A security analysis of the NTP protocol version 2

The network time protocol (NTP) is being used throughout the Internet to provide an accurate time service. The author examines the security requirements of such a service, analyzes version 2 of the NTP protocol to determine how well it meets these requirements, and suggests improvements where appropriate. Five types of security attacks on a time service are possible. An attacker could cause a nontime server to impersonate a time server (masquerade), an attacker could modify some (or all) time messages sent by a time server (modification), an attacker could resend a time server´s time messages (replay), an attacker could intercept a time server´s time messages and delete them (denial of service), and an attacker could delay the time messages by, for example, deliberately flooding the network, thereby introducing large transmission delays (delay)