I see people asking “How do I prevent a DOS” on just about every kind of server/network Q&A site.  And as expected, people post links to all kinds of scripts/applications/module/rules/etc.
How do you plan to prevent something you are not in control of?  There is no DOS switch you can just turn off.  As the administrator of a server or network, all you can do is prepare your server(s) and network(s) to weather the storm that a DOS is.