SYN flood

SYN Flood Definition

A SYN flood is a type of denial-of-service (DoS) attack where an attacker sends a large number of SYN (synchronize) requests to a target server, overwhelming it with connection requests and causing it to become unresponsive to legitimate traffic. This flood of requests exhausts the server's resources and prevents it from processing genuine connections.

How SYN Flood Works

A SYN flood attack takes advantage of the three-way handshake process used to establish a connection between two devices. Here's how it works:

1. The attacker initiates the attack by sending a large number of SYN packets to the target server. Each SYN packet contains the attacker's IP address and a randomly generated sequence number.
2. Upon receiving the SYN packet, the server allocates resources for a potential connection and sends back a SYN-ACK (synchronize-acknowledgment) packet to the attacker's IP address.
3. In a normal scenario, the attacker's device should respond to the server's SYN-ACK packet with an acknowledgment (ACK) packet to complete the handshake and establish a connection. However, in a SYN flood attack, the attacker does not send the final ACK packet.
4. The server, expecting the final ACK, keeps the half-open connections in a queue, waiting for the acknowledgment. As a result, the server's resources, such as memory and connection slots, get exhausted over time, leaving it unable to handle legitimate connection requests.

Prevention Tips

To protect against SYN flood attacks, consider implementing the following measures:

1. Implement SYN cookies: SYN cookies are a technique where the server doesn't allocate any resources until it receives the final ACK from the client. This mitigates the impact of SYN flood attacks by avoiding the accumulation of half-open connections.
2. Configure firewalls: Configure firewalls to detect and drop malicious SYN packets. Firewalls with stateful packet inspection capabilities can analyze the traffic at the network and transport layers, identifying and preventing suspicious SYN flood patterns.
3. Use rate-limiting measures: Implement rate-limiting mechanisms to prevent an overwhelming number of connection requests from a single source. This can help regulate the rate at which SYN packets are received, reducing the impact of an attack.
4. Employ DoS protection services: Consider using DoS protection services that specifically detect and mitigate SYN flood attacks. These services can offer additional layers of defense to your network infrastructure.
5. Keep network devices up-to-date: Regularly update and apply security patches to network devices, including routers, switches, and firewalls. Patching helps address vulnerabilities that attackers may exploit to launch SYN flood attacks.

Related Terms

• Denial-of-Service (DoS) Attack: A cyberattack that disrupts services or networks, making them unavailable to users. SYN flood attacks are a specific type of DoS attack.
• Firewall: A network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Firewalls can play a critical role in detecting and preventing SYN flood attacks.
• SYN Cookies: A technique used to mitigate SYN flood attacks by not allocating resources until the final acknowledgment is received. SYN cookies help prevent resource exhaustion caused by half-open connections in the server.