Differentiate between DoS and DDoS attack.
Denial of Service (DoS) is an attack where the attacker floods the victim resource with illegitimate requests from a single source, making the resource appear busy and unavailable to authorized users. It can be countered by blocking requests from that single source, and recovery is possible by restarting the server.
Distributed Denial of Service (DDoS) is a variant of DoS where the flooded requests come from compromised computer systems called Zombies distributed across the globe. The attacker installs malicious software called Bot on these machines and activates a network known as Bot-Net to carry out the attack. DDoS is very difficult to resolve as the attack originates from multiple distributed locations.
Explanation
The textbook clearly distinguishes between DoS and DDoS attacks. DoS originates from a single source and can be blocked or recovered by restarting the server. DDoS, however, uses a network of compromised Zombie computers (Bot-Net) distributed globally, making it much harder to detect and resolve since blocking one source is insufficient when attacks come from multiple locations.