What is the Undisciplined Local Clock?

Environment

• Red Hat Enterprise Linux (RHEL)
• NTP

Issue

• What is the Undisciplined Local Clock?
• Should the undisciplined local clock be used for NTP clients?
• Is the undisciplined clock used by default?
• Is the local clock used by default?

Resolution

• The Undisciplined Local Clock is a fake driver that can simulate a reference clock when no external synchronization sources are available.
• It allows a designated time server to act as a primary server to provide synchronization to other clients on the network.
• If a server with this driver is connected directly or indirectly to the public Internet, there is some danger that it can destabilize other clients.
• Don't use it in NTP clients.

From /etc/ntpd.conf in RHEL 5:

# Undisciplined Local Clock. This is a fake driver intended for backup
# and when no outside source of synchronized time is available. 
server	127.127.1.0	# local clock
fudge	127.127.1.0 stratum 10	

In RHEL 6, the default configuration file, /etc/ntp.conf, now has the following lines commented:

#server 127.127.1.0 # local clock
#fudge 127.127.1.0 stratum 10

Additional information:
/usr/share/doc/ntp-*/html/drivers/driver1.html

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