Linux® platforms have various spam filters, including whitelist or verification filters, distributive adaptive blacklists, and Bayesian trigram filters. TDMA and Razor are examples of each method. SpamAssassin uses sophisticated algorithms to analyze emails for spam patterns.
Linux® platforms, the result of the open source movement, have a wide variety of spam filters to choose from. To choose the best Linux® spam filter, it’s important to know a little about how the different types work. An aggressive, but almost completely accurate, method of eliminating spam is to use a whitelist or verification filters. For users who find the whitelist method too inflexible, distributive adaptive blacklists are a versatile and widely available solution. More sophisticated anti-spam techniques use Bayesian trigram filters or similar word parsing algorithms as the main Linux® spam filter.
Whitelists or verification filters work according to a very simple but efficient principle; they only allow mail from designated senders into the customer’s mailbox. A popular Linux® spam filter that uses this technique is TDMA. When a user initially sets up TDMA, a safe recipients list is defined and all subsequent mail is checked against this list. If a recipient is not on this list, a verification email is sent to determine if the sender is a legitimate source. The challenge email is designed to be accountable only by individuals, so if a correct response is received, the sender’s email address is added to the white-list.
The problem with the white-list technique as Linux®’s primary spam filter is that it places an unnecessary burden on senders, even if they are legitimate. Adaptive distributive blacklists work in reverse of this principle, but in a much more flexible way. In this technique, messages from certain email addresses known to belong to spammers are blocked based on a centralized database. If enough users mark a particular email address as spam, those email addresses are updated to the centralized blacklist. An example of a Linux® spam filter that uses this method of spam protection is Razor.
Both the whitelist verification method and the distributive adaptive blacklist method analyze the email address, comparing them to databases to determine if they are potentially dangerous. Bayesian trigram filters are used by some spam filters, such as the popular Linux® spam filter SpamAssassin, to analyze the structure of the emails themselves. These filters use sophisticated algorithms to examine the text of emails and determine if they have patterns of sentence construction and word usage typically used by spammers. By eliminating the need for user-generated databases, this technique allows for greater adaptability. Many Linux® spam filter developers are implementing this spam detection method in their programs.
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