How to protect SYN flood? Detailed explanation of three practical protective measures

behind the high-speed operation of internet services, cyber security threats always go hand in hand. SYN flood, as a typical distributed distributed denial-of-service method, has become the first choice for many black-product attacks due to its low cost and high destructive power. This type of attack uses up server semi-connection queue resources by forging a large number of TCP connection requests, eventually leading to normal users unable to access services. This article will combine actual operation and maintenance scenarios to explain in detail a variety of practical SYN flood protection methods to help operation and maintenance personnel build effective defense barriers.

SYN flood protection core: SYN cookie verification

the core principle of SYN flood attack, SYN cookie verification is a classic defense scheme starting from TCP connection logic, which can avoid the exhaustion of semi-connected queues from the root.

1, SYN Cookie work logic

when the server receives a SYN request, it will not directly store it in the semi-connection queue, but generate a cookie value through a specific algorithm, including timestamp, server port and other information, and return it to the requester with the serial number of SYN + ACK packets. Only when the requester returns an ACK message carrying the correct cookie, the server will establish a complete connection for it, fundamentally avoiding the queue resource consumption caused by SYN flood attacks.

2, SYN cookie deployment points

most Linux system kernels already support the SYN cookie function, which can be enabled by modifying the sysctl parameters without additional software installation. When activating, you need to pay attention to adjusting the effective time of the cookie. If it is too short, it may lead to a timeout of the normal connection, and if it is too long, it will reduce the defense flexibility. At the same time, for high concurrency scenarios, it is necessary to cooperate with traffic monitoring tools to automatically enable this function when the SYN flood attack triggers the threshold to balance the defense effect and service performance.

SYN flood pre-protection: boundary flow cleaning

in addition to optimizing from the server, intercepting malicious traffic in advance at the network boundary is a key pre-means to reduce the impact of SYN flood attacks on core servers.

1, professional flow cleaning equipment deployment

enterprises can deploy professional DDoS traffic cleaning equipment at the core network entrance, which can identify SYN flood attacks based on traffic characteristics, such as a large number of SYN requests from the same IP in a short period of time, abnormal message formats, etc. The device will directly discard malicious traffic, only the verified normal traffic is forwarded to the back-end server, which greatly reduces the defensive pressure of the server.

2, cloud platform traffic cleaning service

for enterprises using Cloud as a Service, they can directly use the traffic cleaning service provided by cloud vendors without deploying hardware equipment. The cloud platform will accurately identify the SYN flood attack traffic based on global traffic data, complete the cleaning through distributed nodes, and support elastic expansion. It can cope with large-scale SYN flood attack scenarios and is suitable for small and medium-sized enterprises to quickly build a protection system.

SYN flood optimization: kernel parameter tuning

adjust the TCP connection related parameters from the server kernel level, which can improve the server's ability to resist SYN flood attacks and is the basic link in the defense system.

1, semi-connected queue and fully connected queue expansion

expand the capacity of the semi-connected queue by modifying the tcp_max_syn_backlog parameters, and adjust the somaxconn parameters to expand the full-connected queue, so that the server can accommodate more pending connection requests. However, this method can only mitigate small-scale SYN flood attacks. If it encounters a large-scale attack, it still needs to cooperate with other protection measures to avoid service interruption caused by queue overflow.

2, SYN message retransmission parameter adjustment

adjust the tcp_synack_retries parameters to reduce the number of retransmissions of SYN + ACK packets, shorten the timeout time of half connections, and allow the server to release invalid half-connection resources faster. At the same time, set the tcp_syncookies parameter to 1 to open the kernel-level SYN cookie verification, which complements the queue expansion and further enhances the server's tolerance to SYN flood attacks.

SYN flood auxiliary protection: abnormal flow monitoring

effective traffic monitoring can help operation and maintenance personnel find the precursors of SYN flood attacks in time, buy time for the start of protective measures, and avoid large-scale impact of attacks.

1 Establishing traffic baseline thresholds

collect data such as the daily SYN request volume and connection success rate of the server through monitoring tools to establish a normal traffic baseline. When the SYN request volume exceeds 2-3 times of the baseline in a short period of time, or the connection success rate plummets, it can be determined as a suspected SYN flood attack, triggering an alarm to notify the operation and maintenance personnel.

2, multi-dimensional traffic analysis

in addition to the traffic size, the traffic needs to be analyzed from the dimensions of request source and message characteristics. If a large number of SYN requests are found to come from the same IP segment, or the packets lack the necessary fields, it can be directly determined as SYN flood attack traffic, and the abnormal IP segment is temporarily blocked through firewall rules to quickly curb the attack situation.

To sum up, the protection of SYN flood attack is a multi-level system, the core is to build a defensive barrier from the four dimensions of connection logic, traffic interception, kernel optimization, and monitoring and early warning. SYN cookie verification avoids resource consumption from the root cause, intercepts malicious requests before border traffic cleaning, enhances server tolerance by tuning kernel parameters, and provides timely early warning for protection by abnormal monitoring. Operation and maintenance personnel can choose the appropriate protection combination according to their own business scale and attack risk to ensure the stability and reliability of network services.