What is SYN flood? Analysis of the core concepts and principles of network attacks

in the current digital office and daily Internet scenarios, network attacks have become a common risk that threatens system stability and data security. SYN flood, as a classic distributed distributed denial-of-service method, is often used by attackers to paralyze target servers. This article will analyze the core principles, identification methods, and defense strategies of SYN flood to help readers systematically understand such attacks, master the key knowledge of SYN flood, and build a defense line for their own cyber security.

How did SYN flood launch the attack?

to understand the harm of SYN flood, we must first clarify its attack logic, which is inseparable from the understanding of the TCP three-way handshake mechanism.

1, exploiting the vulnerability of TCP three-way handshake

TCP protocol establishes a reliable connection through a three-way handshake: the first step is that the client side sends a SYN request message to the server, the second step is that the server responds to the SYN + ACK confirmation message and waits for the client side to reply, and the third step is that the client side sends the ACK message to complete the connection. SYN flood is the use of the second step of the waiting window. The attacker forges a large number of SYN requests for false Internet Protocol Addresses. The server cannot receive the ACK message after replying to SYN + ACK, and can only continue to occupy the connection resources and wait. When the waiting queue is full, the request of the legitimate user will be rejected.

2, the large-scale implementation of the attack

a single attacker has limited internet bandwidth, it is difficult to initiate a large-scale SYN flood, so the attacker usually controls a group of compromised devices to form a botnet and send fake SYN requests to the target server. These botnets are distributed in different network nodes and can send massive requests in a short time, quickly depleting the server's connection resources and paralyzing the target server.

What are the identification characteristics of SYN flood attack?

timely identification of SYN flood attacks is the premise of defense, such attacks will show obvious abnormal characteristics in network traffic and server status.

1, server connection queue abnormal overflow

under normal circumstances, the server's TCP half-connection queue is in a dynamic balance state, and when encountering SYN flood attacks, the half-connection queue will be filled quickly. By looking at the server system log or network monitoring tools, it can be found that the number of half-connections far exceeds the normal threshold. At the same time, the server will frequently appear "connection timeout" "insufficient resources" error message.

2, abnormal changes in network traffic

SYN flood attack will lead to a surge in the number of inbound SYN packets on the target server. Compared with normal traffic, the proportion of SYN packets will increase significantly. At the same time, the source Internet Protocol Address shows the characteristics of dispersion and irregularity, and some source IPs are even false addresses that do not exist. Through traffic analysis tools, it can be found that the header information of these SYN packets is incomplete and lacks the necessary identification for subsequent connections.

What are the specific hazards of SYN flood attacks?

SYN flood attack will not only affect the normal operation of the server, but also cause multi-dimensional harm to enterprises and individuals relying on network services.

1, causing the target service to be completely paralyzed

when the SYN flood attack reaches a certain scale, the server's connection resources will be completely exhausted, unable to handle any legitimate user requests, whether it is the enterprise's official website, e-commerce platform or office system, will fall into an inaccessible state, directly affecting the normal development of business, bringing considerable economic losses to the enterprise.

2, causing chain network failure

SYN flood attack generated massive false traffic will also occupy internet bandwidth, resulting in congestion throughout the network link, not only the target server can not work properly, other devices within the same network will also be affected, network speed slows down, delay increases and other issues, and even cause the entire LAN chain failure, expand the scope of the attack.

4. How to effectively defend against SYN flood attacks?

the attack principle and characteristics of SYN flood, a mature defense system has been formed in the industry, and there are corresponding protection means from server configuration to network architecture.

1 Optimize server TCP connection parameters

administrators can improve the server's ability to deal with SYN floods by adjusting the server's TCP half-connection queue length and reducing the half-connection timeout time. For example, reducing the half-connection timeout from the default 75 seconds to 30 seconds can quickly release invalid half-connection resources and avoid the queue being filled with false requests.

2, using SYN cookie verification mechanism

SYN cookie is a mainstream defense technology, it does not require the server to store semi-connection information, but generates the serial number in the SYN + ACK message through a specific algorithm. When the client side replies to the ACK message, the server verifies the legitimacy of the serial number through the algorithm. Only by verification will a connection be established, which avoids the exhaustion of semi-connected resources by SYN flood attacks from the root.

3, deploy professional network protection equipment

enterprises can deploy intrusion detection systems, load balancing devices or professional anti-DDoS firewalls, these devices can monitor network traffic in real time, identify and filter false packets generated by SYN flood attacks, forward legitimate requests to the server, and through traffic cleaning function, abnormal traffic will be stripped from normal traffic to ensure the smooth flow of network links.

To sum up, SYN flood is a typical Distributed Denial-of-service initiated by exploiting TCP protocol vulnerabilities. Its core is to exhaust server connection resources through false requests. We need to master the attack principle and identification characteristics of SYN flood in order to find the attack signal in time; at the same time, by optimizing server parameters, adopting SYN cookie mechanism, deploying protective equipment and other means, the attack risk of SYN flood can be effectively reduced, and the stable operation of the network system can be guaranteed.