Mar 30, 2016

This page will attempt to provide you with as much port information as possible on UDP Port 123. UDP Port 123 may use a defined protocol to communicate depending on the application. A protocol is a set of formalized rules that explains how data is communicated over a network. Identify the UDP port that Network Time Protocol (NTP Jan 29, 2019 How to check TCP or UDP ports using netcat (nc) command

NTP용 UDP 123 포트 - 제타위키

Dec 18, 2016 unix - NTPD: use an unrestricted port for communication

Judging by the specific output Connection to Connection to 53 port [udp/domain] succeeded! you are using openbsd-netcat. Looking at the code for that the test is to bind to the UDP socket, i.e. there is an open connection:

Here is a short post to check port [TCP/UDP] connectivity from a Linux server. A TCP/IP network connection may be either blocked, dropped, open, or filtered. These actions are generally controlled by the IPtables firewall the system uses and is independent of any process or program that may be listening on a network port. Port 123 (tcp/udp) :: SpeedGuide Port(s) Protocol Service Details Source; 123 : udp: NTP: Network Time Protocol (NTP) - used for time synchronization Security Concerns: It provides both information and possible avenue of attack for intruders. Info gathered can include system uptime, time since reset, time server pkt, I/O & memory statistics and ntp peer list. NTP - The Wireshark Wiki