Currently, in order to provide basic DDoS protection and filter aggressive bots, some form of Content Delivery Network (CDN) is usually the simplest and most cost-effective way to protect a web application. In a typical setup, DNS is used to point clients to the CDN, and the CDN will then forward the request to the actual web server. There are a number of companies offering services like this, and cloud providers will usually have solutions like this as well.
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