*1.2.2 Defending against DDoS ("denial service attack")*

A denial of service attack is called a DoS attack on a host. It is a type of cyberattack that disrupts hosted Internet services, making the host machine or network resource on the host unavailable to intended users. DoS attacks attempt to overload the host system or network resource on the host by flooding it with unnecessary requests, thereby blocking the execution of legitimate services. The DDoS attack refers to a "distributed" DoS attack, meaning that the flooding attack of incoming traffic to a victim comes from many disparate sources spread across the Internet [11]. A DDoS attacker can compromise and use one person's computer to attack another

computer by taking advantage of security vulnerabilities or weaknesses. By taking advantage of a set of compromised computers in this way, a DDoS attacker can send huge amounts of data to a hosting website or send spam to specific email addresses. Therefore, it is very difficult to prevent the attack by simply jamming the individual sources one by one. The arm wrestling depends on the rate of repair of these compromised nodes versus the success rate of compromising computer nodes in the network. The major concern in a DDoS attack is the availability of the blockchain and is related to the question of whether a DDoS attacker can make the blockchain unavailable by taking down part or all networks. The answer to this question is no, thanks to the fully decentralized construction and maintenance of the blockchain, particularly the bitcoin system that has a large network (interconnected node), as well as the consensus protocol for generating new blocks and adding them to the blockchain, which ensures that the processing of blockchain transactions can continue even if several blockchain nodes are offline. For a cyberattacker to successfully take the blockchain offline, he or she must gather sufficient computing resources to compromise a very large portion of the blockchain nodes on the entire blockchain network. The larger the blockchain network, the harder it is to pull off such a large DDoS attack [11]. This is the case with the bitcoin blockchain network, which continues to grow and now has 14719 nodes worldwide retrieved on Sat Dec 18 16:24:33 2021 +01 in bitnodes.io and as shown in **Figure 2**.
