When a website or application suddenly becomes extremely slow or even completely inaccessible, while server CPU and memory usage are low, it's highly likely a distributed denial-of-service (DDoS) attack has occurred. This type of attack overwhelms the target with massive amounts of fake traffic, preventing it from handling requests from legitimate users. Faced with attack traffic ranging from gigabytes to hundreds of gigabytes or even terabytes, ordinary servers and network bandwidth will be instantly overloaded. DDoS protection services have emerged to address this, and their core capabilities are often summarized as "three-pronged defense." This "three-pronged defense" doesn't refer to three specific technologies, but rather represents a collaborative defense system targeting three different dimensions: the network layer, the transport layer, and the application layer, forming a deep defense line.
First Defense: Flood Resistance at the Network and Transport Layers
The most direct method used by attackers is to use spoofed IP addresses to send massive amounts of UDP or ICMP packets to the target server, aiming to completely block the server's ingress bandwidth. This is a network layer attack. Another common technique is SYN Flood, where attackers send a large number of TCP connection requests, but disappear after completing the first step of the three-way handshake, causing a large number of half-open connections to hang on the server, eventually exhausting resources. This is a transport layer attack.
To combat such attacks, the primary task of DDoS protection services is "traffic scrubbing." When traffic enters the DDoS cluster, the system first performs anomaly detection. By analyzing indicators such as source IP distribution, packet characteristics, and request rates, and comparing them with a baseline model, attack traffic is quickly identified. Subsequently, the scrubbing equipment employs multiple filtering technologies. For example, for SYN floods, mechanisms such as SYN cookies or TCP first-packet dropping are deployed to verify the authenticity of connections, forwarding only complete and legitimate connections to the origin server. A basic iptables rule can be used for auxiliary mitigation on the server side, but this is only the last line of local defense; the main defense should be deployed forward to the DDoS protection network.
# Example: Setting thresholds using iptables locally on the server to help mitigate SYN Flood (emergency measure)
iptables -A INPUT -p tcp --syn -m limit --limit 1/s --limit-burst 3 -j ACCEPT
iptables -A INPUT -p tcp --syn -j DROPSecondary Defense: Precise Identification and Interception at the Application Layer
Application layer attacks are more covert and complex. Attackers simulate real users, continuously sending high-frequency HTTP/HTTPS requests to dynamic pages of a website (such as login, search, and submission). These requests appear legitimate, but their purpose is to exhaust the server's CPU, database connections, and other computing resources, thereby causing a denial-of-service attack.
The core of defending against application layer attacks is "precision." High-defense services need to have deep packet inspection capabilities, able to parse HTTP/HTTPS protocols and understand request methods such as GET and POST. Based on this, defense strategies can be very granular. For example, independent access frequency limits can be set for specific URL paths (such as `/api/login`), far lower than the frequency limit for the website's homepage. For unusually high-frequency requests originating from a single IP address or IP range within a short period, a CAPTCHA challenge can be triggered. If the user passes the CAPTCHA, they are added to a whitelist and allowed access for a short period; if they fail or continue the attack, the IP address is blacklisted.
These rules are typically configured via a graphical interface on the DDoS protection service console. The underlying logic is similar to configuring rate limiting in Nginx, but with greater scale and granularity.
``nginx
# Analogy: Example of configuring a specific API access rate limit in Nginx
http {
limit_req_zone $binary_remote_addr zone=api:10m rate=10r/s;
server {
location /api/ {
limit_req zone=api burst=20 nodelay;
proxy_pass http://backend;
}
}
}Third Defense: Source Tracing and Suppression of Protocol Vulnerabilities and Reflection Amplification
The third defense targets reflection amplification attacks that exploit vulnerabilities in internet protocols. Attackers spoof the target server's IP address, sending small query requests to numerous open network services (such as DNS, NTP, and Memcached servers). If these services are misconfigured, they can return response data tens to hundreds of times larger than the requests, creating massive attack traffic.
