docs: update for chronyc in centos recipes; add config option to allow query
from local computes
HPC systems rely on synchronized clocks throughout the system and the
NTP protocol can be used to facilitate this synchronization. To enable NTP
-services on the SMS host with a specific server \texttt{\$\{ntp\_server\}},
+services on the SMS host with a specific server \texttt{\$\{ntp\_server\}}, and
+allow this server to serve as a local time server for the cluster,
issue the following:
% begin_ohpc_run
@@ -8,6 +9,19 @@ issue the following:
\begin{lstlisting}[language=bash,literate={-}{-}1,keywords={},upquote=true,keepspaces]
[sms](*\#*) systemctl enable chronyd.service
[sms](*\#*) echo "server ${ntp_server}" >> /etc/chrony.conf
+[sms](*\#*) echo "allow all" >> /etc/chrony.conf
[sms](*\#*) systemctl restart chronyd
\end{lstlisting}
% end_ohpc_run
+
+\begin{center}
+\begin{tcolorbox}[]
+\small Note that the ``allow all'' option specified for the chrony time daemon
+allows all servers on the local network to be able to synchronize with the SMS
+host. Alternatively, you can restrict access to fixed IP ranges and an example
+config line allowing access to a local class B subnet is as follows:
+\begin{lstlisting}[language=bash]
+allow 192.168.0.0/16
+\end{lstlisting}
+\end{tcolorbox}
+\end{center}
