SYN Flood is a type of Denial of Service (DoS) attack that aims to overwhelm a network resource by sending a large number of TCP SYN packets, exceeding the capacity that the protocol implementation can handle.

What is a TCP SYN packet?

To fully understand a SYN Flood attack, it is helpful to know what a TCP SYN packet is. TCP (Transmission Control Protocol) is one of the main protocols used to transmit data over the Internet. When two computers want to communicate via TCP, they begin with a “three-way handshake.” The first phase of this handshake is the sending of a SYN (synchronize) packet by the client to the server to request the opening of a connection.

How does a SYN Flood attack work?

During a SYN Flood attack, the attacker sends a massive number of connection requests (SYN packets) to a target server, often using spoofed IP addresses. When the server receives these SYN packets, it responds with a SYN-ACK (acknowledgment) packet and waits for a final response to complete the connection. However, the attacker never sends this final response, leaving the connections in a “half-open” state.

Each incomplete connection occupies server system resources, such as memory and communication ports. If the number of these partial connections exceeds the server’s capacity, the server is no longer able to handle new legitimate connections, causing a service disruption.

Effects of a SYN Flood attack

The effects of a SYN Flood attack can vary depending on the server configuration and the security countermeasures in place. Generally, the effects include:

• Performance degradation: The server becomes slow in responding to legitimate requests.
• Denial of service: The server may be unable to respond to any legitimate requests, making the service inaccessible.
• Resource exhaustion: Excessive consumption of system resources can lead to a server crash.

Countermeasures and prevention

To protect against SYN Flood attacks, organizations can adopt several strategies:

• Increasing resources: Allocating more resources to the server can help handle a larger number of requests.
• Packet filtering: Using firewalls and routers to filter and block suspicious traffic.
• SYN Cookies: This technique allows the server to respond to SYN requests without allocating resources until the connection is fully established.
• Reducing the timeout for incomplete connections: Configuring the server to reduce the time it waits for a final response from a partially open connection.

SYN Flood attacks represent a significant threat to the availability of network services. Understanding these attacks and implementing appropriate security measures are fundamental to maintaining the stability and security of network infrastructures.