An HTTP flood DDoS (Distributed Denial of Service) attack is a type of cyberattack aimed at overwhelming a targeted server with a massive volume of HTTP requests. This attack is designed to make a web service or website unavailable by saturating it with a flood of requests, rendering it unable to respond to legitimate traffic. Here’s how an HTTP flood attack works and how it can be mitigated:

How an HTTP Flood Attack Works

1. Type of Attack: HTTP flood attacks fall under “layer 7” DDoS attacks. Layer 7 refers to the application layer of the OSI model, which includes internet protocols like HTTP. In this attack, malicious actors exploit the HTTP protocol to flood a target with requests.

2. Application Layer Targeting: These attacks target the application layer, making them complex to mitigate because the malicious traffic closely resembles normal web traffic.

3. Botnets: Attackers often use botnets, networks of compromised devices, to maximize the impact of the attack. These botnets allow attackers to coordinate a large volume of malicious traffic.

Varieties of HTTP Flood Attacks:

There are two main varieties of HTTP flood attacks:

1. HTTP GET Attack: In this attack, multiple devices send numerous requests for assets like images or files from a targeted server. The flood of incoming requests overwhelms the server, leading to a denial-of-service for additional legitimate requests.

2. HTTP POST Attack: When a form is submitted on a website, the server must process and store the data, typically in a database. Handling POST requests is resource-intensive. Attackers exploit this by sending a large number of POST requests to the server, saturating its capacity and causing a denial-of-service.

Mitigating HTTP Flood Attacks:

Mitigating HTTP flood attacks is a complex task, but several strategies can help:

1. Challenge Mechanisms: Implementing challenge mechanisms that test the requesting machine to differentiate between bots and legitimate users. This can include JavaScript computational challenges or CAPTCHAs.

2. Web Application Firewall: Using a WAF to filter and block malicious traffic based on predefined rules and patterns.

3. IP Reputation Database: Managing an IP reputation database to track and selectively block known malicious sources.

4. On-the-Fly Analysis: Employing skilled engineers who can perform real-time analysis and adjust mitigation strategies.

5. Content Delivery Networks (CDNs): Utilizing CDNs like Cloudflare, which have the advantage of scale and the ability to analyze traffic from various sources, enabling the rapid development of mitigation strategies to counter application layer DDoS attacks.

In conclusion, HTTP flood DDoS attacks pose a serious threat to websites and web services. A multifaceted strategy combining challenge mechanisms, web application firewalls, IP reputation databases, and real-time analysis is needed to mitigate these attacks. In order to effectively defend against these attacks, it is also crucial to be able to scale and utilize resources like CDNs.