if all you want to do is browse the net safely you can use this mini-script:

in /home/knoppix create a file called "blockall" and place the following script inside it:

#!/bin/sh
iptables -F
iptables -A INPUT -p all -s 127.0.0.1 -j ACCEPT
iptables -A INPUT -p tcp --syn -j DROP

(==>>note: this is the edited & corrected version of the script! <<==)

then run the following commands:

chmod +x blockall
sudo ./blockall

and that's it. All your ports are closed, you cannot run any server, but you can surf safely.

to reopen everything (to stop this script) simply execute:

iptables -F


HTH