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Enabling Security Groups on Rackspace Public Cloud servers (PublicNet connection) unexpectedly blocks NTP.
NTP is not whitelisted by default, and it typically uses UDP (stateless) transport. While Security Groups do allow inbound connections that are initiated by your Cloud Server (stateful rules), they cannot work with UDP-based traffic.
All Rackspace DNS servers are fully functional NTP servers as well. Thus, we recommend using the Rackspace DNS servers as your NTP servers.