Anti-Spam Filtering - D-Link DFL-1660 User Manual

6.2.5. The SMTP ALG
• Dropping email which has a very high probability of being spam.
• Letting through but flagging email that has a moderate probability of being spam.
The NetDefendOS Anti-Spam Implementation
SMTP functions as a protocol for sending emails between servers. NetDefendOS applies Spam
filtering to emails as they pass through the NetDefend Firewall from an external remote SMTP
server to a local SMTP server (from which local clients will later download their emails). Typically,
the local, protected SMTP server will be set up on a DMZ network and there will usually be only
one "hop" between the sending server and the local, receiving server.
DNSBL Databases
A number of trusted organizations maintain publicly available databases of the origin IP address of
known spamming SMTP servers and these can be queried over the public Internet. These lists are
known as DNS Black List (DNSBL) databases and the information is accessible using a standardized
query method supported by NetDefendOS. The image below illustrates all the components involved:
DNSBL Server Queries
When the NetDefendOS Anto-Spam filtering function is configured, the IP address of the email's
sending server is sent to one or more DNSBL servers to find out if any DNSBL servers think the
email is from a spammer or not. NetDefendOS examines the IP packet headers to do this.
The reply sent back by a server is either a not listed response or a listed response. In the latter case
of being listed, the DSNBL server is indicating the email might be spam and it will usually also
provide information known as a TXT record which is a textual explanation for the listing.
Creating a DNSBL Consesus
The administrator can configure the NetDefendOS SMTP ALG to consult multiple DNSBL servers
in order to form a consensus opinion on an email's origin address. For each new email, configured
Figure 6.5. Anti-Spam Filtering
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Chapter 6. Security Mechanisms