13.1.3. Artificial ignorance
Artificial ignorance is a method to detect anomalies. When applied to log analysis, it means that you ignore the regular, common log messages - these are the result of the regular behavior of your system, and therefore are not too interesting. However, new messages that have not appeared in the logs before can sign important events, and should be therefore investigated. "By definition, something we have never seen before is anomalous" (Marcus J. Ranum).
The syslog-ng application can classify messages using a pattern database: messages that do not match any pattern are classified as unknown. This provides a way to use artificial ignorance to review your log messages. You can periodically review the unknown messages — syslog-ng can send them to a separate destination, and add patterns for them to the pattern database. By reviewing and manually classifying the unknown messages, you can iteratively classify more and more messages, until only the really anomalous messages show up as unknown.
Obviously, for this to work, a large number of message patterns are required. The radix-tree matching method used for message classification is very effective, can be performed very fast, and scales very well. Basically the time required to perform a pattern matching is independent from the number of patterns in the database. For sample pattern databases, see Section 13.2.2, Downloading sample pattern databases.