Flood attacks, particularly distributed denial-of-service (DDoS) and its variations, have become one of the most prevalent and dangerous types of cyberattacks targeting network infrastructures today. These attacks aim to overwhelm a network or server with excessive traffic, causing legitimate requests to be delayed or dropped. The results can range from degraded performance to complete network downtime, leading to financial losses, reputational damage, and critical service disruptions.
In this comprehensive guide, we’ll dive deep into the technical aspects of flood attacks, examine common types of attacks, and explore proven strategies to mitigate and defend against these sophisticated threats.
Flood Attacks explanation
Flood attacks are a category of DoS (Denial-of-Service) attacks where an attacker generates massive amounts of data or connection requests to overwhelm the resources of a target server or network infrastructure. By exceeding the network’s bandwidth or the server’s capacity to handle incoming connections, legitimate users are unable to access the service.
The primary types of flood attacks include:
- SYN Flood Attacks
- UDP Flood Attacks
- ICMP Flood Attacks (Ping Flood)
- HTTP Flood Attacks
- DNS Amplification Attacks
- NTP Amplification Attacks
Let’s explore each of these types in more detail.
2. Types of Flood Attacks
SYN Flood Attacks
A SYN flood is a TCP-based attack that exploits the TCP handshake process, which consists of three steps:
- The client sends a SYN (synchronize) message to the server.
- The server responds with a SYN-ACK (synchronize-acknowledge).
- The client sends an ACK (acknowledge) message back to establish the connection.
In a SYN flood attack, the attacker sends numerous SYN packets but never completes the handshake. The server waits for the ACK message, keeping the connection open in a half-open state, and consumes its memory and resources for these incomplete requests.
Mitigation Techniques:
- SYN Cookies: A technique where the server encodes necessary state information in the TCP sequence number field, eliminating the need to store half-open connections in memory.
- TCP Intercept: Used by firewalls to proxy the TCP handshake and only forward completed handshakes to the server.
- Connection Rate Limiting: Restricts the rate at which SYN packets are processed.
UDP Flood Attacks
A UDP (User Datagram Protocol) flood attack targets a server or network by sending a large number of UDP packets. Since UDP is connectionless, the target system processes every incoming UDP packet without performing connection validation, consuming both bandwidth and processing power.
Mitigation Techniques:
- Filtering Unnecessary UDP Ports: Close unnecessary UDP ports at the firewall or router level.
- Rate Limiting: Apply rate limits to UDP traffic.
- UDP Protocol Anomalies Detection: Intrusion detection/prevention systems (IDS/IPS) can identify and drop anomalous or excessive UDP packets.
ICMP Flood (Ping Flood)
ICMP flood, or ping flood, overwhelms a target with ICMP Echo Request (ping) packets. Since ICMP packets are designed for diagnostics and are typically processed by the CPU, these attacks can lead to high CPU usage and network saturation.
Mitigation Techniques:
- ICMP Rate Limiting: Control the rate of ICMP Echo Requests.
- Block Unnecessary ICMP Traffic: Disable or limit ICMP Echo Requests on routers and firewalls for non-essential hosts.
- Disabling ICMP for Public Interfaces: For services not requiring ICMP, consider disabling it at the public-facing interface.
HTTP Flood Attacks
HTTP flood attacks overwhelm web servers by sending a large volume of legitimate-looking HTTP GET or POST requests, often through bots or hijacked devices. Since these requests seem normal, distinguishing between genuine user requests and attack traffic can be difficult.
Mitigation Techniques:
- Web Application Firewalls (WAFs): Deploy WAFs to analyze and filter malicious HTTP requests.
- CAPTCHA Challenges: Use CAPTCHA challenges to differentiate between bots and legitimate users.
- Behavior-Based Detection: Monitor user behavior, such as the frequency of requests and session patterns, to detect anomalies.
DNS Amplification Attack
In a DNS amplification attack, the attacker sends DNS requests with a spoofed source IP (the victim’s IP) to open DNS resolvers. The DNS server responds with a significantly larger response packet (amplifying the traffic), which is sent to the victim’s server. This leads to an overload of traffic that saturates the victim’s bandwidth.
