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There are many effective methods to prevent or mitigate DDoS attacks. This article will cover a few suggestions.
No method can be 100% effective in preventing DDoS attacks.
Enabling security related modules/plugins such as mod_security and mod_evasive can be extremely beneficial when preventing DDoS attacks.
Some services such as 1)CloudFlare provide a proxy-type of protection. It acts as a middle-man to determine whether or not your "visit" should be served the content.