Question

I am new to socket progamming and I am trying to implement a stripped down FTP like program.
It uses two TCP connections, one as a control connection and other as a data connection. The problem is that I do not know how to use the sever to connect to the client’s N+1 port using its port 20. Please refer here, to fund out more.

Answer 1

To create the active mode data connection, you:

• Find the local address of the control connection with getsockname();
• modify this address by changing the port number to 20;
• create another socket with socket();
• bind the new socket to the port 20 address created with bind();
• connect the socket to the client’s address/port with connect().

Note that the bind() will likely fail if your daemon is not running as root, because binding a low port number is a privileged operation.