If your server is running slow because of the load, and not because of the bandwidth of the attack that might be a viable route. It is trivial to use iptables t

If your server is running slow because of the load, and not because of the bandwidth of the attack that might be a viable route. It is trivial to use iptables to block all UDP traffic except from your configured DNS server. Most of the cases though it's as simple as attack is 200 mbit and connection is 100mbit -> no amount of dropping packets once they reach the server will do anything, as the connection is entirely saturated.