Not really any software for this,
If the DDoS is taking down the server by making it too busy, then there is hardware DDoS protection.
If the DDoS is taking it down by using so much bandwidth its impossible to get legitimate traffic through, then there is no protection to stop it, other then upgrading the servers connection (May not help seeing as I've heard of attacks that were over 40 Gbps, which currently no single connection is capable of.)