How to use PORT command while writing an FTP client with JAVA

Question

I am trying to write an FTP client using java.

As far as I understand I need to send my IP adress with PORT command when I need to establish an active connection but, How do I get that IP address?

I am trying to do something like this but it is obviously wrong:

ServerSocket ssock = new ServerSocket();
ssock.bind(null);
ssock.ssock.getLocalPort();
ssock.getInetAddress();

Last line returns null.

So again how can I send my IP address and port to server?

Thank you.

Answer 1

You need to connect first, then call the socket.getLocalAddress() to get the correct local address. When you have multiple IPs, you only know the correct address to reach a specific destination after connection is established.

Sun JRE comes with a FTP client sun.net.ftp.FtpClient. You can check out how they do it.

With wide-spread use of NAT and proxies, PORT command rarely works. You should try passive mode first. As you can see, Sun's client only does PORT when PASV fails.