Traditional DDoS attacks often aim to consume bandwidth, for example, by flooding network links with traffic. While these attacks are extremely powerful, they are relatively easy to defend against through bandwidth expansion, traffic scrubbing, and hardware firewalls. However, as attack methods evolve, application-layer DDoS attacks have become a primary tactic. These attacks don't simply flood servers with traffic; instead, they overload the servers' business logic by simulating normal user behavior, resulting in slow website access and even service interruptions. Compared to traditional attacks, application-layer DDoS attacks are more subtle and difficult to defend against.
The core characteristic of application-layer DDoS attacks is that they appear normal. They target specific website interfaces, such as search functions, login interfaces, product search, file downloads, or requests for dynamic content. Attackers impersonate legitimate users and frequently and regularly access these functions. Because each request is small and doesn't quickly fill up bandwidth like a volume-based attack, it's difficult to distinguish between malicious and legitimate requests at the network level. In other words, application-layer attacks aren't about overwhelming the server with sheer volume, but rather about cleverly consuming it. They can inflict enormous computational and resource consumption on the server with relatively little traffic.
One reason why servers struggle to defend against application-layer DDoS attacks is that they're difficult to identify. Traditional traffic-based attacks have distinct characteristics, such as short-term bandwidth spikes and extremely abnormal request volumes, which can trigger monitoring alerts. However, the packets in application-layer attacks closely resemble normal traffic, with their request frequency and access paths mimicking real user behavior. Overly restrictive defense strategies can easily harm legitimate users and disrupt normal website operations. Overly permissive defense strategies can effectively block attack traffic. Therefore, striking a balance between user experience and security is a major challenge in practical defense.
Another factor is the varying patterns of server resource consumption. Application-layer DDoS attacks typically focus on resource-intensive operations, such as database searches, dynamic page generation, and complex business logic. These operations consume significantly more CPU, memory, and database I/O than loading a simple static page. An attacker can easily overload a server with just a few requests, impacting legitimate user access. This is completely different from traditional bandwidth attacks, as while bandwidth may appear to be idle, backend computing resources are already overwhelmed. Many webmasters mistakenly believe that a fully utilized bandwidth means the website is secure, when in reality, it has already fallen prey to an application-layer attack.
The stealthiness of application-layer DDoS attacks is also reflected in the distribution of attack traffic. Attackers often utilize botnets to initiate requests from thousands of different IP addresses. Each IP address generates a small number of requests, making it appear as a legitimate user. This distributed nature makes it difficult to address the attack through IP blocking, as blocking one or a few IP addresses will not significantly impact the site's performance. Directly blocking a large number of IP addresses would likely block legitimate users as well, causing damage to the website's operations.
Furthermore, application-layer DDoS attacks are often persistent. Attackers may not flood the site with traffic in a short period of time, but rather engage in low-frequency, persistent attacks. For example, they may continuously issue requests to certain endpoints over several hours or even days, gradually consuming server resources. This "boiling frog"-like attack is extremely difficult to detect, as the traffic volume at a single point may not appear abnormal, but overall, the website's response speed will gradually decrease, severely impacting the user experience.
From a technical defense perspective, the cost of responding to application-layer attacks is extremely high. Traditional firewalls and traffic scrubbing devices primarily filter packets at the network and transport layers and are incapable of addressing the complex logic at the application layer. Even some high-defense services offer application-layer defense capabilities, requiring deep inspection to determine request legitimacy, which in turn consumes significant computing resources. Improperly designed defense strategies can even increase server pressure.
The difficulty of defending against application-layer attacks is also related to the complexity of the business itself. Different websites have distinct business logic, allowing attackers to target specific attack points. For example, e-commerce websites have product search and ordering interfaces, forums have posting and commenting features, and video streaming interfaces. These functions are core to the website's operations and cannot be simply disabled or restricted without impacting the user experience. Attackers exploit this dilemma, posing a dilemma for defenders: either relax restrictions, resulting in degraded server performance, or impose stricter restrictions, preventing users from using the functions normally.
To effectively defend against application-layer DDoS attacks, a combination of methods is required. First, traffic behavior analysis is needed. Using logs and monitoring, abnormal patterns can be identified, such as frequent IP accesses to specific interfaces within a short period of time, or significant anomalies in certain request parameters. Secondly, intelligent protection mechanisms can be introduced, such as verification codes, human-machine authentication, and access frequency limits. These can effectively distinguish machine requests from real users. However, these measures can also impact user experience and require careful design.
In addition, optimizing website architecture is crucial. Using a CDN to cache static resources can reduce pressure on the origin server; load balancing can distribute requests across multiple servers to avoid single point overload; and database optimization and caching can reduce the computational overhead of each request, thereby improving resilience. If conditions permit, professional high-security cloud services can be used, outsourcing application-layer traffic distribution and detection to a third-party security platform.
In practice, small and medium-sized websites are often the primary victims of application-layer attacks. Large websites have dedicated security teams and high-security services, allowing them to quickly identify and mitigate attacks. However, small and medium-sized enterprises, lacking security investment, are often helpless when faced with application-layer DDoS attacks, resulting in prolonged slow access or website downtime. Therefore, proactive planning and deployment of defensive measures are crucial to avoiding losses.
In summary, servers are difficult to defend against application-layer DDoS attacks because they are highly concealed, difficult to identify, and employ diverse and flexible attack methods. Furthermore, these attacks target resource-intensive business logic, making traditional defenses ineffective. Only through a combination of traffic analysis, architectural optimization, and multi-layered defenses can these threats be mitigated to some extent. Application-layer attacks have become the mainstream form of DDoS attacks, posing a serious threat to website stability and security. Website administrators must remain vigilant and proactively prepare for these challenges.
