The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks.
Default port: 123/udp
PORT STATE SERVICE REASON123/udp open ntp udp-response
ntpq -c readlist <IP_ADDRESS>ntpq -c readvar <IP_ADDRESS>ntpq -c monlist <IP_ADDRESS>ntpq -c peers <IP_ADDRESS>ntpq -c listpeers <IP_ADDRESS>ntpq -c associations <IP_ADDRESS>ntpq -c sysinfo <IP_ADDRESS>
nmap -sU -sV --script "ntp* and (discovery or vuln) and not (dos or brute)" -p 123 <IP>
NTP protocol by design uses UDP to operate, which does not require any handshake like TCP, thus no record of the request. So, NTP DDoS amplification attack begins when an attacker crafts packets with a spoofed source IP to make the packets appear to be coming from the intended target and sends them to NTP server. Attacker initially crafts the packet of few bytes, but NTP responds with a large amount of data thus adding to amplification of this attack.
MONLIST command: It is a NTP protocol command which has very little use, but it is this command which is the main culprit for this attack. However, the use of MONLIST command is to give details of the last 600 clients that have connected to the NTP time service. Below is the command syntax:
ntpdc -n –c monlist <IP or hostname of time server>