Scanning Foreign Host's Ports

Question

I Im writing a port scanner in java as an educational exercise and I have run into a problem. My program works fine when im scanning the ports on my local machine, however when i try to scan for ports on foriegn computers the process just freezes. Here is the code for my scan method:

public void scan(InetAddress ad, int start, int end){
    for(int i=start; i<=end; i++){
        try{
            socket = new Socket(ad, i);
            System.out.println("Port "+i+"is open");
        }catch(Exception e){
            System.out.println("Port "+i+"is not open");
        }
    } 
}

So far it is working when the host name is my loopback address or just "localhost", it even works with my IP Address. But when I for instance resolve the IP for www.google.com or even give it the IP address of my other computer on the same network, the program will freeze after scanning the first port.

I am resolving the IP address like this:

inetAddressObject = InetAddress.getByName(hostNameString);

Can anyone tell me if I am missing a big difference between scanning ports on a local machine and scanning ports on a foreign host? Or is there just something simple going wrong with my code ?

Answer 1

The way you are calling the Socket constructor the connection will be made right away. The problem is that if the connection times out then you will have to wait a while (OS default). The way you should do it is the following:

Socket s = new Socket();
s.connect(new InetSocketAddress(ad, i), timeout);

The separate reason is why it is timing out. This is because your machine is likely behind a firewall of some sorts and is unrelated to your code example.