I'm trying to sync the clock of our production server located in a data center with pool.ntp.org. 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

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

Not realizing ntp.org is using round robin DNS and the servers being voluntary, they changes from time to time. In fact the IP I've got from 3.asia.ntp.org 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.