The default configuration in Debian Edu is to keep the clocks on all machines synchronous but not necessarily correct. NTP is used to update the time. The clocks will be synchronised with an external source by default. This can cause machines to keep the external Internet connection open if it is created when used.
If you use dialup or ISDN and pay per minute, you want to change this default setting.
To disable synchronisation with an external clock, the file /etc/ntp.conf on the main server needs to be modified. Add comment ("#") marks in front of the server
entries. After this, the NTP server needs to be restarted by running service ntp restart
as root. To test if a machine is using the external clock sources, run ntpq -c lpeer
.