TcpCatcher - WorkShop n°4 : Synchronous hook example, Blacklisting a host

WorkShop n°4 : Synchronous hook example, Blacklisting a host

In this workshop, we are going to demonstrate how to build your own plugin to perform a synchronous operation on each packet going through TcpCatcher.
For each HTTP request going through TcpCathcher, we are going to check if the requested destination belong to a list before serving it.
For demonstration purposes, we are going to forbid access to google.

Import the TcpCatcher jar (which contains the interface class) into your java project and write a class implementing the CustomHook interface and that may look like this one
Note the your class must not belong to any package in order to be plugged at runtime.
import tcpcatcher.TcpCatcherPacket; public class ForbiddenHost implements tcpcatcher.CustomHook { static char a = 13; static char b = 10; static final protected String line_delimit = "" + a + b; static final protected String header_delimit = "" + a + b + a + b; static final String content = "<html>TcpCatcher has blocked this address.</html>"; static final String refuseReponse = "HTTP/1.1 200 OK" + line_delimit + "Content-Type: text/html" + line_delimit + "Content-Length: " + content.length() + header_delimit + content; public void modifyPacket(TcpCatcherPacket tcpcatcherPacket) { // let's look at requests only if (tcpcatcherPacket.fromClient) { // Note that header is already extracted by TcpCatcher but // the field tcpcatcherPacket.ipHost is not yet known when the // synchronous hook is called if (tcpcatcherPacket.httpHeader != null) { int x = 5 + tcpcatcherPacket.httpHeader.indexOf("Host:"); int y = tcpcatcherPacket.httpHeader.indexOf(line_delimit, x); String host = tcpcatcherPacket.httpHeader.substring(x, y); System.out.println("host=" + host); if (host.indexOf("google") < 0) return; // we are not going to forward that request tcpcatcherPacket.bounceBack = true; // instead we create an html refusal response page tcpcatcherPacket.packetbytes = refuseReponse.getBytes(); } } } public void setUp() { // TODO Auto-generated method stub } public void shutDown() { // TODO Auto-generated method stub } }

Start TcpCatcher, check "Apply an automatic transformation to packets" and select your .class file after clicking on the "Choose Class" button.
A message should tell you that the hook has been successfully set up.
Because checking host must be done before packet transmission, let's Check "Synchronous callback".
Note also that because the hook is synchronous, any other modification on packet payload could be done here (and with potential delay on packet delivery).

Click "Apply settings and start", and try visiting google