Why can't my socket programs connect to the internet? - sockets

Question

I have begun my first course on networking this semester.
My problem is that whenever I try to connect my socket to a host outside my LAN,
network unreachable error is returned. Whether they be BSD sockets or Java sockets.
Moreover, my nmap probes also return the same error.
Once I asked a similar question here about ping probes and the answer was that my LAN proxy is rejecting ICMP requests. But there are no ICMP requests in establishing TCP connections right?
Why, on the other hand, my browser can connect to any host... although that it uses the same proxy...
Also,(please pardon me for this long doubt) when I give host name as www.google.com in my sockets, "unknown host" is returned. But my browser happily recognizes the same host.
Thank you in advance...
import java.net.*;
import java.io.*;
class Whois {
public static void main(String args[]) throws Exception{
int c;
Socket s=new Socket(args[0],Integer.parseInt((args[1])));
InputStream in=s.getInputStream();
while( (c=in.read()) != -1) {
System.out.print((char) c);
}
s.close();
}
}

Answer 1

Unfortunately, there may be many reasons for this behavior.
My best shot is that you're behind a firewall that's blocking any connections that are not going to port 80. In this case, you may try to connect with your program to the same network, but port 80.
Not sure why you wouldn't get name resolution for google. I'm guessing it's a bug in your code, but can't tell for sure.
Hope it helps.

Answer 2

Investigate and understand your network setup. How are you connected to the internet? Your workstation probably has a default route pointing to some machine again probably doing Network Address Translation (NAT) and/or running a firewall and maybe that proxy server. Find out how you do name resolution (DNS). One you figure these out you might find what you need to do to connect outside.
On the other hand, errors in the code we don't see in the question are also likely :)

Answer 3

There is likely a firewall that is blocking all connections to outside hosts, and an internal DNS server that does not lookup external hostnames. THis is why your socket programs can neither lookup outside hostnames nor connect to outside services.
The same restriction applies to your browser; it's just that your browser is set up to use a proxy server. This means that the browser isn't directly looking up outside hostnames or connecting to outside hosts - it is only looking up the proxy name and making TCP connections to the proxy server. The proxy then is doing the hostname lookups and making the TCP connections to the outside world, on behalf of your browser.