D-Link NetDefendOS User Manual page 538

Drop Threshold applies.
• Specify a textual tag to prefix to the Subject field of email designated as spam.
• 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 are inserted into the header of these emails.
Caching Addresses for Performance
To speed processing, the SMTP ALG maintains a cache of the most recently looked-up sender
"From" IP addresses in local memory. If the cache becomes full then the oldest entry is written
over first. There are two SMTP ALG properties which can be configured for this address cache:
• Cache Size
This is the number of entries that the cache can contain. If set to zero, the cache is not used.
Increasing the cache size increases the amount of NetDefendOS memory required for
anti-spam.
• Cache Timeout
The timeout 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 default value if 600 seconds.
The address cache is emptied when NetDefendOS restarts or a reconfiguration operation is
performed.
For the DNSBL subsystem overall:
• Number of emails checked.
• Number of emails spam tagged.
• Number of dropped emails.
For each DNSBL server accessed:
• Number of positive (is spam) responses from each configured DNSBL server.
• Number of queries sent to each configured DNSBL server.
• Number of failed queries (without replies) for each configured DNSBL server.
The dnsbl CLI Command
The dnsbl CLI command provides a means to control and monitor the operation of the spam
filtering subsystem. It is only used in conjunction with SMTP anti-spam.
The dnsbl command on its own without options shows the overall status of all ALGs. If the name
of the SMTP ALG object on which DNSBL spam filtering is enabled is my_smtp_alg then the
output would be:
gw-world:/> dnsbl
538
Chapter 6: Security Mechanisms