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SYN Flood: How to Mitigate an Attack?

SYN Flood: How to Mitigate an Attack?

 By Charles Joseph | Cybersecurity Researcher
 Published on August 10th, 2023
This post was updated on November 25th, 2023

A SYN flood is a type of Denial-of-Service (DoS) attack in which an attacker sends many SYN (synchronize) requests to a target system’s services but intentionally does not complete the three-way handshake process to establish a connection. The aim is to consume all the available resources of the target system, thereby rendering it unresponsive to legitimate requests.

Here’s a brief overview of how it works:

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  1. Normal Three-Way Handshake:
    • Step 1: A client sends a SYN packet to request a connection with a server.
    • Step 2: The server responds with a SYN-ACK packet, acknowledging the request.
    • Step 3: The client returns an ACK packet to the server, and the connection is established.
  2. SYN Flood Attack:
    • Step 1: The attacker sends a flood of SYN packets to the target server.
    • Step 2: Thinking these are legitimate connection requests, the server sends SYN-ACK packets in response to each of them and awaits the final ACK to complete the handshake. The server reserves resources for each of these half-open connections.
    • Step 3: The attacker either doesn’t respond to the ACK or sends them very slowly. This leaves many half-open connections on the server, consuming its resources.

As a result, the server can become overwhelmed, leading to decreased performance or even a complete halt in its ability to handle new or existing connections.

What Are Some Countermeasures?

Countermeasures against SYN flood attacks include using firewalls to filter out malicious packets, reducing the timeout for half-open connections, and deploying SYN cookies, a technique that eliminates the need for a server to maintain half-open connections.

What Is a SYN Cookie?

A SYN cookie is a defense mechanism against SYN flood attacks, a type of Denial-of-Service (DoS) attack.

When a server receives a SYN packet initiating a connection, it computes a SYN cookie instead of allocating resources based on the packet’s attributes.

This cookie is embedded in the initial sequence number of the server’s SYN-ACK response. If the client is legitimate, it responds with an ACK packet.

The server recalculates the SYN cookie using the ACK’s attributes.

If it matches, the server acknowledges the connection and allocates resources. If not, the connection is ignored, saving resources.

This technique effectively prevents resource exhaustion from malicious SYN floods.

SYN Flood Examples (A Less Technical Explanation)

1. Telephone Jam

In this example, an attacker attempts to obstruct a particular business’s phone line. Their method is relatively straightforward — they dial the business number incessantly, rendering the line perpetually busy. This continuous activity then prevents actual customers from connecting and communicating with the business, causing a significant hindrance to customary operations.

This scenario mirrors the mechanism of a SYN Flood attack but in a digital environment. Here, the phone line is symbolically the target server, and the flood of calls represents the surge of SYN requests. Both actions aim to overwhelm the system so legitimate activity is blocked or severely delayed.

2. Mailroom Mayhem

Let’s look at the scenario of a mailroom in a large corporate building. The personnel in the mailroom are meant to receive, sort, and distribute many pieces of mail and packages daily. Now, imagine a day when they get swarmed with junk mail and false packages – priorities get mixed up, and essential parcels may be delayed or lost.

Similarly, a SYN Flood attack operates in the same disruptive way in a digital setup. Here, the mailroom is equivalent to the target server system, and the onslaught of unnecessary packages corresponds to the bombardment of SYN requests. This distracting flood of unnecessary server requests causes genuine requests to get neglected or delayed, significantly affecting network performance.


A SYN Flood is a cyber attack aimed at overwhelming a target’s resources by sending a deluge of SYN requests, causing the server to become unresponsive to legitimate traffic. Much like jamming a phone line, distracting a celebrity, or swamping a mailroom with junk, it’s a tactic to prevent proper functioning and disrupt communication.

Key Takeaways

  • A SYN Flood is a type of cyber attack designed to overload a server with bogus requests, which can cause it to become unresponsive to legitimate traffic.
  • Various real-world scenarios can serve as useful analogies for understanding SYN Flood attacks, including a jammed phone line, a celebrity overwhelmed by fans, or a mailroom overloaded with junk packages.
  • The main goal of a SYN Flood attack is to disrupt communications by preventing a server from processing genuine requests.
  • We can better protect our digital systems from cyber threats by comprehending how these attacks function.
  • Dealing effectively with a SYN Flood involves various strategies, including limiting the number of half-open connections, employing firewalls, or using SYN cookies.

Related Questions

1. How can a server prevent SYN Flood attacks?

There are several strategies to mitigate SYN Flood attacks. These include implementing SYN cookies, limiting the number of incomplete connections a server will accept, and configuring firewalls to recognize and block rapid sequence SYN requests.

2. What purpose does a SYN Flood attack serve in cybercrime?

A SYN Flood attack can be utilized as a form of disruption, making a server unavailable to users. They are often used in combination with other attacks to compromise the security of a system or network.

3. Does a SYN Flood target individual devices or networks?

A SYN Flood can target individual devices and entire networks if they’re connected to the internet. The attacker aims to overwhelm system resources to disrupt normal operations.

4. Are SYN Flood attacks easy to identify?

Identifying a SYN Flood can be challenging because the incoming flood of requests may appear as legitimate traffic. However, tools and software exist to monitor network traffic and detect unusual activity that could be indicative of a SYN Flood.

5. Can a SYN Flood attack be traced back to the source?

Tracing a SYN Flood back to its source is typically difficult because attackers often spoof IP addresses to make the flood of requests appear as if they’re coming from multiple sources. While it’s not impossible to trace these attacks, it often requires substantial time, knowledge, and resources.

"Amateurs hack systems, professionals hack people."
-- Bruce Schneier, a renown computer security professional
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