Question

I am writing a client-side FTP program, and so far, after a successful connection the server will run in extended passive mode. Using the port number returned from the EPSV command, I can create client-side sockets like this:

void create_data_channel() {
  if ((data_sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    perror("Cannot create client socket for data connection :(");
    exit(1);
  }

  data_server_addr.sin_family = AF_INET;  
  data_server_addr.sin_port = htons(port);
  data_server_addr.sin_addr = *((struct in_addr *)ftp_server->h_addr);
  bzero(&(data_server_addr.sin_zero),8); 

  // Connect to the ftp server at given port for data connection
  if (connect(data_sock, (struct sockaddr *)&data_server_addr, 
    sizeof(struct sockaddr)) == -1) {
    perror("Cannot connect to the ftp server for data connection :(");
    exit(1);
  }
}

Now, whenever I want to send a command involving the data channel (e.g. LIST), I can first open a new socket using the method above, and get/send whatever data I need from/to the ftp server. Then, I close the data connection using close(data_sock).

This works well for the first LIST command. However, if I were to try to run two or more LIST command, the program fails with my error message "Cannot connect to the ftp server for data connection :(". Why is this so? What am I missing here?

Answer 1

通常，FTP服务器不接受到同一动态端口的多个连接。因此，需要在每次数据传输之前完成PASV或EPSV命令，以便服务器创建一个新的侦听套接字并将其端口号返回给客户端。

TCP Connection: Recreating a socket that has been closed

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