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serv-u相关的防火墙设置（Serv-U related firewall settings）.doc

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serv-u相关的防火墙设置（Serv-U related firewall settings）

serv-u相关的防火墙设置（Serv-U related firewall settings） Three, Serv-u related firewall settings: One of the most common challenges about firewall processing is the difference between active FTP and passive FTP, as well as how to configure firewalls and support them perfectly. Many administrators may find that using FTP transport on a firewall server always has such a small problem, and sometimes data transmission is not smooth enough. Fortunately, this article can help you figure out how to support FTP in a firewall environment. FTP services are just TCP based services that do not support UDP. The difference is that FTP uses 2 ports, a data port, and a command port (also known as a control port). Typically, these two ports are command ports (21) and data ports (20). But when we find that the data port is not always 20, the new problem comes out, depending on the way (FTP work). Active FTP: The active way of FTP is this: the client connects to the command port of the FTP server, that is, the 21 port, from an arbitrary, non privileged port, N (N; 1024). The client then starts listening to port N+1 and sends the FTP command port N+ 1 to the FTP server. The server then connects to the data port (N+1) specified by the client from its own data port (20). For the firewall in front of the FTP server, you must allow the following communication to support the active FTP Any port to the 21 port of the FTP server (client initialization connection S) 21 port of FTP server to port greater than 1023 (server responds to clients control port S-C) FTP server 20 port to port greater than 1023 (server side initialization data connect to clients data port S-C) 20 port larger than 1023 port to FTP server (client sends ACK response to servers data port S) The main problem with active FTP is actually the client. The FTP client does not actually build a connection to the server data port. It simply tells the server to listen to its port number, and the server comes back to connect to the specified