What is SYN Flood Protection?
SYN flood protection involves techniques like SYN cookies, rate limiting, and load balancing to defend against DDoS attacks that overwhelm servers with half-open TCP connections, ensuring legitimate users can still connect by verifying requests and filtering malicious traffic:
How SYN flood attacks work:
- Overwhelming the TCP handshake: Attackers send a high volume of SYN packets to start TCP handshakes but never complete them, leaving connections half-open.
- Filling the connection queue: Each half-open connection consumes a slot in the server’s backlog queue. Once full, new connections, including legitimate ones, are dropped.
- Exhausting system resources: The server wastes memory and CPU handling fake requests, especially during large or distributed attacks.
- Using spoofing and botnets: Attackers often spoof IP addresses or use botnets to hide their identity and scale the attack, making mitigation harder.
Key techniques for SYN flood protection include:
- SYN cookies: Instead of storing connection state, the server sends a special SYN-ACK with a unique "cookie" (a hash of connection details) and waits for the client to return it in an ACK, only allocating resources for confirmed valid connections.
- Rate limiting: Firewalls and servers can restrict the number of new SYN requests allowed per second from a single source or destination IP, preventing saturation.
- Load balancing & CDNs: Distributing traffic across multiple servers or using a Content Delivery Network (CDN) absorbs attacks and filters bad traffic before it hits the core infrastructure.
- Increased backlog: Temporarily increasing the server's queue for incoming connections can handle short bursts, but is a limited solution.
- Firewall rules: Deploying firewalls with built-in DoS protection to drop SYN packets based on source/destination IPs and defined rates.
- Traffic anomaly detection: Monitoring baseline traffic patterns to identify unusual surges in SYN requests, triggering automated responses.
- Behavioral DDoS protection and real-time mitigation: Using behavioral thresholds to identify and block DDoS attacks.
This is part of a series of articles about DDoS protection.
In this article:
A SYN flood attack exploits the TCP three-way handshake, which consists of SYN, SYN-ACK, and ACK messages. Attackers send a high rate of SYN packets to a target server, each representing a request to initiate a new TCP connection, but typically do not complete the handshake.
The server allocates memory and other resources for each incoming SYN, waiting for the corresponding ACK to finish the handshake, often exhausting the system’s available resources. When the backlog queue is full, the server rejects new, legitimate requests. This disruption is effective because TCP is designed to handle connections reliably but allocates resources upon receipt of an initial SYN packet.
Attackers can spoof IP addresses, making mitigation difficult by hiding the sources of malicious requests. These attacks can be automated and distributed from multiple devices, amplifying their effectiveness. By overwhelming the server’s connection-handling mechanisms, SYN flood attacks can quickly cause service outages, network slowdowns, and operational headaches for organizations unprepared to handle them.
Here are some of the most important consequences of a SYN flood attack:
- Service disruption: A SYN flood fills the server’s connection queue with half-open connections, preventing it from accepting legitimate requests. This leads to slow response times, dropped connections, or complete service outages.
- Business and operational impact: In environments with strict uptime requirements such as eCommerce platforms, online banking, and SaaS applications, service disruptions can result in revenue loss, reputational damage, and customer dissatisfaction. For critical infrastructure and public services, outages can affect large user populations and lead to violations of service-level agreements (SLAs).
- Resource exhaustion: SYN flood attacks can consume upstream bandwidth and CPU resources, particularly when launched from multiple sources in a distributed denial-of-service scenario. Systems are forced to allocate processing capacity to evaluate and respond to illegitimate connection attempts.
- Attack concealment and secondary risk: Attackers may use SYN floods as a smokescreen to divert attention while probing for other vulnerabilities within the network, increasing overall security exposure.
- Long-term availability risk: Without adequate protection, repeated SYN flood attacks create ongoing availability issues and increase the cost and complexity of maintaining stable and secure services.
1. SYN Cookies
SYN cookies are a defense mechanism implemented in the TCP stack that mitigate resource exhaustion during the handshake. When the server detects that its SYN backlog is full or under attack, it doesn’t allocate memory for the half-open connection. Instead, it encodes session state into the SYN-ACK sequence number sent to the client as a “cookie.”
