Wireshark is a free open source packet analyzer. It is used in computer network troubleshooting and analysis, protocol development, education, and reverse engineering. Wikipedia
An attack by denied service (Denial Of Service attack, hence the abbreviation DoS) is a computer attack whose aim is to make a service unavailable, to prevent legitimate users of a service from using it. This may involve the disruption of connections between two machines, preventing access to a particular service, the flooding of a network in order to prevent its operation, the obstruction of access to a service at a person in particular, also the fact of sending billions of bytes to an internet box. A denial of service attack can block a file server, make access to a web server impossible or prevent the distribution of email within a company. This mode of attack is widely used by hackers via security vulnerabilities “Zero Days” or the absence of automatic updates.