If you Google "Server Data Security", there will be much information returned for your consideration. Apart from preventing a denial-of-service attack, the biggest consideration for security of servers is the sensitive Data therein - and judging by today's situation - lots of data.
We spend lots of money and resources on IT security products and practices to protect our precious data. However, we still come across media reports on issues about data leakages, data theft etc - and the victims are large organizations including government departments. They have all the money and resources to ensure comprehensive and robust IT security ... so what are we missing?
One age-old concern about the security of servers, is the prevention of the data stored in there being viewed by an over-zealous or busy-body "super-user" system administrator. There are some tools out there that try to prevent this, including a host firewall. But so far the attempts have been cumbersome or impractical. It is also quite hard to implement full-disk encryption on a server, or backup, or for that matter, cloud services.