If the client replies with the correct ACK, the server reconstructs the session state using information encoded in the cookie and completes the handshake. This technique makes it difficult for attackers to consume server resources using fake or spoofed SYN packets. SYN cookies allow stateless handling of TCP handshakes up to the point of the client’s ACK response, reducing the server’s vulnerability window to SYN flood attacks.
2. Rate Limiting
Rate limiting restricts the number of new incoming SYN packets a server or network appliance will process within a given timeframe. By setting thresholds, administrators can limit exposure to volumetric attacks, ensuring that in the event of a SYN flood, a reserve of connection slots remains available for legitimate users.
Rate limiting can be applied globally, per source IP, or by more granular criteria such as geographic region or subnet. These thresholds can be tuned to match normal traffic patterns for the protected service. Implementing rate limiting requires careful analysis of expected usage so that protection does not inadvertently block real users during periods of legitimate traffic spikes.
3. Load Balancing and CDNs
Load balancing spreads incoming connections across multiple back-end servers or network resources, distributing risk and preventing a single host from becoming a bottleneck. By redirecting SYN packets to different servers or geographically distributed nodes, load balancers make it much harder for attackers to overwhelm any single system.
Hardware and software load balancers can implement SYN proxying and connection multiplexing, both of which can absorb or filter out malicious SYN traffic before it reaches critical back-end infrastructure. Content delivery networks (CDNs) add another layer of protection by terminating TCP connections at their edge nodes, absorbing attack traffic with their scale and capacity. Legitimate requests are then proxied to origin servers only as needed.
4. Increased Backlog
Configuring a larger backlog queue increases the number of half-open connections a server can hold, delaying the point at which the server must drop new connection attempts. This gives legitimate clients a better chance to complete the handshake during heavy SYN traffic and allows more time for SYN flood detection and response mechanisms to engage.
Most major operating systems allow administrators to tune the maximum connection backlog values based on available system memory and expected load. However, simply increasing the backlog is not a long-term solution, as attackers can also scale their SYN floods to match higher limits. This technique should be used in conjunction with stronger protections like SYN cookies and rate limiting.
5. Firewall Rules
Firewall rules are one of the primary defenses against SYN flood attacks. Administrators can use access control lists (ACLs) and stateful inspection to limit SYN packets from unusual sources or block known malicious IP ranges. Modern firewalls can analyze traffic patterns and drop connections that exceed expected SYN rate thresholds or show indications of spoofing.
Some appliances feature SYN proxy features, where the firewall itself completes the handshake before forwarding it to the protected server. Deploying granular firewall rules requires ongoing management and attention to legitimate user behavior to avoid locking out real clients. Automated threat intelligence feeds, geolocation filtering, and adaptive rule sets can enhance responsiveness to emerging threats.
6. Traffic Anomaly Detection
Traffic anomaly detection employs analytics and heuristics to identify patterns in network traffic that deviate from established baselines. These systems monitor metrics such as SYN packet rates, connection attempts per IP, and the ratio of completed to incomplete handshakes. When deviations suggesting a SYN flood are detected, security tools can trigger automated mitigation.
Intelligent anomaly detection can identify both traditional volumetric SYN floods and more subtle, distributed attacks that evade signature-based firewalls. For best results, detection algorithms must be trained on real traffic over time and continually refreshed to account for normal variations in usage.
7. Behavioral DDoS Protection and Real-Time Mitigation
Behavioral DDoS protection uses machine learning and advanced analytics to classify inbound traffic according to expected patterns of legitimate user behavior. These systems profile normal handshake sequences, connection rates, and protocol usage for each endpoint or application. During a SYN flood, deviations from these behavioral models can trigger real-time mitigation actions, such as CAPTCHA challenges or rate limiting specified signatures.
Real-time mitigation means that suspicious traffic can be blocked or redirected to sandbox environments for further inspection before reaching mission-critical systems. When deployed at the edge of the network or in the cloud, behavioral DDoS technologies provide scalable protection and adapt to evolving attack methodologies.
Here are some of the ways that organizations can better protect themselves against SYN flood attacks
1. Enforce Stateless Handshake Validation Where Possible
Stateless handshake validation is a practice where the server minimizes resource commitments until a TCP handshake is fully validated. Enabling features like SYN cookies helps servers process handshake attempts without allocating memory for half-open connections, significantly reducing vulnerability to SYN flood attacks.
