NTP service is stopped, still my CentOS machine is sending out NTP packets to a NTP server:
# service ntpd status ntpd is stopped
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In addition to traditional ntpd your system may have an alternative tool installed, configured and running that syncs your clock using the NTP protocol.
"chrony" comes to mind but most likely systemd is synching your clock with ntp, check with
[sudo] timedatectl status
and if that is the case expect an output like the following :
Local time: Fri 2019-09-13 13:29:18 CEST Universal time: Fri 2019-09-13 11:29:18 UTC RTC time: Fri 2019-09-13 11:29:18 Time zone: Europe/Amsterdam (CEST, +0200) NTP enabled: yes NTP synchronized: yes RTC in local TZ: no DST active: yes Last DST change: DST began at Sun 2019-03-31 01:59:59 CET Sun 2019-03-31 03:00:00 CEST Next DST change: DST ends (the clock jumps one hour backwards) at Sun 2019-10-27 02:59:59 CEST Sun 2019-10-27 02:00:00 CET