To counter such attacks, DDoS protection services employ a "combined strategy." First, protocol compliance verification filters out packets that clearly do not conform to protocol specifications or are malformed. Second, traffic shaping and rate limiting are implemented, with particularly strict monitoring and rate limiting of UDP traffic pointing to certain suspicious ports. More importantly, many large DDoS protection providers collaborate with global network operators to establish real-time threat intelligence sharing systems. When persistent reflection attacks are detected, they can intervene closer to the source of the attack traffic (i.e., the reflection servers being exploited) or implement traffic diversion and cleaning on the backbone network; this is known as "near-source cleaning." This layer of defense relies on the infrastructure scale of the DDoS protection provider and the partnerships with network operators, and cannot be accomplished independently by ordinary users.
Three-pronged approach: The actual operation of defense-in-depth
A complex hybrid attack may employ multiple of these methods simultaneously. A mature DDoS protection service doesn't operate in isolation; rather, it's a dynamically coordinated, automated system. Its workflow typically follows this pattern: Traffic entering the DDoS protection network nodes first undergoes network layer scrubbing to filter out obvious spam and protocol attacks. Surviving traffic then enters the application layer deep analysis engine, which, based on preset rules and AI behavioral analysis models, identifies and intercepts abnormal patterns such as CC attacks. Simultaneously, a global threat awareness system continuously monitors the system, immediately initiating appropriate source tracing and suppression strategies upon detecting characteristics of reflection amplification attacks. Finally, requests verified as legitimate are securely transmitted to the customer's actual server via the DDoS protection service provider's dedicated origin link.
When choosing a DDoS protection service, one should not only focus on its claimed protection capacity but also examine the actual depth and synergy of its three-pronged defense capabilities. It's essential to understand whether the service has verifiable case studies and specific mitigation solutions for dealing with large-scale SYN Floods, complex CC attacks, and reflection attacks exploiting the latest protocol vulnerabilities. Furthermore, the service's availability, manageability, and handling procedures for false positives are also crucial. True DDoS protection provides more than just bandwidth; it offers an intelligent defense system from the edge to the core, from traffic to protocols. # DDoS Protection Service: A Comprehensive Analysis of its Three-Tier Defense: What Does It Protect Against?
When a website or application suddenly becomes extremely slow or even completely inaccessible, while server CPU and memory usage are low, it's highly likely that it's experiencing a Distributed Denial-of-Service (DDoS) attack. This type of attack overwhelms the target with massive amounts of fake traffic, preventing it from handling requests from legitimate users. Faced with attack traffic ranging from several gigabytes to hundreds of gigabytes or even terabytes, ordinary servers and network bandwidth will be instantly overloaded. DDoS protection services emerged to address this, and their core capabilities are often summarized as "three-tier defense." This "three-tier defense" doesn't refer to three specific technologies, but rather represents a collaborative defense system targeting three different dimensions: the network layer, the transport layer, and the application layer, forming a deep defense line.
First Defense: Resisting the Flood of Attacks at the Network and Transport Layers
The most direct method used by attackers is to use forged IP addresses to send massive amounts of UDP or ICMP packets to the target server, aiming to completely block the server's ingress bandwidth. This is a network layer attack. Another common tactic is SYN Flood. Attackers send a large number of TCP connection requests, but disappear after completing the first step of the three-way handshake, causing a large number of half-open connections to hang on the server, eventually exhausting resources. This is a transport layer attack.
To combat this type of attack, the primary task of DDoS protection services is "traffic scrubbing." When traffic enters the DDoS protection cluster, the system first performs anomaly detection. By analyzing indicators such as source IP distribution, packet characteristics, and request rates, and comparing them with a baseline model, attack traffic is quickly identified. Subsequently, the scrubbing equipment employs multiple filtering technologies. For example, for SYN Flood, mechanisms such as SYN cookies or TCP first-packet dropping are deployed to verify the authenticity of connections, forwarding only complete and legitimate connections to the origin server. A basic iptables rule can be used for auxiliary mitigation on the server side, but this is only the last line of defense locally; the main defense should be moved forward to the DDoS protection network.
