NTP is a network protocol used to synchronize the time in our systems. NTP was designed and currently maintained by Dave Mills. This protocol is organized of a hierarchical way, the main server is called stratum-0, this servers has the highest level of accuracy of the time. Within the stratum-0 group, there are some servers with a bit lower accuracy due that receive the time over GPS satellites or radio stations and have to correct the time depending on the transmission time. The main servers stratum-0 provide the time to the stratum-1 servers, this servers are public and are the primary servers to provide the time in Internet. In a lower level we have secondary servers called stratum-2, this servers send request to the startum-1 servers and use NTP algorithm to obtain the best data. Stratum-3 servers are below startum-2 servers and are used to send request to stratum-2 and act as a server for lower strata in the hierarchy.

Install ntp server and ntp utilities

# yum install ntp ntp-perl ntpdate

Configuring the NTP server

– With this configuration I’ll use three stratum-2 public servers, if the connection fails with the public servers ntp will use hardware clock to obtain the time, using a lower priority setting a stratum 10 value. Also I’ll allow the local network 192.168.1.0/24 hosts can synchronize with the server. Edit /etc/ntp.conf:

#drift file is used by ntp to store the frequency error from ntp servers
driftfile /var/lib/ntp/drift
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default kod nomodify notrap nopeer noquery
# Permit all access over the loopback interface. This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1
# Hosts on local network are less restricted.
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
# Use public servers from the pool.ntp.org project.
server 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org
# 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

– Add iptables rule:

-A INPUT -m state --state NEW -m udp -p udp --dport 123 -j ACCEPT

– Starting ntpd service:

# service ntpd start
# chkconfig --levels 235 ntpd on

Configuring the ntp client

– Edit configuration file /etc/ntp.conf:

driftfile /var/lib/ntp/ntp.drift
server 192.168.1.250
restrict -4 default kod notrap nomodify nopeer noquery
restrict 127.0.0.1

– Restart ntp service:

# service ntpd restart

Checking the ntp server

– From the client send a ntp query to verify the server operation with ntpq command:

# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
192.168.1.250 173.255.226.203 11 u 1 64 1 0.572 -56.051 0.000

– ntpdate command is used to set local time over a ntp server. With -d option will show more debug information of every step with the conversation with the server:

# ntpdate -d 192.168.1.250
1 Sep 19:53:52 ntpdate[30609]: ntpdate [email protected] Sun Oct 17 13:45:14 UTC 2010 (1)
transmit(192.168.1.250)
receive(192.168.1.250)
transmit(192.168.1.250)
receive(192.168.1.250)
transmit(192.168.1.250)
receive(192.168.1.250)
transmit(192.168.1.250)
receive(192.168.1.250)
transmit(192.168.1.250)
server 192.168.1.250, port 123
stratum 3, precision -21, leap 00, trust 000
refid [192.168.1.250], delay 0.02625, dispersion 0.00009
transmitted 4, in filter 4
reference time: d3ecc9e8.4f754141 Sat, Sep 1 2012 19:52:40.310
originate timestamp: d3ecca36.52cecf88 Sat, Sep 1 2012 19:53:58.323
transmit timestamp: d3ecca36.58b5d137 Sat, Sep 1 2012 19:53:58.346
filter delay: 0.02625 0.02628 0.02640 0.02640
0.00000 0.00000 0.00000 0.00000
filter offset: -0.02386 -0.02381 -0.02367 -0.02365
0.000000 0.000000 0.000000 0.000000
delay 0.02625, dispersion 0.00009
offset -0.023868
1 Sep 19:54:00 ntpdate[30609]: adjust time server 192.168.1.250 offset -0.023868 sec
Time synchronization with NTP
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