The DDOS (Distributed Denial of Service) attack is a network attack. The attacker sends a large amount of requests or data traffic to the target server to make the server overload and cannot handle legal requests normally, which makes the target server unable to provide services outside.

　　There may be many reasons for the server to be attacked by DDOS, such as:

　　Port attack: The attacker sends a large amount of requests or packets to a certain port on the server to make the server overload.

　　SYN attack: The attacker uses the vulnerability in the TCP protocol to send a large number of SYN requests to the server to make the server unable to handle legal requests.

　　DNS attack: The attacker sends a large number of requests to the DNS server to operate the DNS server overload, which affects the normal operation of the server.

　　If the server is attacked by DDOS, the following are some response measures:

　　Using firewall: Setting a firewall can filter out a lot of malicious flow to protect the server from attack.

　　Use CDN: Use CDN (content distribution network), you can distribute the content of the website to multiple nodes to disperse the attack power of the attacker.

　　Increasing bandwidth: Increasing the bandwidth of the server can withstand more traffic, thereby reducing the impact of DDOS attacks.

　　Use DDOS protection service: Using professional DDOS protection services can help reduce the impact of DDOS attacks and ensure the normal operation of the server.