Multi-user, multi-tasking operating systems are subject to ``denial of service'' attacks where one user can render the system unusable for legitimate users by ``hogging'' a resource or damaging or destroying resources so that they cannot be used. Denial of service attacks may be caused deliberately or accidentally. Taking precautions to prevent a system against unintentional denial of service attacks will help to prevent intentional denial of service attacks.
Systems on a network are vulnerable to overload and destructive attacks as well as other types of intentional or unintentional denial of service attacks. Three common forms of network denial of service attacks are service overloading, message flooding, and signal grounding. It is important for system administrators to protect against denial of service threats without denying access to legitimate users. In general, denial of service attacks are hard to prevent. Many denial of service attacks can be hindered by restricting access to critical accounts, resources, and files, and protecting them from unauthorized users.