BZ1887404: Add allowlist firewall info for NTP servers and custom DNS

Author: skrthomas

modules/configuring-firewall.adoc

@@ -151,3 +151,13 @@ the routes work. If you are the cluster administrator and do not want to allow
 * `console-openshift-console.apps.<cluster_name>.<base_domain>`, or the host name
 that is specified in the `spec.route.hostname` field of the
 `consoles.operator/cluster` object if the field is not empty.
+
+. If you use a default Red Hat Network Time Protocol (NTP) server allow the following URLs:
+* `1.rhel.pool.ntp.org`
+* `2.rhel.pool.ntp.org`
+* `3.rhel.pool.ntp.org`
+
+[NOTE]
+====
+If you do not use a default Red Hat NTP server, verify the NTP server for your platform and allow it in your firewall.
+====