Skip to content

Bypass a Blocked NAT

A few years ago, I was staying in a student accommodation, where I had to pay to connect devices to the internet.  Since I was doing a Master in Computer Science, I had many devices to connect and decided to buy a router / switch Cisco Linksys E2000 and use the Network Address Translation to connect my devices to the internet.

Once the router set up, and configured I received the following message on all my devices  “NAT was detected […] Illegal […]”. I suddenly found this challenging and interesting to investigate, and thought that it would be cool to see how their system was working, and how to bypass it.


Since it was a student accommodation I pictured the connection like this ( To make it easy! )


After a few readings ( RFC of NAT) and a few tests with Wireshark, I discovered that the only field which was modified by the router was the TTL (Time To Live) field, and that’s how I came with the idea to upgrade the TTL from 2 on my machines. To achieve that  I used “iptables” with the following command:

and this is how I bypassed the TTL check from my ISP, I hope this will help all the Students in student accommodations in the UK.


{ 2 } Comments

  1. Fatal error: Uncaught Error: Call to undefined function ereg() in /home/users4/n/noktec/www/noktec/wp-content/themes/barthelme/functions.php:178 Stack trace: #0 /home/users4/n/noktec/www/noktec/wp-content/themes/barthelme/comments.php(34): barthelme_commenter_link() #1 /home/users4/n/noktec/www/noktec/wp-includes/comment-template.php(1554): require('/home/users4/n/...') #2 /home/users4/n/noktec/www/noktec/wp-content/themes/barthelme/single.php(44): comments_template() #3 /home/users4/n/noktec/www/noktec/wp-includes/template-loader.php(106): include('/home/users4/n/...') #4 /home/users4/n/noktec/www/noktec/wp-blog-header.php(19): require_once('/home/users4/n/...') #5 /home/users4/n/noktec/www/noktec/index.php(17): require('/home/users4/n/...') #6 {main} thrown in /home/users4/n/noktec/www/noktec/wp-content/themes/barthelme/functions.php on line 178