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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.


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