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2Across half of China to sleep with you电视和网络上充斥着本该死去的人在全世界复活的消息。不过魏队长说的是，真金不怕火炼。不过，等到和其他水军同行交流后，这个水军顿时平衡了。When a client accesses a server's web service, The client sends a message to the network card, The tcp/ip stack is part of the kernel, Therefore, the client information is transferred to the web service in user space through the kernel's TCP protocol, At this time, the destination destination of the client message is on the socket (IP: Port) that the web service is listening for. When a web service needs to respond to a client request, The destination destination of the response message sent by the web service is the client, At this time, The IP and port that the web service listens on becomes the origin instead, As we said, Netfilter is the real firewall, It is part of the kernel, Therefore, if we want the firewall to achieve the goal of "fire prevention", You need to set up checkpoints in the kernel. All incoming and outgoing messages have to pass through these checkpoints. After checking, those that meet the release conditions can be released, while those that meet the blocking conditions need to be blocked. As a result, input checkpoints and output checkpoints appear. These checkpoints are not called "checkpoints" but "chains" in iptables.