How to defend against SYN flooding attacks? Detailed explanation of 7 practical protection measures

in the high-speed operation of Internet services, network attacks have become one of the core risks threatening the stability of services, among which SYN flooding attack is a very representative type of distributed Denial-of-service. It takes advantage of the TCP three-way handshake protocol vulnerability, consumes server resources by sending a large number of forged SYN requests, and eventually leads to normal users unable to establish connections. This article will start from technical principles and actual configuration to disassemble seven practical protection measures against SYN flooding attacks for readers, and help business entities of different sizes to build effective defense barriers.

, how to resist SYN flooding attacks through system configuration?

operating system level is the basic line of defense against SYN flooding attacks, which can quickly improve the server's anti-attack ability without additional hardware investment.

1, adjust the SYN queue length parameter

in the Linux system, you can expand the capacity of the semi-connected queue by modifying the tcp_max_syn_backlog parameters, so that the server can accommodate more SYN requests to be processed; at the same time, the tcp_syncookies parameters are turned on, and when the queue is full, an encrypted SYN cookie will be generated instead of the semi-connected storage to avoid SYN flooding attacks. The Windows system can adjust the SynAttackProtect parameter through regedit to enable the attack protection mode.

2, shorten the half connection timeout time

default, the server will reserve a long timeout time for the semi-connected state, which will leave an opportunity for SYN flooding attacks. By modifying the tcp_synack_retries parameters to reduce the number of retransmissions of SYN + ACK packets and shorten the timeout time of tcp_fin_timeout parameters, invalid semi-connected resources can be quickly released, reducing the risk of continuous resource occupation.

Second, how to use network equipment to intercept SYN flooding attacks?

professional network hardware equipment has stronger data packet processing capabilities and is the core line of defense against large-scale SYN flooding attacks.

1. Enable the SYN protection function of the firewall

most enterprise firewalls have built-in SYN flood attack detection module, which can automatically trigger the interception mechanism when the number of SYN packets per unit time exceeds the set value by setting the SYN request threshold. Some firewalls can also identify the forged source Internet Protocol Address and directly discard the SYN request from the abnormal address, effectively filtering the attack traffic.

2. Deploy load balancing equipment shunt

load balancing device can distribute user requests to multiple backend servers, which can not only improve business processing efficiency, but also disperse the pressure of SYN flooding attacks. When a single server is attacked, load balancing will automatically adjust the traffic allocation strategy to avoid single node resource exhaustion. At the same time, some high-end load balancing devices also have SYN proxy function, which can replace the backend server to complete three handshakes and filter out invalid attack requests.

Which network architectures are optimized to defend against SYN flooding attacks?

adjusted from the network architecture level, it can reduce the impact range of SYN flooding attack from the root cause and improve the fault tolerance of the overall defense.

1, using CDN node shunt cleaning

traffic access to the CDN network, the user request will first reach the nearest CDN node, CDN will clean the traffic in real time, identify and intercept SYN flood attack traffic, and only forward normal requests to the source server. This method can not only hide the real Internet Protocol Address of the source server, avoid attacks directly hitting the core node, but also use the distributed nodes of CDN to disperse the attack pressure.

2, build a multi-node cluster architecture

by building a distributed server cluster, the business load is distributed to multiple nodes. Even if some nodes encounter SYN flooding attacks, the rest of the nodes can still provide services normally. At the same time, with the health detection function of the cluster management system, the attacked abnormal nodes can be automatically isolated to ensure the availability of the overall business.

IV. How to deal with SYN flooding attacks through protocol optimization?

for the inherent vulnerability of TCP protocol, through protocol optimization or alternative, we can reduce the opportunity of SYN flooding attack from the root cause.

1, using TCP fast handshake extension

enable the TCP Fast Open extension function, so that the server caches the client's verification information during the first handshake, and subsequent connections do not require a complete three-way handshake process, reducing the resource consumption of the half-connection state. This method can not only improve the establishment efficiency of normal connections, but also reduce the proportion of resources occupied by SYN flooding attacks.

2, try to use QUIC protocol instead of TCP

QUIC protocol is based on UDP implementation, using 0-RTT or 1-RTT handshake, without going through the three-way handshake process of TCP, fundamentally avoiding the protocol vulnerabilities exploited by SYN flooding attacks. At present, mainstream browsers and Cloud as a Service vendors have gradually supported QUIC protocol, and latency-sensitive businesses can try to deploy this protocol to improve anti-attack capabilities.

sum up, to resist the SYN flood attack requires the construction of a multi-layer defense system, from system parameter optimization, network equipment interception to architecture adjustment, protocol replacement, seven measures cover multiple dimensions from basic configuration to advanced architecture. Operations personnel can flexibly combine these protection measures according to the business scale and attack risk level, while ensuring service availability, effectively reducing the resource consumption and business interruption risk caused by SYN flood attacks.