Shorewall: setup Geo-IP filtering
Firewall
Route to nowhere - Blocking attackers using Nullroutes
If someone attacks a linux-system, most administrator would block the ip-address of the attacker using iptables-rules. But there is another method to block the address of an attacker: nullroutes. A Nullroute simply directs to nowhere.
Firewall Evasion-Techniques
I found some very interesting papers about firewall evasion: