I.The Evolution of DDoS Attacks: From "Traffic Flood" to "Surgical Strike"

II. The underlying defense architecture of high-defense CDN: three-dimensional protection from edge to core

Different from the single-point defense of traditional IDC, high-defense CDN relies on distributed architecture to build a protection barrier. Its core technical modules include:

3. Central cleaning cluster: "precision screen" for application layer attacks

• Seven-layer deep detection:Based on regular expression matching (Regex Matching) and semantic analysis, identify application layer threats such as HTTP Flood and CC attacks. For example: trigger verification code challenge for IPs with URI access frequency > 500 times/minute;By verifying the Referer header, block requests with illegal Referer (such as localhost).
• AI behavior modeling:Collect normal business traffic data for more than 7 days, train and generate dynamic baseline models (including 18 dimensions such as geographical distribution, request time period, user equipment, etc.), and trigger secondary detection for traffic that deviates from the baseline by more than 30% (the false alarm rate can be controlled below 0.05%).

III. Practical steps for defending against large-scale DDoS attacks (taking a 2.5Tbps attack as an example)

Step 1: Attack detection and feature analysis (0-3 minutes)

• Traffic abnormality warning:
  The following indicators are monitored in real time through a self-developed monitoring system (such as Zabbix customized template):
  • Inbound traffic exceeds 200% of the bandwidth peak (e.g., daily peak is 500G, triggering the warning threshold of 1T);
  • The number of connections in the SYN_RECV state exceeds 80% of the TCP stack capacity (the default net.ipv4.tcp_max_syn_backlog in Linux is 4096, and an alarm is triggered when it exceeds 3276).
• Protocol analysis positioning:
  Use Wireshark to capture packets and analyze them. If the proportion of UDP packets is greater than 60% and the destination ports are concentrated on 53 (DNS) and 123 (NTP), it is initially determined to be a reflective DDoS attack. If the proportion of HTTP POST requests increases suddenly and carries a large amount of invalid form data, it is a hybrid application layer attack.

Step 2: Emergency interception by edge nodes (3-8 minutes)

• IP-level fast blocking:
  Through the hardware ACL of the edge node, the malicious IP library of the threat intelligence platform (such as FireEye) is synchronized in real time, and the attack source IP segment (such as /24 subnet) is blocked, with a response time of <100ms.
• Traffic cleaning policy adjustment:
  For reflective attacks, enable source IP validation (Source IP Validation) to discard all traffic whose source IP is inconsistent with the return packet address; for HTTP Flood, shorten the keep-alive timeout from the default 15s to 5s to force the release of server connection resources.

Step 3: Global traffic scheduling and expansion (8-15 minutes)

• Node load balancing:Through the Anycast routing system, traffic that exceeds the processing capacity of a single node (such as 1.5T) is automatically diverted to nodes in adjacent regions (such as when East Asian traffic overflows, it is dispatched to Sydney and Tokyo nodes for cleaning), ensuring that the load of a single node does not exceed 70% of the peak value.
• Elastic bandwidth expansion:Trigger the backup cleaning cluster (pre-allocate 3T redundant bandwidth), dynamically announce the new cleaning node IP through the BGP protocol, and achieve minute-level defense capacity expansion (in a financial customer's actual combat, the defense peak was increased from 5T to 8T within 12 minutes).

Step 4: Deep protection of the source site (15-30 minutes)

• Back-to-source path encryption:Enable TLS 1.3 to encrypt back-to-source traffic, and use quantum key distribution (QKD) technology to ensure that the cleaned traffic is not tampered with or eavesdropped during the back-to-source process (applicable to highly sensitive businesses such as finance and medical care).
• Connection pool optimization:Configure Nginx connection pool on the source server, set proxy_max_temp_file_size 0 to prohibit disk caching, and limit the number of concurrent connections for a single IP (such as limit_conn_zone $binary_remote_addr zone=perip:10m; limit_conn perip 100) to prevent resources from being maliciously exhausted.

