Wanting to withstand a DDOS attack is kinda like wanting to withstand an Earthquake at ground zero.

A good foundation helps alot, you've already shown this by wanting to know about server software (i think thats what you were meaning), but also look at your network setup.

If you setting up, or preparing a network, most likely some computers will need a connection to the internet to serve data. Ie Web Server / Email Server etc etc. So you will require a DMZ or a semi-DMZ. Also, if you read alot about DDOS attacks, many successful (well as successful as they can be) are done when blocking to packets at the ISP. Made the time to speak to the techies or setup a worese-case plan ?

[Also, as i was writing this, it also occured to me that some devices/software have some wierd effects to DDOS attacks, ie resetting to default settings etc etc. So check google, and other sites for the hardware/software your looking for. But a simple, patch / firmware upgrade should fix it]*




* The comments in the [] are based on little research and many assumptions. So do abit of research on that topic first. Sorry if i manage to mislead you.