30. NTP
30.1 Overview
1. Network Time Protocol
o Uses a distributed hierarchy to synchronize time to UTC (Universal Coordinated Time).
o Each server is at a certain stratum. The lower the stratum, the closer it is to an external source of UTC.
o Stratum 1 servers have direct access to an external UTC source. (e.g. a radio clock synchronized to time signal broadcasts).
o A stratum 2 server gets its time from a stratum 1 server. A stratum 3 gets it from a stratum 2, and so on and so on.
o To avoid synchronization problems, the maximum number of strata is 15.
o Ideally, NTP likes to have at least 3 sources of time available to synchronize to.
o NTP never runs a system clock backwards, but can slow it down if it’s running too fast.
o When NTP is first started, it starts to compute the frequency of the clock on the computer it’s running on. It usually takes a day or so for NTP to determine the error or “drift” of the local clock. This “drift” is stored in a local file so it doesn’t have to be recomputed if NTP is restarted.
2. Packages
3. Port
UDP 123
30.2 Configuration
1. /etc/ntp.conf
o Primary configuration file.
o Example:
o server rackety.udel.edu
o server umd1.umd.edu
o server lilben.tn.cornell.edu
o driftfile /etc/ntp/drift
The “server” keyword is used to indicate the servers that should be used to synchronize to UTC. This host can receive synchronization from one of the listed servers, but cannot provide it to them.
The “driftfile” directive indications the file that contains the current value of the frequency error of the clock on the computer.

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