Step 5: Attack tracing and policy iteration (24 hours after the attack)

• In-depth log analysis:
  Export the attack log of CDN nodes (including IP ownership, ASN number, and attack tool characteristics), visualize the attack chain through Maltego, and locate the attack infrastructure (such as the IP of a botnet control node: 185.153.224.12).
• Defense strategy upgrade:
  In response to the characteristics of this attack, add recognition rules (such as specific HTTP header fields and abnormal URL parameters) to the AI ​​model, and simulate similar attacks through Chaos Engineering exercises to verify the effectiveness of the defense strategy.

IV. Five hard-core indicators for selecting high-defense CDN (engineer's guide to avoid pitfalls)

1. Cleaning node density:
   The number of nodes in the target area must be greater than 10/continent (e.g., the Asia-Pacific region must cover at least Hong Kong, Singapore, Tokyo, and Sydney) to ensure localized cleaning of attack traffic and avoid increased latency caused by cross-continental transmission.
2. Defense elasticity:
   Service providers are required to provide minute-level elastic expansion capabilities, and the defense peak after expansion must be no less than 3 times the daily peak (e.g., daily 5T, burst can be expanded to 15T).
3. Protocol support:
   Attack detection of new protocols such as HTTP/3 and QUIC must be supported, and compatible with national encryption algorithms (SM2/SM3/SM4) to meet domestic compliance requirements.
4. False blocking rate control:
   The false blocking rate of high-quality service providers should be less than 0.01%, which can be verified by actual measurement: inject 5% of simulated attack traffic into normal business traffic and observe the access blocking rate of legitimate users.
5. Emergency response speed:
   When an attack occurs, the technical team needs to provide customized defense strategies (such as protection rules for specific APIs) within 15 minutes, and support 24/7 telephone + work order dual-channel response.

5. Actual combat case: A cross-border e-commerce company resisted a 5.2Tbps UDP reflection attack

In March 2024, a cross-border e-commerce company encountered the largest UDP reflection attack in history, with a peak of 5.2Tbps. The attack traffic mainly came from NTP server reflections in North America. The defense process is as follows:

1. Edge node interception: Block the top 1,000 attack source IPs through hardware ACL and clean 3.8Tbps of traffic;
2. Intelligent scheduling: Divert the remaining 1.4T traffic to European and Asian nodes to avoid single node overload;
3. Source station protection: Enable UDP port rate limit (only allow necessary ports such as 53 and 67 to pass), cooperate with CDN's UDP checksum verification to ensure that normal business (such as DNS resolution) is not affected;
4. Attack tracing: Through log analysis, it was found that the attack exploited the monlist vulnerability of the old version of the NTP server, assisted the customer in submitting a vulnerability warning to CERT, and finally traced it to 3 botnet control centers.
   Defense effect: During the attack, the website availability remained at 99.99%, and the order conversion rate only dropped by 1.2%, which is far lower than the industry average (20%-30%).

6. Daily protection checklist for operation and maintenance engineers

VII. Summary:

Defense against large-scale DDoS attacks is never a victory of a single technology, but a systematic project of architecture design, strategy optimization, and emergency response. High-defense CDN builds a three-dimensional defense line of "edge interception - center cleaning - source station reinforcement" for enterprises through the deep integration of distributed cleaning, intelligent scheduling, and AI detection.

As a network security engineer, you must always remember that the best defense is not only to intercept attacks, but also to make attacks "unable to find, unable to hit, and unable to afford".

Action suggestions: When choosing a high-defense CDN, require the service provider to provide real attack interception logs and node performance measurement data to avoid falling into the propaganda trap of "theoretical defense". For enterprises with larger business scale, it is recommended to deploy a multi-CDN redundancy solution (primary service provider + backup service provider) to ensure service continuity under extreme attacks.