On a Linux/Unix system you can mitigate the effects of an attack by blocking the communication with the attacking ip addresses. You can either do this by creating IPTables Rules or via a null route also known as a black-hole route. This article will help you to block Denial of Service (DoS) with null route on server.
First verify the load on server, If the load on server looks like unexpected then execute mention command to verify the connections. It will help you understand that load on server is normal or is there any DoS attach.