The NTP program is configured using either the /etc/ntp.conf or /etc/xntp.conf file depending on what distribution of Linux you have. I won't go into too much detail on how to configure NTP. Instead I'll just cover the basics.
An example of a basic ntp.conf file would look like:
# --- GENERAL CONFIGURATION --- server aaa.bbb.ccc.ddd server 127.127.1.0 fudge 127.127.1.0 stratum 10 # Drift file. driftfile /etc/ntp/drift
The most basic ntp.conf file will simply list 2 servers, one that it wishes to synchronize with, and a pseudo IP address for itself (in this case 127.127.1.0). The pseudo IP is used in case of network problems or if the remote NTP server goes down. NTP will synchronize against itself until the it can start synchronizing with the remote server again. It is recommended that you list at least 2 remote servers that you can synchronize against. One will act as a primary server and the other as a backup.
You should also list a location for a drift file. Over time NTP will "learn" the system clock's error rate and automatically adjust for it.
The restrict option can be used to provide better control and security over what NTP can do, and who can effect it. For example:
# Prohibit general access to this service. restrict default ignore # Permit systems on this network to synchronize with this # time service. But not modify our time. restrict aaa.bbb.ccc.ddd nomodify # Allow the following unrestricted access to ntpd restrict aaa.bbb.ccc.ddd restrict 127.0.0.1It is advised that you wait until you have NTP working properly before adding the restrict option. You can accidental restrict yourself from synchronizing and waste time debugging why.
NTP slowly corrects your systems time. Be patient! A simple test is to change your system clock by 10 minutes before you go to bed and then check it when you get up. The time should be correct.