iptables and several NTP servers

[shen mode]

Hi,

I'm setting up a NTP server which sync its time with
some public NTP servers.

According to publich NTP server list, those servers
seem to use DNS names as public name which may resolve
to several IP addresses. 

I want to allow ntp port communication between our NTP
server and three or more public NTP servers. How can I
set up iptables to allow for this?

Must I use IP address ?

thanks.


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