How to protect a web application from DDoS by the implementation of Web Application firewalls?
How effective is this method?
You might want to clarify if you're looking for DoS or DDoS protection. See this answer for more details.
In a typical web-application architecture, the WAF stands in front of your web-application, either in your network zone (e.g. DMZ) or within an external service provider network that filters the traffic for you. In case of a DDoS attack, the WAF will be hit by the traffic load before your web-application, and it can even become a point of failure in the network flow. Keep in mind that the objective of a DDoS attack is to flood your bandwidth or resources. Configuring your WAF to reject or block incoming traffic based on rules/patterns/signature (e.g. source IP addresses, protocol etc.) might help, but it might not be able to handle the huge volume of incoming requests or connections.
There are various solution in the market which can be used to protect from DDoS attack and depends on the device means what are methods used by the device to protect from layer 7 DDoS. If you want to use products available in the market then its fine because most of the products uses same and good techniques or if you are planning to use iptables then you need to check the logs for the IP's which is used to request your application frequently. In this method you have to use some threshold value such that when the threshold value is reached then add that IP in blacklist and DROP the requests from that IP's. Some products in the market uses single packet authorization or port knocking mechanism which is itself a great idea to prevent from DoS and DDoS attacks.