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I'm trying to sync the clock of our production server located in a data center with For security reason, our servers has no internet access unless we requested to open specific ip/port explicitly. I worked out a list of IPs based on

Not realizing is using round robin DNS and the servers being voluntary, they changes from time to time. In fact the IP I've got from last month is no longer working now.

I'm wondering if there's a publicly known NTP server that doesn't change as often or if there's a way to go around this without having to request an update to the firewall on a monthly basis. I believe many admin is facing the same issue here.

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Call your ISP. They probably have a local NTP server on a fixed IP that you can use, and they will have the smallest latency/jitter. Many ISPs have a NTP server, they just don't advertise it very much. – Zoredache Sep 6 '12 at 1:38
Or you can setup one or two NTP servers of your own (perhaps in a DMZ), have those synchronize with, and point your production server to your owhn NTP server(s). – Ansgar Wiechers Sep 6 '12 at 2:29
up vote 3 down vote accepted provides a list of primary and secondary time servers. I think it's reasonable to assume these will generally be static addresses, especially compared to the NTP pool which intentionally rotates every hour.

Please note that not all are open/public access and abide by the rules of engagement, which suggest using secondary servers over primary among other things.

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I once ran a script that did lookups on some of those pool addresses and collected the IP addresses every hour. After a couple weeks I had collected over 200 addresses. This is not a practical resource in limited IP access situation. The ISP time servers should be used. Any decent colo provider has a couple of them. – Skaperen Sep 6 '12 at 4:05
Thanks, I found that some of the servers has Use DNS = No, which means IP would most like be static. – faulty Sep 6 '12 at 11:15

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