock~. You can use the --unixsock argument for overwriting the default.


The PID file where a running server stores its PID, currently ~pid_file~. You can use the --pidfile argument for overwriting the default.


IspMailGate is running as this user and group, by default ~mail_user~ and ~mail_group~.


The host to use for passing mails after processing them by the mail filter. By default 'localhost' is used, in other words, the mails are immediately passed back to sendmail.

To omit a possible loop problem, sendmail must be ready for handling email addresses like user@domain.ispmailgate. For such addresses it must rip off the .ispmailgate and guarantee not to feed the mails back into ispMailGate. See "SENDMAIL CONFIGURATION" below.


An array ref with list of possible recipients/senders and filter lists that describe how to handle mails being sent from the senders to the recipients.

Each element of the list is a hash ref with the following elements:


A regular expression (Perl regular expression, that is) for matching the recipient address. An empty string matches any recipient.


A regular expression (Perl regular expression, again) for matching the sender address. An empty string matches any sender.


An array ref to a list of filters. A mail will be fed into that list (from the left to the right) and the final result will be returned to sendmail. See Mail::IspMailGate::Filter(3) for a description of creating filters.

The recipient list will be read top to bottom, the first match decides which rule to choose. See the example configuration below for some example rules.


If no element of the recipients list matches an emails senders and recipients, the filters from this variable will be choosen. By default it contains a dummy filter:

  $cfg->{'default_filter'} = ['Mail::IspMailGate::Filter::Dummy'];

This variable belongs to the Packer module. See Mail::IspMailGate::Packer(3) for details.


These belong to the VirScan module. See Mail::IspMailGate::VirScan(3) for details.


This variable belong to the PGP module. See Mail::IspMailGate::PGP(3) for details.

Example Configuration

It might help to look at a commented example of the configuration file:

    package Mail::IspMailGate::Config;

    $Mail::IspMailGate::Config::config = bless( {
    # Config file version
      'VERSION' => '1.100',
    # Path of external virus scanner or empty
      'antivir_path' => '/usr/bin/antivir',
    # List of filters to use by default
      'default_filter' => [
    # Facility to use for syslog
      'facility' => 'mail',
    # Path of the gzip binary (for extracting .gz files) or empty
      'gzip_path' => '/usr/bin/gzip',
    # Path of the LhA binary (for extracting .lha files) or empty
      'lha_path' => '',
    # GID under which sendmail is executing external binaries
      'mail_group' => 'mail',
    # Mail host (with optional :port) to use for final delivery
      'mail_host' => 'localhost:26',
    # UID under which sendmail is executing external binaries
      'mail_user' => 'daemon',
    # Configuration of the packer
      'packer' => {
        'gzip' => {
          'neg' => '$gzip_path -cd',
          'pos' => '$gzip_path -c'
    # PID file to use for the server
      'pid_file' => '/var/run/',
    # E-Mail address of the administrator
      'postmaster' => '',
    # List of senders/recipients and associated filters
      'recipients' => [
          'filters' => [
          'recipient' => '[@\\.]$'
    # Path of the tar binary (for extracting .tar files) or empty
      'tar_path' => '/usr/bin/gtar',
    # Directory to use for temporary files
      'tmp_dir' => '/var/spool/ispmailgate',
    # Path of the unarj binary (for extracting .arj files) or empty
      'unarj_path' => '',
    # Path of the Unix socket to connect to a server
      'unix_sock' => '/var/run/ispmailgate.sock',
    # Domain to assume for email adresses without @domain
      'unqualified_domain' => '',
    # Path of the unzip binary (for extracting .zip files) or empty
      'unzip_path' => '/usr/bin/unzip',
    # Configuration of the virus scanner
      'virscan' => {
        'scanner' => '$antivir_path -rs -nolnk -noboot $ipaths',
        'deflater' => [
            'cmd' => '$gzip_path -cd $ipath | $tar_path -xf -C $odir',
            'pattern' => '\\.(?:tgz|tar\\.gz|tar\\.[zZ])$'
            'cmd' => '$gzip_path -cd $ipath >$opath',
            'pattern' => '\\.(?:gz|[zZ])$'
            'cmd' => '$gzip_path -cd $ipath >$opath',
            'pattern' => '\\.tar$'
            'cmd' => '$unzip_path $ifile -d $odir',
            'pattern' => '\\.zip$'
            'cmd' => '$gzip_path -cd $ipath | $tar_path -xf -C $odir',
            'pattern' => '\\.(?:tgz|tar\\.gz|tar\\.[zZ])$'
            'cmd' => '$gzip_path -cd $ipath >$opath',
            'pattern' => '\\.(?:gz|[zZ])$'
            'cmd' => '$gzip_path -cd $ipath >$opath',
            'pattern' => '\\.tar$'
            'cmd' => '$unzip_path $ifile -d $odir',
            'pattern' => '\\.zip$'
            'cmd' => '$gzip_path -cd $ipath | $tar_path -xf -C $odir',
            'pattern' => '\\.(?:tgz|tar\\.gz|tar\\.[zZ])$'
            'cmd' => '$gzip_path -cd $ipath >$opath',
            'pattern' => '\\.(?:gz|[zZ])$'
            'cmd' => '$gzip_path -cd $ipath >$opath',
            'pattern' => '\\.tar$'
            'cmd' => '$unzip_path $ifile -d $odir',
            'pattern' => '\\.zip$'
    }, 'Mail::IspMailGate::Install' );


This module is

    Copyright (C) 1998         Amar Subramanian
                               Grundstr. 32
                               72810 Gomaringen

                               Phone: +49 7072 920696

                       and     Jochen Wiedmann
                               Am Eisteich 9
                               72555 Metzingen

                               Phone: +49 7123 14887

    All Rights Reserved.

You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.


Mail::IspMailGate::Filter(3), Mail::IspMailGate::Packer(3), Mail::IspMailGate::VirScan(3), Mail::IspMailGate::PGP(3) and MIME::Entity(3)