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这次的头名张牧之是除杨长贵外第一个得铃的，人们不由得紧张起来，这铃不仅要挂，还要早挂，挂得越早越管用。秀水河畔纷纷雨，闲谈故人酒一盅。有些旧事或许能在时光的罅隙里，被如今之人窥到端倪，翻出了一两张旧时稚嫩的字画以作凭证，而有些，或许只能葬在时间的洪流里，像祭在坟前的那壶酒，只够醉了偶尔经过的春风。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.香儿抬头看向对面，娇声道：大叔，你想问什么，就问我吧。能力和号召力都极强的路桥川幸运的当上了班长，哪知道这个沉重的头衔给路桥川带来了荣耀，却也带来了数不清的麻烦和巨大的压力。在屡屡的失败面前，路桥川打起了退堂鼓，然而，同学和朋友们的信任让路桥川再度振作了起来，一转眼大一就要接近尾声。One problem with the factory method pattern is that the creation of the class depends on the factory class, that is to say, if you want to expand the program, you must modify the factory class, which violates the closure principle. Therefore, from the design point of view, there are certain problems, how to solve them? We use the abstract factory mode to create multiple factory classes, so that once new functions need to be added, we can directly add new factory classes without modifying the previous code. Because the abstract factory is not very easy to understand, let's look at the diagram first, and then it is easier to understand with the code.哥哥真是守信之人啊。