This approach is particularly effective for high-traffic environments, as it prevents attackers from exhausting server-side state and ensures connection slots remain available for real clients when under duress. In addition to using SYN cookies, system administrators should evaluate their stack for opportunities to process as much handshake negotiation as possible without allocating resources or state.
2. Deploy Multi-Layer Network Filtering and Edge Protection
Multi-layer filtering involves placing security mechanisms at several points in the network path, such as routers, firewalls, load balancers, and edge appliances. This layered approach ensures SYN packets are screened as early as possible, reducing attack volume before it reaches sensitive back-end resources.
By employing both network-level filtering (IP ACLs, geographic restrictions) and application-aware firewalls (stateful inspection, SYN proxy), defenders can block or rate-limit suspicious SYN traffic quickly and efficiently. Edge protection via cloud-based scrubbing services or content delivery networks (CDNs) allows organizations to shield origin infrastructure from the brunt of attack traffic.
3. Monitor Connection Queues and System Resource Baselines
Continuous monitoring of connection queues and server resource consumption is critical to detect SYN flood activity early. By tracking metrics like half-open connection counts, queue saturation rates, and unusual spikes in SYN packet volumes, administrators can identify attack onset before normal users are impacted.
Modern monitoring tools and network analytics platforms can trigger alerts or invoke automated scripts when defined thresholds are approached, enabling fast response. Maintaining resource baselines like memory usage, CPU load, and network throughput helps organizations recognize deviations linked to SYN floods and aids in capacity planning.
4. Harden Configurations of Firewalls, Load Balancers, and Hosts
Securing the configuration of all security infrastructure is essential in preventing successful SYN flood attacks. Firewalls should be updated with the latest firmware, and their rule sets must block known malicious IPs and control SYN rates effectively. Load balancers and network appliances should be configured to handle proxying, queue management, and SYN rate control, while operating systems must have recommended kernel settings applied .
Configuration hardening includes disabling unused network services, applying the principle of least privilege to administrative access, and reviewing documentation for vendor-specific SYN flood defense recommendations. Periodic audits and testing of these settings will ensure they remain aligned with evolving threats and infrastructure changes.
5. Regularly Test SYN Flood Readiness Using Controlled Simulations
Proactive testing with controlled SYN flood simulations is a best practice for verifying defense effectiveness and response protocols. Regular exercises reveal weaknesses in current configurations, expose bottlenecks, and validate monitoring and mitigation workflows. By simulating attacks using safe tools in a non-production environment or during scheduled maintenance windows, security teams can assess how solutions react under stress.
Lessons learned from simulation should drive continual improvement; findings must be documented and used to update playbooks, adjust thresholds, and automate remediation where possible. Organizations that rehearse DDoS response, including SYN flood scenarios, consistently outperform those that rely solely on vendor promises or theoretical protection.
Radware helps organizations defend against SYN flood attacks using real-time behavioral detection, automated mitigation, and scalable protection that preserves service availability even during high-volume, multi-vector campaigns.
Radware DefensePro provides inline, behavior-based DDoS protection that detects SYN floods and related connection-exhaustion attacks in real time. Rather than relying only on static thresholds, DefensePro baselines legitimate traffic patterns and identifies abnormal surges in handshake behavior, half-open connections, and SYN rate anomalies. It then applies targeted mitigation, such as SYN proxying and challenge-response techniques, to validate handshakes and prevent state exhaustion while allowing legitimate sessions to complete. This approach complements traditional measures like SYN cookies and rate limiting by adding adaptive controls that respond to changing attack patterns.
For large-scale attacks that can overwhelm upstream bandwidth or cloud infrastructure, Radware Cloud DDoS Protection Service provides high-capacity scrubbing to mitigate SYN floods before they reach the protected environment. This helps absorb massive bursts, reduces strain on AWS/Azure/on-prem edge resources, and supports resilient architectures where always-on or on-demand cloud mitigation is paired with on-prem controls. Together, DefensePro and Radware’s cloud mitigation capabilities provide layered SYN flood protection, combining fast edge detection with scalable upstream filtering to maintain availability and performance during sustained attacks.