Firewall Bypass a Secret Tip

You will never become interesting as Skype exchanges data between computers, and how is it possible that the person who sits at a computer for Firewall protection enables packets or traffic away from it?

There is little that knows not everyone. The company Skype bypasses the Firewall deception that is, they use a simple trick. I'll try to explain it to you in the simplest language by the example of two Linux systems.

Peer to peer software nightmare of every system administrator because instead of the normal exchange of data they create a vulnerability in Firewall, and open channel for the output packet. This idea and uses Skype-known company, and this is not very new concept, but rather a work even today.

Bypass Firewall Restriction in Linux?

We wake up to use standard programs on Linux systems like netcat and hping .

To begin open the port UDP 14141 on Local / 1 which is under Firewall protection.

local / 1 # nc -u -l -p 14141

To attack the computer attempts to connect to the first ( the Local / 1 )

remote # echo "hello" | nc -p 53 -u local-fw 14141

As expected Local / 1 does not receive packets because is under Firewall protection. 
Now, on the attacking computer use hping who will carry out the hole thus bypassing the firewall.

local / 2 # hping2 -c 1 -2 -s -p 14141 53 remote

When trying to connect it will give us that port unreachable (port unreachable), but do not worry this is normal.
If the second attempt to establish a connection

remote # echo "hello" | nc -p 53 -u local-fw 14141

Now we see that the Local / 1 in netcat caught a UDP packet from the outside which is passed through the firewall. 
As you can see Firewall bypassing possible on Linux distributions.

At the moment, there is no perfect protection for Linux on this trick. Be careful and use wisely this guide.