Table of Contents

• What is a SYN Flood Attack?
• Understanding the TCP Handshake
• What Are SYN Cookies?
• How SYN Cookies Mitigate SYN Floods
• Pros and Cons of SYN Cookies
• Comparison of SYN Cookie Implementations
• Implementing SYN Cookies
• Best Practices for Using SYN Cookies
• Conclusion
• Frequently Asked Questions

What is a SYN Flood Attack?

A SYN flood attack is a type of Denial of Service (DoS) attack that targets a web server’s ability to establish new connections. It exploits a fundamental part of how servers communicate over the internet using the TCP (Transmission Control Protocol) handshake. By sending a flood of fake connection requests, attackers overwhelm the server’s resources, making it slow or completely unavailable to legitimate users.

Imagine a mailbox stuffed with thousands of fake letters, leaving no room for real mail. In a SYN flood, the server’s connection queue gets filled with fake requests, preventing real users from connecting. This can disrupt websites, online services, or even critical infrastructure, causing downtime and frustration.

Understanding the TCP Handshake

To grasp how SYN cookies work, we first need to understand the TCP handshake, the process that establishes a connection between a client (like your browser) and a server. The handshake has three steps:

• SYN (Synchronize): The client sends a SYN packet to the server, requesting a connection.
• SYN-ACK (Synchronize-Acknowledge): The server responds with a SYN-ACK packet, acknowledging the request and reserving a spot in its connection queue.
• ACK (Acknowledge): The client sends an ACK packet to confirm the connection, and communication begins.

In a SYN flood attack, the attacker sends countless SYN packets but never completes the handshake by sending the ACK. Each incomplete connection takes up space in the server’s queue, which is limited. When the queue fills up, the server can’t accept new connections, effectively shutting down access for legitimate users.

What Are SYN Cookies?

SYN cookies are a clever technique to protect servers from SYN flood attacks without using up valuable resources. Instead of storing every incoming SYN request in a queue, the server encodes key connection details into a “cookie” (a small piece of data) and sends it back to the client in the SYN-ACK packet. If the client is legitimate and completes the handshake by sending an ACK, the server uses the cookie to reconstruct the connection details without needing to store them initially.

Think of SYN cookies as a restaurant reservation system where, instead of holding a table for every caller, the restaurant gives each caller a ticket with all the details. Only when the guest shows up with the ticket does the restaurant prepare a table. This saves space and ensures only real guests get served.

How SYN Cookies Mitigate SYN Floods

SYN cookies work by eliminating the need to store pending connections during a TCP handshake, which is the main vulnerability exploited by SYN floods. Here’s a step-by-step explanation:

• Receiving a SYN Packet: When a client sends a SYN packet, the server doesn’t store it in the connection queue as usual.
• Creating the Cookie: The server generates a SYN cookie, a unique value based on the client’s IP address, port number, a timestamp, and a secret key known only to the server. This cookie is embedded in the sequence number of the SYN-ACK packet.
• Sending SYN-ACK: The server sends the SYN-ACK packet with the cookie to the client and discards the SYN request from memory, freeing up the queue.
• Validating the ACK: If the client is legitimate, it sends an ACK packet back. The server uses the cookie in the ACK to verify the connection and recreate the necessary state without ever having stored it.
• Ignoring Fakes: If the client is an attacker and doesn’t send an ACK, the server doesn’t waste resources, as no queue space was used.

This approach ensures the server can handle legitimate connections even during a flood of fake SYN requests, keeping the service available.

Pros and Cons of SYN Cookies

Like any technology, SYN cookies have strengths and weaknesses. Understanding these helps you decide when and how to use them.

• Pros:
  • Saves server memory by not storing incomplete connections.
  • Effective against SYN flood attacks, even large-scale ones.
  • Transparent to clients; no changes needed on their end.
  • Supported by most modern operating systems (e.g., Linux, Windows).
• Cons:
  • Limited data in cookies means some TCP options (e.g., window scaling) may not be supported during the handshake.
  • Requires computational resources to generate and verify cookies, which can strain servers under heavy load.
  • Not effective against other types of attacks, like application-layer floods.
  • May require configuration tweaks for optimal performance.

Comparison of SYN Cookie Implementations

Different operating systems and tools implement SYN cookies in slightly different ways. Below is a comparison to help you choose the right setup.

Comparison of SYN Cookie Implementations

Different platforms and tools handle SYN cookies in unique ways, each with its own setup process and performance considerations. This table compares key options based on the latest available information as of September 2025.

This comparison helps server administrators choose the best implementation for their needs, balancing ease of use with performance efficiency.