Mitigation Techniques:
- Rate Limiting DNS Traffic: Apply limits on DNS query rates.
- Disable Open DNS Resolvers: Ensure DNS resolvers are not open to public queries unless necessary.
- DNS Response Filtering: Use filtering mechanisms to drop unusually large DNS responses.
NTP Amplification Attack
NTP amplification attacks involve sending small NTP (Network Time Protocol) requests to NTP servers that respond with large packets, amplifying the attack volume directed at the target. This is similar in nature to DNS amplification attacks but uses the NTP protocol instead.
Mitigation Techniques:
- Update and Secure NTP Servers: Disable monlist commands, which return extensive lists of clients that have interacted with the NTP server.
- Rate Limiting NTP Traffic: Apply rate limits and filtering to reduce the impact of NTP responses.
- Switch to NTP Authentication: Use secure authentication features to prevent unauthorized NTP queries.
Best Practices for Defending Against Flood Attacks
Intrusion Detection and Prevention Systems (IDS/IPS)
Intrusion Detection Systems (IDS) monitor network traffic in real-time, looking for patterns that indicate flood attacks or other anomalies. Intrusion Prevention Systems (IPS) go a step further by automatically blocking or filtering malicious traffic.
- Signature-Based Detection: IDS/IPS can detect known attack signatures (e.g., certain types of malformed packets).
- Anomaly-Based Detection: The system detects deviations from normal traffic patterns, which can indicate the presence of a flood attack.
DDoS Mitigation Services
Cloud-based DDoS protection services, such as Cloudflare, Akamai, and AWS Shield, offer scalable protection by absorbing and mitigating large-scale attacks. These services reroute traffic through scrubbing centers where attack traffic is filtered out before reaching the target infrastructure.
Rate Limiting and Traffic Shaping
Rate limiting controls the amount of incoming traffic, allowing servers and applications to handle traffic bursts more gracefully. Traffic shaping prioritizes specific types of traffic, ensuring that critical services remain functional even under attack.
- Ingress Filtering: Filtering inbound traffic based on known attack patterns.
- Outbound Filtering: Used to prevent spoofed traffic (especially important for amplification attacks).
Stateful Firewalls
Unlike stateless firewalls, stateful firewalls keep track of active connections. They can differentiate between legitimate ongoing connections and malicious flood attacks by monitoring the state of each connection.
Anti-DDoS Appliances
Many hardware-based solutions, such as those from Arbor Networks or Radware, are designed to mitigate DDoS attacks. These appliances sit at the network’s edge and filter malicious traffic before it reaches the internal network.
Building a Multi-Layered Defense Strategy
The best defense against flood attacks involves a multi-layered approach that combines several tools and strategies. This defense-in-depth approach should incorporate the following:
- Proactive Monitoring: Implement continuous monitoring using network traffic analysis, IDS/IPS systems, and log management to detect unusual traffic patterns early.
- Traffic Scrubbing Centers: Utilize third-party DDoS mitigation services for scrubbing traffic during large-scale attacks.
- Redundancy and Failover: Deploy multiple data centers or cloud regions to distribute traffic and ensure continuous service during attacks.
- Rate Limiting and Protocol Hardening: Apply rate limits to susceptible protocols such as HTTP, TCP, and DNS, and enforce protocol-specific security measures like SYN cookies or DNS rate limiting.
- Firewalls and Filters: Use stateful firewalls and ingress/egress filters to block malicious traffic early in the network stack.
Conclusion
Flood attacks pose a significant threat to the availability and reliability of networked systems. They exploit the fundamental limitations of internet protocols, such as TCP, UDP, and HTTP, and aim to overwhelm the targeted infrastructure with massive volumes of traffic. A robust and multi-layered defense strategy is essential to mitigate these attacks, combining traffic filtering, rate limiting, DDoS protection services, and constant network monitoring.
By employing proactive defense measures, utilizing cloud-based mitigation, and ensuring proper protocol hardening, organizations can significantly reduce the impact of flood attacks and maintain continuous service availability even under duress.