On raspberry, I was looking at the NTP configuration file under /etc/systemd/timesyncd.conf. All lines are commented (by default) but when I run the timedatectl status command, I get:
...
Network time on : yes
NTP synchronized : yes
...
How NTP works without configuration in conf file ? Where are NTP server links ?
Raspberry pi 3 ; Raspbian 9.13 (stretch)
The NTP management of systemd is called systemd-timesyncd. On my Ubuntu system, the corresponding systemd service is /lib/systemd/system/systemd-timesyncd.service. Typically, when you don't have items activated in a configuration file, daemons use default values. Moreover, if you look at the comment in the configuration file, you will see that the commented items are actually default values:
Entries in this file show the compile time defaults. You can change settings by editing this file. Defaults can be restored by simply deleting this file.