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ExpressionEngine has a Throttling feature that permits you to limit the number of page requests by a single IP address within a given time interval. This feature is intended to help control Denial of Service attacks, where your pages are maliciously loaded repeatedly (usually through the use of automated scripts) in an effort to cause undue load to your web server.
A server-wide DOS attack preventative is beyond the scope of what ExpressionEngine can do, but EE’s Throttling feature can help keep your particular site from being as susceptible.
The preferences in the Throttling page enable you to set various parameters to control the threshold at which the feature kicks in, and the action that should be taken when the threshold is exceeded. Since the throttling check happens very early on during ExpressionEngine’s system initialization it can deal with DOS attacks before server resources are unduly used.