D-Link NetDefend DFL-210 User Manual page 150

6.2.5. SMTP
Logging
There are three types of logging done by the SPAM filtering module:
• Logging of dropped or SPAM tagged emails - These log messages include the source email
address and IP as well as its weighted points score and which DNSBLs caused the event.
• DNSBLs not responding - DNSBL query timeouts are logged.
• All defined DNBSLs stop responding - This is a high severity event since all email will be
allowed through if this happens.
Network Setup
Setup Summary
To set up DNSBL SPAM filtering in the SMTP ALG, the following list summarizes the steps:
• Specify which DNSBL servers are to be used. There can be multiple and they can act both as
backups to each other as well as confirmation of a sender's status.
• Specify a weight for each server which will determine how important it is in deciding if email is
SPAM or not in the calculation of a weighted sum.
• Specify the threshold for designating an email as SPAM. If the weighted sum is equal or greater
than this then an email will be considered to be SPAM.
• Specify a textual tag to prefix to the Subject field of email designated as SPAM.
• Specify the Drop threshold. If the weighted sum is equal or greater than this then an email will
be dropped entirely. This threshold should be greater or equal to the SPAM threshold. If they are
equal then the Drop threshold will have precedence so that all email will be dropped when that
threshold is reached.
• Optionally specify an email address to which dropped email will be sent (as an alternative to
simply discarding it). Optionally specify that the TXT messages sent by the DNSBL servers that
failed it be appended to these emails.
Caching Addresses for Performance
To speed processing NetDefendOS maintains a cache of the most recently looked-up sender
addresses in local memory. If the cache becomes full then the oldest entry is written over first.
The Address Timeout value for the cache can be changed by the administrator. This determines how
long any address will be valid for once it is saved in the cache. After this period of time has expired,
a new query for a cached sender address must be sent to the DNSBL servers.
The cache is emptied at startup or reconfiguration and its size of this cache can be controlled by the
administrator.
The dnsbl CLI Command
The dnsbl CLI command provides a means to control and monitor the operation of the SPAM
filtering module. The dnsbl command on its own without options shows the overall status of all
ALGs. If the SMTP ALG name on which DNSBL SPAM filtering is enabled is my_smtp_alg then
the output would be:
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Chapter 6. Security Mechanisms