How can a client lie about its IP address to the server (HTTP)

Question

I'm trying to set the user agent property to other than Java/1.4.2_02, and also send requests to the server that should appear coming from ip specified by me. In another words i am trying that I could hit the server with variety of different setting of user-agent and ip address. I have this piece of code that works fine for the user-agent. But how can I send request to the server mimicking different ip addresses. Any ideas?thanks
I don't to download the page. I just have to send the requests.
Here is the code that sends different user-agent configurations to the server:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import javax.swing.text.html.HTMLDocument;
public class teste {
private static HttpURLConnection conn = null;
private static URL url = null;
private static HTMLDocument html = new HTMLDocument();
public static void main(String args[]) {
BufferedReader in = null;
String str = "";
try {
url = new URL("http://www.google.com/search?hl=de&ie=ISO-8859-1&q=test");
} catch(MalformedURLException e) {
System.out.println(e);
System.exit(-1);
try {
conn = (HttpURLConnection) url.openConnection();
conn.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.7.2) Gecko/20040803");
conn.connect();
in = new BufferedReader(new InputStreamReader(url.openStream()));
} catch(IOException e) {
System.out.println(e);
System.exit(-1);
}

Answer 1

There is bad news, and their is worse news.
You need raw sockets. Java doesn't do raw sockets.
Windows doesn't eithier (XP SP2), though there ARE ways arround it, the problem is you will need to be administrator and it will be throttled (read SLOW) by the OS itself (no way arround this) and it will refuse to send Spoofed IPs (no sensible way arround this).
Linux you need to be root.
You need to write a JNI that will drop your requests onto the network.
It's horrible, and you need to consider whether java is the right language for this. Sounds like you are just doing some NATting or proxying or such, which would be much easier to do in pure c. Unfortunately I have to have raw packets from java so pain and suffering for me.