DOS (DENIAL of Service) attack is different from DDOS (Distributed Denial of Service) attack. Although their goals are to make the target system unable to provide services normally, the methods and scale of attacks are different.

　　DOS attack (refusal service attack):

　　DOS attack means that the attacker sends a large number of requests or occupy system resources to the target system, making the system unable to respond to the request of legal users normally. This attack is usually sponsored by a single host or a few hosts in order to exhaust the bandwidth, processing capacity or other key resources of the target system.

　　DDOS attack (distributed service attack):

　　DDOS attack is a larger and more complicated form of attack. Unlike DOS attacks, the DDOS attack involves multiple attackers (hosts) in collaboration, and drowns the target system by sending a large amount of traffic to the target system at the same time. These attackers are usually located in different geographical locations, forming a distributed attack network, making defense more difficult.

　　Key differences:

　　Attack scale:

　　DOS attacks are usually initiated by a single host or a few hosts, and the attack scale is small. DDOS attack involves multiple attackers, and the scale of attack may far exceed a single attack.

　　Attack source:

　　DOS attack comes from a single host or a group of hosts. DDOS attack involves multiple attackers distributed globally, forming a distributed attack network.

　　Coordination:

　　DOS attacks are usually directly launched by a single attacker. DDOS attacks need to coordinate multiple attackers, which may involve means to control servers, botnets.

　　It is difficult to defend:

　　DDOS attacks are relatively difficult to defend, because the attack traffic comes from many different sources, and the attacker may use various means to disguise its true identity and intention.

　　DDOS attack is a more complicated and challenging attack form. It requires professional DDOS protection measures to effectively respond. DOS attacks are usually relatively simple, and its defense can use some basic network security measures.