Denial of Service often shortened to DoS is a situation in which legitimate access to systems, devices, or network resources is impossible due to a malicious actor. The most common method is to flood network resources with more traffic than it can handle. An attacker will send many requests, usually illegitimate with spoofed return addresses to hide their origin, rendering the victim overwhelmed and unable to handle requests from legitimate users.
With regards to viruses, worms, and other self-replicating programs, Denial of Service may be an unintended consequence of the code spreading and using system resources, or it may be a deliberate part of its payload. When multiple systems are used in a coordinated manner to knock out an Internet service, it is certainly
Viruses and Worms that can cause Denial of Service
This is a non-exhaustive list of viruses and worms that cause a Denial of Service.
Accidental
Intentional
Common Types of DoS
Denial of Service may describe anything from a worldwide botnet coordinating an attack on a certain internet-connected server, to simply yanking the cord for a system (assuming no uninterruptible power supply).
Smurf Attack
For a Smurf Attack, the attacker sends ICMP (Internet Control Message Protocol) broadcast packets to hosts with a spoofed source address to look like they come from the ultimate target of the attack. This causes the hosts to respond to the requests, and flood the target with the responses.
SYN Flood
For a SYN Flood, the attacker sends a SYN request to connect to the target server, but does not complete the three-way handshake process used in Transmission Control Protocol (TCP) that creates the connection between the host and server. This leaves the target port in an occupied state unable to receive further requests. The attacker will flood all available open ports making it impossible for legitimate users to connect. This method was used by the Randex worm.
Ping of Death
In a ping of death attack, the attacker sends a packet larger than the maximum allowable size using the common "ping" utility that is usually used to test network connections. Under normal circumstances, the program simply sends a short message to the target, and if there is a connection between the source and target, the target will send a reply. This naturally does not require a large packet size, and the largest allowable packet is 65,535 bytes. An attacker can create a much larger packet, which gets broken into smaller segments while going through the network and the target must reassemble, causing it to crash, freeze, or reboot when a buffer overflow occurs.
Distributed Denial of Service Attack (DDoS)
Distributed Denial of Service Attack, abbreviated to DDoS, is a common tactic of botnets and often their primary purpose. A DDoS can use any of the above mentioned methods to accomplish the denial of service. Multiple machines coordinate to attack a specific target. Botnets compromising unaware users are a common tool used to coordinate such attacks. In these cases, the compromised systems performing the attack are considered victims because of the hijacking of their systems as well as the fact that they can potentially experience slow-downs and other problems as a result of the use of their resources. Naturally, coordinate botnet DDoS attacks can be extremely effective in bringing down the target system.
Sources
Cybersecurity & Infrastructure Security Agency, Understanding Denial-of-Service Attacks. 01-FEB-2021
Imperva, DDoS Attacks.
Cloudflare, Ping of death DDoS attack.