ePrism User Guide - EdgeWave

DCC (Distributed Checksum Clearinghouse)
DCC (Distributed Checksum Clearinghouse)
DCC is based on a number of servers that maintain databases of message checksums derived from
numeric values that uniquely identify a message. DCC provides a simple but very effective way to
successfully identify spam and control its disposition while updating its database with new spam
message types.
Mail users and ISPs all over the world submit checksums of all messages received. The database
records how many of each message is submitted. If requested, the DCC server can return a count
of how many instances of a message have been received. ePrism uses this count to determine the
disposition of a message.
A DCC server receives no mail, address, headers, or any similar information, but only the
cryptographically secure checksums of such information. A DCC server cannot determine the text
or other information that corresponds to the checksums it receives. It only acts as a clearinghouse
of counts of checksums computed by clients.
DCC interacts with ePrism’s other spam controls as follows:
• Mail is checked by DCC after it has been filtered by Specific Access Patterns and Pattern Based
Message Filters. Messages that trigger an "accept" rule will not be processed by DCC.
• All messages classified as "bulk" by DCC (those that exceed the locally set threshold) are passed
to the STA engine for analysis as spam unless the specified action is "reject".
Note: You must allow a connection on UDP port 6277 on your firewall or router to allow
communications with a DCC server. If this port is not available, DCC server calls will fail
and slow down mail delivery.
DCC Considerations
When implementing DCC, consider the following:
• Educate your user community about this tool and request them to submit mailing lists and
other bulk mail sources that need to be whitelisted. This step is crucial if DCC and STA are to
work properly.
• Configure your initial disposition for bulk mail to be Modify Subject Header. Users will see all the
bulk mail and will quickly identify any sources of mail they want to whitelist. Users can also
create local filter rules in their mail clients to put all tagged mail into a folder.
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