Go2Linux shows you how to protect your server with Fail2Ban

D.O.S. or (denial of service) attack is according to Wikipedia:

A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer resource unavailable to its intended users. Although the means to carry out, motives for, and targets of a DoS attack may vary, it generally consists of the concerted efforts of person or persons to prevent an Internet site or service from functioning efficiently or at all, temporarily or indefinitely. Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers. The term is generally used with regards to computer networks, but is not limited to this field; for example, it is also used in reference to CPU resource management.[1]
One common method of attack involves saturating the target machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

So, as you can see you can suffer from this kind of attacks on any port where you are serving something, name it email, web content, ftp or any other service running on a port.
But most of the times this kind of attacks are attempted against web servers, and that is the one I'm going to show you how to protect using fail2ban.

  Fail2ban protect web server http DOS attack