A DoS attack is a malicious attempt to render a service, system, or network unusable by its legitimate users. Unlike most other hacks, a DoS does not require the attacker to gain access or entry into the targeted server. The primary goal of a DoS attack is instead to deny legitimate users access to the service provided by that server.
Attackers achieve their DoS objective by flooding the target until it crashes, becomes unreachable from the outside network, or can no longer handle legitimate traffic. The actual volume of the attack traffic involved depends on the type of attack traffic payload used. With crafted payload such as malformed IP fragments, several such packets might be sufficient to crash a vulnerable TCP/IP stack; on the other hand, it might take a very large volume of perfectly conforming IP fragments to overwhelm the defragmentation processing in the same TCP/IP stack. Sophisticated attackers might choose to use a mixture of normal and malformed payloads for a DoS attack. DoS attacks can vary in impact from consuming the bandwidth of an entire network, to preventing service use of a single targeted host, or crashing of a single service on the target host.
Most DoS attacks are flood attacks; that is, attacks aimed at flooding a network with TCP connection packets that are normally legitimate, but consume network bandwidth when sent in heavy volume. The headers of malicious packets are typically forged, or spoofed, to fool the victim into accepting the packets as if they are originating from a trusted source.