network socket programing. ftp服务器程序，有USER,PASS,PASV,PORT,LIST,RETR命令，在telnet下可以调试。-network socket programing. Ftp server, have USER, PASS, PASV, PORT, LIST, RETR command, In telnet can be debugging.

很经典的ftp服务器实现，实现为一个类，附带一个命令行的demo， 可跨平台-Classic implementation of the ftp server, a type of implementation, with a command-line demo, can be cross-platform

FTP服务器 能够实现监听 打开目录-In active mode FTP the client connects from a random unprivileged port (N > 1024) to the FTP server’s command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server.