What is a SYN flood attack?

Companies face an ever-growing threat of disruptive DDoS attacks. These digital blockades are very easy to launch, but can have a devastating impact, often leading to costly downtime, impacting productivity and causing significant financial losses. 

Among these threats, the SYN flood attack stands out as a particularly potent risk. In 2023, it ranked as the second most common type of DDoS attack, accounting for 13.89% of all incidents. Only UDP flood attacks were more prevalent, making up 54.99% of cases.

Read on to learn what a SYN flood attack is, how it operates, and how to strengthen your defenses against such disruptive online assaults.

What is a SYN flood attack?

A SYN flood attack is a denial of service where an attacker overwhelms a target system by continuously sending SYN (Synchronize) requests. The primary objective of a SYN flood attack is to exhaust the target server's resources, rendering it incapable of responding to legitimate traffic. This malicious strategy exploits a fundamental aspect of internet communication: the TCP three-way handshake, which is crucial for establishing connections in TCP/IP networks.

By manipulating this essential protocol, attackers can effectively paralyze a system, disrupting normal operations and potentially causing significant damage to businesses relying on stable network communications. Understanding the mechanics of SYN flood attacks is the first step in developing robust defense strategies against these pervasive threats.

The basics of a TCP handshake

To effectively identify and understand a SYN flood attack, it's essential to grasp the fundamentals of a typical TCP handshake. Understanding this process helps you notice when something isn't right, potentially alerting you to a SYN flood attack in progress. 

A normal TCP handshake consists of three key steps:

1. SYN (Synchronize): Your device initiates communication by sending a SYN packet to the intended server, requesting a connection.

2. SYN-ACK (Synchronize-Acknowledge): The server responds with a SYN-ACK packet, confirming its readiness to communicate. This packet also includes the server's own Synchronization information.

3. ACK (Acknowledge): Your device sends a final ACK packet to the server, completing the handshake. This step establishes the connection, allowing actual data exchange to begin between your business systems and your customers or employees.

By familiarizing yourself with the normal flow of network communications, you're better equipped to protect your systems from malicious exploits that target this fundamental protocol.

How does a SYN flood attack work? 

A SYN flood attack overwhelms the target server with a high volume of SYN packets, initiating connection requests without completing the TCP three-way handshake. The attacker sends multiple SYN packets, ignoring the server's SYN-ACK or using spoofed IP addresses, ensuring the final ACK never reaches the server. Consequently, the server's connection table fills with incomplete requests, consuming resources and preventing legitimate connections. This overload blocks legitimate users from accessing the system as the server struggles with pending, incomplete connections.

A SYN flood attack can occur in four different ways, namely:

1. Direct attack

As the name suggests, the attacker targets your business's systems directly, inundating your servers with a barrage of SYN packets to clog your network's connection tables. The direct nature of these attacks poses a severe risk, quickly overwhelming systems that are not adequately prepared.

2. Spoofed attack

In this method, attackers alter the source IP addresses on the SYN packets, making the requests appear to come from various locations rather than from the actual perpetrator. This alteration complicates pinpointing the source, requiring your security team to sift through numerous potential origins. Blocking a single source does little to halt the traffic flow, as the attacks do not originate from a fixed location.

3. Botnet attack

In a botnet attack, multiple compromised internet-connected devices are commanded to send SYN requests to your network simultaneously. The sheer volume of traffic from diverse sources can overwhelm even the most robust systems. This overload consumes most available resources, significantly hindering your network's ability to manage legitimate connection attempts from your workforce and customers.

4. Coordinated DDoS attack

This DDoS attack involves multiple attackers combining their efforts to maximize disruption. By merging traffic from various sources, these attacks amplify the impact and complicate the distinction between legitimate and malicious requests for your network.

5 Ways to mitigate a SYN flood attack

SYN flood attacks can severely disrupt network services by blocking legitimate users from establishing connections. Swift action is critical to minimize the damage these attacks can inflict. Here are some strategies your business can implement:

1. SYN cookies

When receiving a SYN request, your server generates a unique code, or "cookie," and includes it in the SYN-ACK response. The client must return this code in the subsequent ACK to verify the connection's legitimacy. The advantage of implementing SYN cookies is that the server does not allocate resources for the connection until the cookie is validated. This process helps differentiate between genuine users and attackers, preserving resources for authentic connections.

2. Load balancing

A network or transport layer load balancer helps manage and mitigate the impact by balancing traffic before it reaches the application layer.

3. Rate limiting

By setting maximum connection rates, you can prevent any single source from overwhelming the system. This method blocks excessive traffic from a potential attacker. Configuring rate limits according to your business's specific needs, such as limiting requests from a single IP address to 60 per minute, can provide adequate control.

4. Increasing backlog

Enhancing your server's backlog capacity allows more SYN requests to queue up without timing out, thus optimizing resource use and improving the overall management of incoming connections.

5. Content Delivery Networks

A Content Delivery Network (CDN) helps mitigate SYN flood attacks by absorbing and filtering malicious traffic, distributing load across multiple servers, and providing always-on DDoS protection. This preserves the origin infrastructure's resources and ensures availability even for organizations with limited capacity. CDNs act as a buffer, offloading attack traffic from the origin servers.

See how Fastly's rate limiting and filtering features can dramatically reduce these kinds of attacks and ease resource load. Fastly was specifically able to impact, CPU consumption on load balancers dropping from 90% to manageable levels. 

How Fastly can help with SYN flood attacks

SYN flood attacks pose a significant threat to your business, potentially leading to service disruptions and costly downtime. To safeguard your network, you need robust prevention strategies. While techniques like SYN cookies and rate limiting are effective, partnering with a top-tier edge cloud platform like Fastly offers comprehensive protection against DDoS attacks.

Fastly's edge cloud platform provides robust solutions to shield your business:

• Smart Traffic distribution: Fastly spreads incoming requests across multiple edge locations. This approach absorbs the impact of large-scale attacks before they reach your servers, keeping your network running smoothly.

• Intelligent SYN proxy: The platform completes the TCP handshake for you. Only legitimate, fully established connections reach your origin servers, creating a solid barrier against SYN floods.

• Enhanced protection: By handling traffic at the edge, Fastly creates a protective barrier between attackers and your origin servers.

• Improved performance: Load distribution across edge locations not only improves security but also enhances overall network performance.

• Visibility and analytics: Comprehensive logging and analytics provide insights into traffic patterns and potential security threats.

• Scalability: Whether from legitimate users or attack attempts, Fastly's global network scales quickly to handle sudden traffic spikes.

• Customizable rules: Fastly allows for implementing custom rules to address specific threats or traffic patterns.

Learn more about how Fastly's solutions prevent SYN flood attacks to help reduce server load and improve user experience.