How to fool Web Filters.

Information wants to be free.

Introduction

Web filters allow companies to forbid their employees to access certain web sites they deemed inappropriate. They also allow parents to control which sites their children connect to. There are several versions, some work inside the computer, some in the router/firewall. They all have the problem that they tend to censor too much. For example, some censored any page which the word "Dick", which is actually short for Richard. Others work sites that only give information like 2600, but I guess that is dangerous. Web filters have been on the news because they are required in Public Libraries in the US. Web filters are stupid and only work because people are stupid and don't know how to do math (they are afraid) and how the Internet work (they are lazy).

Beat that filter.

The most elegant way to beat a filter is described below, there are others but I like this one. Caveat: This won't work on a Mac, because of the way TCP streams are used, it should work in any other system/browser. I learn this trick from the Magazine 2600... Oh! I see why information is dangerous!

It works like this most names of web sites have the form www.site.xxx, for example this page is part of the web site www.groupot.org. You put that in a browser and you get to GroupOT Home Page. But this machine also has a number. You can discover this number by issuing the command "nslookup www.groupot.org" (you can do this in a Unix, or Windoze command line). The command should return 150.186.10.7, this number is called IP number. Now to fool the filter you only need to know math! (Math keeps the world in motion). Transform 150 to base 2. You can do this in a Scientific calculator (or in Mathematica using the command BaseForm) or by hand (but that can be long) just makes sure to feel zeroes in front if the number doesn't have 8 digits in base 2. For example, 150=10010110 (base two) because 150=128*1+64*0+32*0+16*1+8*0+4*1+2*1+1*0. Do the same with the rest of the digits in the IP number. That is, 186=10111010, 10=00001010, 7=00000111. Now put the numbers together to make a huge binary number: 10010110101110100000101000000111, transform this number to base 10 and you get: 2528774663. Put http://2528774663 in your browser and (even if there is a filter) you get to the GroupOT Home Page.

You think this is too hard? You don't have a copy of Mathematica around? Okey, lets make this easier. You get the final number 2528774663, you only have to calculate: 150 256^3+186*256^2+10*256^1+7*256^0. You see the trick? Do you understand why it works? Think about it!

 

Tell me what you think (aoctavio@ivic.ivic.ve)


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Last Modified: February 9, 2001