How do I time synchronize servers without using an external NTP server?
 

Dear All,

Our server are not connected with Internet due to office security policy. But we want to run linux ntp server to synchronize all our devices including Firewall,Router,Switches,Servers. Please help me to setup the ntp server with using external public NTP server update. Does anyone have a working configuration?

Plenty of hits on google. However, bear in mind that unless you have a server with a proper real time clock installed your machines will drift from the correct time possibly by a significant amount depending upon the server designated as the time source. You could get around that by having one machine allowed to pick up the time from an internet source and have all others take their time from that.

Which distro and which version of NTPd are you using?

If it's ntpd from ntp.org then there is a configuration file, /etc/ntp.conf, which contains a list of servers. In that you can on the clients comment out the existing list and replace the list with your local NTP server. Then all machines pointing at that one server will at least be in sync. You can see more information in the manual page for ntp.conf

In order to turn one machine into an NTP server, you need to open port 123 in the machines's own firewall on the machine that will serve. You also need to tell ntpd to serve time, but many default configurations do that for you by default. If you want the NTP server and clients to authenticate when checking time, that will have to be set up.

Also, there are some peripherals you can buy to have your server be a stratum 1 time server.

It's actually quite common, within a corporate network, to create a corporate time-server within that network. Then, only this one machine needs to reach out to an external source to set its own clock. Every other machine synchronizes to that machine's clock, and the messages are simply passing back-and-forth within the company network.

If you want an external time source then you will have to connect some machine to the web. That machine could be a number of devices from soho routers to virtual machine to dedicated hardware.

Almost every major distro has ntp ability and they have help pages on those topics. Here is one example. http://www.thegeekstuff.com/2014/06/...-server-client

If you want to get wild then they make time sources that sync over LF or public TV channel times or GPS.