Network Time Protocol

Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks.

Install and configure ntpd on CentOS

• Install the necessary packages:

$ yum install ntp ntpdate ntp-doc

• Turn on the ntpd service persistently:

$ chkconfig ntpd on

• Synchronize the system clock with 0.pool.ntp.org server:

$ ntpdate pool.ntp.org

• Start the NTP server. The following will continuously adjusts system time from upstream NTP server (i.e., o need to run ntpdate):

$ /etc/init.d/ntpd start
$ #~OR~
$ service ntpd start

• Locate your geographically closest server pools from http://www.pool.ntp.org/en/

$ for i in africa asia europe north-america south-america; do \
  curl -s http://www.pool.ntp.org/zone/$i|sed -n -e '/<pre>/,/<\/pre>/p'|sed -e 's/<[^>]*>//g'; done
server 0.africa.pool.ntp.org
server 1.africa.pool.ntp.org
server 2.africa.pool.ntp.org
server 3.africa.pool.ntp.org

server 0.asia.pool.ntp.org
server 1.asia.pool.ntp.org
server 2.asia.pool.ntp.org
server 3.asia.pool.ntp.org

server 0.europe.pool.ntp.org
server 1.europe.pool.ntp.org
server 2.europe.pool.ntp.org
server 3.europe.pool.ntp.org

server 0.north-america.pool.ntp.org
server 1.north-america.pool.ntp.org
server 2.north-america.pool.ntp.org
server 3.north-america.pool.ntp.org

server 0.south-america.pool.ntp.org
server 1.south-america.pool.ntp.org
server 2.south-america.pool.ntp.org
server 3.south-america.pool.ntp.org

• Since I am in North America, I will add the lines above for North America to the following two files:

$ cat >> /etc/ntp.conf <<EOF
server 0.north-america.pool.ntp.org
server 1.north-america.pool.ntp.org
server 2.north-america.pool.ntp.org
server 3.north-america.pool.ntp.org
EOF
$ cat >> /etc/ntp/step-tickers <<EOF
0.north-america.pool.ntp.org
1.north-america.pool.ntp.org
2.north-america.pool.ntp.org
3.north-america.pool.ntp.org
EOF

• Finally, restart the NTP daemon:

$ service ntpd restart

• You can also list out your server pools and get details with the following:

$ ntpdc -l
$ ntpdc -p

• Use tcpdump to watch the NTP traffic:

$ tcpdump dst port 123

External links

• Official website