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4.3. Content Filtering in MTA
Content filtering method is in the present a well known method used to screen and/or exclude certain defined
information from the Internet or its part. Concerning an e-mail server system the best place to implement content
filtering method is the MTA agent as an e-mail communication traffic nod. The advantage of such an implementation
is that it allows one to scan e-mails inbound as well as outbound in the same implementation algorithm. On the other
hand the content filtering method is MTA dependent. The ESET comes with four content filters built for most common
MTA, i.e. MTA Sendmail, Postfix, Exim and QMail. These are reviewed in the following sections.
4.3.1. Content filtering in MTA Postfix
The MTA Postfix content filter requirements are fulfilled by nod32smtp daemon. To configure nod32smtp for this
purpose the following parameters must be specified in section [smtp] of the main NOD32 configuration file.
agent_enabled = yes
listen_addr = "localhost"
listen_port = 2526
server_addr = "localhost"
server_port = 2525
After reread of the newly created NOD32 configuration using command:
/etc/init.d/nod32d reload
the nod32smtp module will listen on port 2526 and will forward all communication from this port to the local port
2525.
In the next step, add the following specification into the '/etc/postfix/master.cf' configuration file.
localhost:2525 inet n - n - - smtpd
-o content_filter=
-o myhostname=nod32.yourdomain.com
Note that the 'yourdomain.com' part of the
'myhostname' parameter must be in your case replaced
by an appropriate server's domain specification. Note also
that the MTA Postfix does not like when the 'yourdomain.
com' part of 'myshostname' parameter above matches
domain name specified in the parameter 'myhostname'
of the '/etc/postfix/main.cf' configuration file. The trick
to overcome this problem relies on using IP address in
'myhostname' parameter of '/etc/postfix/main.cf' while
DNS name of the domain name used in '/etc/postfix/
master.cf' .
Finally, add ' c ontent_filter' specification into '/
etc/postfix/main.cf' configuration file by entering the
following command.
postconf -e "content_filter = smtp:localhost:2526"
Entire entire process described above is illustrated in figure 4-4.
Figure -. Bidirectional scanning scheme of an nod3smtp
module working as a content filter.
POSTFIX
SMTPD
CLEANUP
PICKUP
SMTPD
TCP port 2525
NOD32SMPT
NOD32 for Linux/BSD Mail Server
LOCAL
QUEUE
SMTP
SMTP
TCP port 2526
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