[CentOS] Re: ntpd stuck on stratum 16 = not synced

[Sean Carolan]

> The zeros in the "reach" column indicate that the server has been unable to
> receive any packets from the upstream servers.
> 
> Is your server inside a firewall? If so, perhaps it is blocking NTP traffic.
> You need to have it allow UDP port 123 in both directions. You don't need
> port forwarding from outside to in, since all incoming packets will be replies
> to outgoing packets.
> 
> If it is not inside a firewall, perhaps you have iptables on the server itself
> blocking UDP port 123 traffic.

Fantastic, Tony.  This is the information I needed.  Our ISP does in fact 
block UDP packets and I suspect this is why the sync is failing.

One question though - how come I can use ntpdate servername to update them by 
hand?  Shouldn't that be blocked as well?