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Luke Sun

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Distributed Denial of Service (DDoS)

| , 3 minutes reading.

1. Definition

Distributed Denial of Service (DDoS) is an availability attack where multiple compromised computer systems (a Botnet) attack a target, such as a server, website, or other network resource, causing a denial of service for legitimate users.

The “Distributed” aspect makes it difficult to stop, as blocking a single source is insufficient.

2. Technical Explanation

DDoS attacks target different layers of the OSI model:

  1. Volumetric Attacks (Layer 3/4): Saturate bandwidth. Examples: UDP Flood, ICMP Flood, DNS/NTP Amplification.
  2. Protocol Attacks (Layer 3/4): Consume server resources (firewall connection tables, load balancers). Examples: SYN Flood (exploiting the TCP handshake).
  3. Application Layer Attacks (Layer 7): Target specific heavy web applications. Examples: HTTP Flood, Slowloris (keeping connections open with partial requests).

3. Attack Flow (Mirai Botnet Style)

flowchart TD
    C2[Attacker C2 Server]
    
    subgraph Botnet
        IoT1[Infected Camera]
        IoT2[Infected Router]
        IoT3[Infected DVR]
    end
    
    Target[Target Web Server]
    
    C2 --"Cmd: Attack Target IP"--> Botnet
    IoT1 --"UDP Flood"--> Target
    IoT2 --"TCP SYN Flood"--> Target
    IoT3 --"HTTP GET /"--> Target

    Target -.-> Result>"Bandwidth Saturated<br/>CPU at 100%<br/>Legitimate users dropped"]

4. Real-World Case Study: Dyn DNS Attack (2016)

Target: Dyn (Major DNS Provider). Attack Type: Massive Volumetric IoT Botnet (Mirai). Volume: Estimated 1.2 Terabits per second (Tbps).

The Attack: The Mirai botnet scanned the internet for IoT devices (cameras, routers) with default passwords (like admin/admin). It infected hundreds of thousands of devices. On October 21, 2016, these devices were commanded to flood Dyn’s DNS infrastructure with TCP and UDP packets on port 53.

Impact: Because Dyn provided DNS for major sites, the attack effectively “broke the internet” for millions of users. Netflix, Twitter, Reddit, CNN, and GitHub became inaccessible across North America and Europe, not because their servers were down, but because browsers couldn’t resolve their domain names.

5. Detailed Defense Strategies

A. Anycast Network Routing

Use an Anycast network (provided by CDNs like Cloudflare, AWS CloudFront, or Akamai).

  • Mechanism: The same IP address is announced from multiple global locations.
  • Defense: Attack traffic is naturally dispersed to the closest data center, preventing any single point from being overwhelmed. The “flood” is diluted across the global network.

B. WAF and Rate Limiting (Layer 7)

For Application attacks:

  • Challenge: Distinguish bots from humans using CAPTCHAs or JS challenges (Browser Integrity Check).
  • Rate Limiting: Restrict requests per IP (e.g., 100 requests/minute).
  • WAF Rules: Block known bot user-agents or malicious patterns.

C. Scrubbing Centers (Layer 3/4)

For Volumetric attacks:

  • Route BGP traffic through a specialized “Scrubbing Center” when an attack is detected.
  • The center filters out malicious packets (malformed UDP, SYN without ACK) and passes only clean traffic to the origin server.

D. Reduce Surface Area

  • Do not expose origin server IPs directly.
  • Use a whitelist to only accept traffic from your CDN provider’s IP ranges.
  • Close all unnecessary ports (SSH/FTP) or restrict them to VPN access.

6. References