# Example: Setting thresholds using iptables locally on the server to help mitigate SYN Flood (emergency measure)
iptables -A INPUT -p tcp --syn -m limit --limit 1/s --limit-burst 3 -j ACCEPT
iptables -A INPUT -p tcp --syn -j DROPSecondary Defense: Precise Identification and Interception at the Application Layer
Application layer attacks are more covert and complex. Attackers simulate real users, continuously sending high-frequency HTTP/HTTPS requests to dynamic pages of a website (such as login, search, and submission). These requests appear legitimate, but their purpose is to exhaust the server's CPU, database connections, and other computing resources, thereby causing a denial-of-service attack.
The core of defending against application layer attacks is "precision." High-defense services need to have deep packet inspection capabilities, able to parse HTTP/HTTPS protocols and understand request methods such as GET and POST. Based on this, defense strategies can be very granular. For example, independent access frequency limits can be set for specific URL paths (such as `/api/login`), far lower than the frequency limit for the website's homepage. For unusually high-frequency requests originating from a single IP address or IP range within a short period, a CAPTCHA challenge can be triggered. If the user passes the CAPTCHA, they are added to a whitelist and allowed access for a short period; if they fail or continue the attack, the IP address is blacklisted.
These rules are typically configured via a graphical interface on the DDoS protection service console. The underlying logic is similar to configuring rate limiting in Nginx, but with greater scale and granularity.
``nginx
# Analogy: Example of configuring a specific API access rate limit in Nginx
http {
limit_req_zone $binary_remote_addr zone=api:10m rate=10r/s;
server {
location /api/ {
limit_req zone=api burst=20 nodelay;
proxy_pass http://backend;
}
}
}Third Defense: Source Tracing and Suppression of Protocol Vulnerabilities and Reflection Amplification
The third defense targets reflection amplification attacks that exploit vulnerabilities in internet protocols. Attackers spoof the target server's IP address, sending small query requests to numerous open network services (such as DNS, NTP, and Memcached servers). If these services are misconfigured, they can return response data tens to hundreds of times larger than the requests, creating massive attack traffic.
To counter such attacks, DDoS protection services employ a "combined strategy." First, protocol compliance verification filters out packets that clearly do not conform to protocol specifications or are malformed. Second, traffic shaping and rate limiting are implemented, with particularly strict monitoring and rate limiting of UDP traffic pointing to certain suspicious ports. More importantly, many large DDoS protection providers collaborate with global network operators to establish real-time threat intelligence sharing systems. When persistent reflection attacks are detected, they can intervene closer to the source of the attack traffic (i.e., the reflection servers being exploited) or implement traffic diversion and cleaning on the backbone network; this is known as "near-source cleaning." This layer of defense relies on the infrastructure scale of the DDoS protection provider and the partnerships with network operators, and cannot be accomplished independently by ordinary users.
Three-pronged approach: The actual operation of defense-in-depth
A complex hybrid attack may employ multiple of these methods simultaneously. A mature DDoS protection service operates its "three-defense" capabilities not in isolation, but as a dynamically coordinated automated system. Its workflow typically follows this pattern: Traffic entering the DDoS protection network nodes first undergoes network layer scrubbing to filter out obvious spam and protocol attacks; surviving traffic then enters the application layer deep analysis engine, which identifies and blocks abnormal patterns such as CC attacks based on preset rules and AI behavioral analysis models; simultaneously, a global threat awareness system continuously monitors the system, and immediately initiates corresponding source tracing and suppression strategies upon detecting characteristics of reflection amplification attacks. Finally, requests verified as legitimate are securely transmitted to the customer's actual server via the DDoS protection service provider's dedicated origin link.
When choosing a DDoS protection service, one should not only focus on its claimed protection capacity, but also examine the actual depth and synergy of its "three-defense" capabilities. It is necessary to understand whether the service has verifiable case studies and specific mitigation solutions for dealing with large-scale SYN Floods, complex CC attacks, and reflection attacks exploiting the latest protocol vulnerabilities. Furthermore, the availability, manageability, and handling procedures for false positives are also crucial.