Implementing SYN Cookies

Enabling SYN cookies depends on your server’s operating system or software. Here’s how to do it on a Linux server, one of the most common platforms:

• Check if SYN Cookies Are Enabled: Run cat /proc/sys/net/ipv4/tcp_syncookies. A value of 1 means it’s enabled; 0 means it’s disabled.
• Enable SYN Cookies: Use the command sudo sysctl -w net.ipv4.tcp_syncookies=1 to enable it temporarily. To make it permanent, add net.ipv4.tcp_syncookies=1 to /etc/sysctl.conf.
• Verify Settings: Restart the network service or reboot the server, then check again to confirm the setting is active.
• Monitor Performance: Use tools like netstat or ss to monitor connection queues and ensure SYN cookies are working as expected.

For other platforms like Windows or NGINX, consult their documentation for specific steps, as the process varies.

Best Practices for Using SYN Cookies

To maximize the effectiveness of SYN cookies, combine them with other security measures:

• Combine with a WAF: A Web Application Firewall can filter application-layer attacks that SYN cookies don’t address.
• Use a CDN: Content Delivery Networks like Cloudflare can absorb attack traffic before it reaches your server.
• Monitor Traffic: Use tools like Nagios or Zabbix to detect unusual traffic patterns and respond quickly.
• Optimize Server Resources: Ensure your server has enough CPU and memory to handle cookie calculations during an attack.
• Regularly Update Software: Keep your operating system and server software updated to benefit from the latest security improvements.

Conclusion

SYN flood attacks can cripple a web server by exploiting the TCP handshake, but SYN cookies offer a smart, resource-efficient defense. By encoding connection details into a cookie and avoiding the need to store pending requests, SYN cookies keep your server running smoothly even under attack. While they have some limitations, such as reduced support for advanced TCP options, their benefits make them a must-have for any server admin. By implementing SYN cookies alongside other protections like WAFs, CDNs, and monitoring tools, you can build a robust defense against SYN floods and keep your website accessible to legitimate users. Stay proactive, and your server will thank you!

Frequently Asked Questions

What is a SYN flood attack?

A SYN flood attack is a type of DoS attack that overwhelms a server with fake TCP connection requests, filling its connection queue and blocking legitimate users.

What are SYN cookies?

SYN cookies are a technique where a server encodes connection details in a cookie sent to the client, avoiding the need to store pending connections.

How do SYN cookies stop SYN floods?

They prevent the server’s connection queue from filling up by not storing SYN requests, only validating legitimate connections when the client responds.

Are SYN cookies enabled by default?

On many Linux systems, SYN cookies are disabled by default but can be enabled with a simple configuration change.

Do SYN cookies work on all servers?

Most modern operating systems, like Linux and Windows, support SYN cookies, but implementation varies by platform.

Can SYN cookies stop all DDoS attacks?

No, SYN cookies only protect against SYN flood attacks, not other types like application-layer or volumetric DDoS attacks.

What is the TCP handshake?

The TCP handshake is a three-step process (SYN, SYN-ACK, ACK) that establishes a connection between a client and a server.

Do SYN cookies affect legitimate users?

No, SYN cookies are transparent to legitimate users and don’t impact their ability to connect.

Can I use SYN cookies with a CDN?

Yes, SYN cookies work well with CDNs, which can further reduce attack traffic before it reaches your server.

How do I enable SYN cookies on Linux?

Run sudo sysctl -w net.ipv4.tcp_syncookies=1 and add it to /etc/sysctl.conf for persistence.

Do SYN cookies use a lot of server resources?

They require some CPU for cookie calculations, but the impact is minimal compared to storing thousands of connections.

What are the limitations of SYN cookies?

They don’t support all TCP options, like window scaling, and are ineffective against non-SYN flood attacks.

Can attackers bypass SYN cookies?

It’s difficult to bypass SYN cookies, as they rely on cryptographic techniques and don’t store exploitable data.

Do I need SYN cookies if I use a WAF?

A WAF helps with application-layer attacks, but SYN cookies are still useful for protecting against SYN floods at the network level.

How can I tell if my server is under a SYN flood attack?

Look for slow performance, high connection queue usage, or spikes in SYN packets using tools like netstat or ss.

Are SYN cookies secure?

Yes, they use cryptographic methods to ensure cookies are hard to forge, making them a secure defense.

Can I test SYN cookies on my server?

Yes, use network testing tools or hire a cybersecurity firm to simulate SYN floods and verify your setup.

Do SYN cookies work with NGINX?

Yes, NGINX supports SYN cookies through its TCP configuration, but you may need to enable it manually.

Is there a performance penalty for using SYN cookies?

There’s a small CPU overhead for generating cookies, but it’s negligible compared to the impact of a SYN flood.

Should I combine SYN cookies with other protections?

Yes, combining SYN cookies with WAFs, CDNs, and monitoring tools provides the best defense against various attacks.