I wanted to setup time synchronization on my Linux router. I then wanted to have my Windows machine synchronize time with the router.

Here is the article about how I accomplished that.

[edit] Setting up a Linux NTP server

Had to install the NTP server software and configure it to the configuration that would work for me.

[edit] yum Package

The applicable yum package is ntp:

yum install ntp

[edit] Configuration file

The applicable configuration file is /etc/ntp.conf. The sections I had to modify pertained to the restrict and server options.

[edit] restrict options

The first restrict options allow the local machine to access the ntp daemon:

restrict 127.0.0.1
restrict -6 ::1

Now, I added a section to allow LAN machines to be able to query the server but not to be able to modify configuration or provide trap service (quoting from the man page: which is intended for use by remote event logging programs):

restrict 192.168.3.0 mask 255.255.255.0 nomodify notrap
restrict 192.168.4.0 mask 255.255.255.0 nomodify notrap

Finally, we need to allow access to the servers we will be using to synchronize time. I got these servers from the pool.ntp.org Project. Same restrictions as the LAN with the added restriction that these servers are not allowed to perform query on our servers:

restrict 0.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 1.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 2.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery

For more information on the restrict options:

man ntp_acc

[edit] server options

Finally, we setup which servers we want to synchronize with. As mentioned above, I got these servers from the pool.ntp.org Project:

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

[edit] Starting NTP service

To start the ntpd service, you would do something like this:

/sbin/service ntpd start

To configure it to start automatically:

/sbin/chkconfig ntpd on

[edit] Testing and troubleshooting

First thing, we should make sure it starts correctly. After starting the server, grep /var/log/messages for ntpd just to make sure things started up fine:

[belminf@home ~]# sudo grep ntpd /var/log/messages

home ntpd[15000]: ntpd 4.2.4p2@1.1495-o Mon Sep 24 14:40:07 UTC 2007 (1)
home ntpd[15001]: precision = 1.000 usec
home ntpd[15001]: Listening on interface #0 wildcard, 0.0.0.0#123 Disabled
home ntpd[15001]: Listening on interface #1 lo, 127.0.0.1#123 Enabled
home ntpd[15001]: Listening on interface #2 eth2, 68.161.230.103#123 Enabled
home ntpd[15001]: Listening on interface #3 eth0, 192.168.3.1#123 Enabled
home ntpd[15001]: Listening on interface #4 eth1, 192.168.4.1#123 Enabled
home ntpd[15001]: kernel time sync status 0040
home ntpd[15001]: frequency initialized 307.091 PPM from /var/lib/ntp/drift

We could then use the ntpq command to view the current status of the server and what other srevers it's interacting with by using the -p option.

When you first start up the server, you should see something like this:

[belminf@home ~]# /usr/sbin/ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 mail2.safire.bz 18.103.0.198     2 u    3   64    7   58.155   -3.329   0.856
 phoenix.netserv 64.113.44.54     2 u   63   64    3   59.387    1.998   0.697
 kiri.nonexiste. 66.36.239.104    3 u   65   64    3   57.072    4.045   0.504
 LOCAL(0)        .LOCL.          10 l   62   64    3    0.000    0.000   0.001

But as the sever synchronizes with other servers, you should see something like this:

[belminf@home root]$ /usr/sbin/ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*mail2.safire.bz 18.103.0.198     2 u   10   64   77   57.713   -2.274   0.890
+phoenix.netserv 64.113.44.54     2 u    4   64   77   59.038    1.368   1.327
+kiri.nonexiste. 66.36.239.104    3 u    8   64   77   56.552    3.564   0.505
 LOCAL(0)        .LOCL.          10 l    6   64   77    0.000    0.000   0.001

For info on the significance of this output, man ntpq.

[edit] Getting a Windows PC to synchronize

[edit] Setting the NTP server

The following command will set your server and then, just to make sure the change was successful, perform a query:

C:\>net time /setsntp:192.168.3.1&& net time /querysntp
The command completed successfully.

The current SNTP value is: 192.168.3.1

The command completed successfully.

[edit] Restarting the service

And probably good to restart the w32time service:

C:\>net stop w32time&& net start
The Windows Time service is stopping.
The Windows Time service was stopped successfully.

The Windows Time service is starting.
The Windows Time service was started successfully.

[edit] Manual time synchronization

If force the time to synchronize, use this command:

w32tm /resync