add round-robin DNS document

docs/dns_setup.md

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+# Configuring DNS for the Avere cluster
+
+This section explains the basics of setting up a DNS system for your Avere cluster.
+
+* If your system is accessed by NFS clients only, using DNS is recommended but not required - it is possible to specify all network addresses by using numeric IP addresses. 
+
+* If your system supports SMB (CIFS) access, DNS is required, because you must specify a DNS domain for the Active Directory server.
+
+* DNS is required if you want to use Kerberos authentication.
+
+## Load Balancing
+
+To distribute the overall load, configure your DNS domain to use round-robin load distribution for client-facing IP addresses.
+
+## Configuration Details
+
+For optimal performance, configure client-facing cluster addresses as shown in the following diagram.
+
+![Diagram showing configuration information for client-facing addresses - detailed description to come](images/AvereDns.png)
+
+The following ``nsupdate`` commands provide an example of configuring DNS correctly:
+
+        update add avere.example.com. 86400 A 10.0.0.10
+        update add avere.example.com. 86400 A 10.0.0.11
+        update add avere.example.com. 86400 A 10.0.0.12
+        update add client10.example.com. 86400 A 10.0.0.10
+        update add client11.example.com. 86400 A 10.0.0.11
+        update add client12.example.com. 86400 A 10.0.0.12
+        update add 10.0.0.10.in-addr.arpa. 86400 PTR client10.example.com
+        update add 11.0.0.10.in-addr.arpa. 86400 PTR client11.example.com
+        update add 12.0.0.10.in-addr.arpa. 86400 PTR client12.example.com
+
+## DNS Settings
+
+DNS parameters are set in [Cluster > Administrative Network](<http://library.averesystems.com/ops_guide/4_7/gui_admin_network.html#gui-admin-network>) settings page. Settings on that page include: 
+
+* DNS server address
+* DNS domain name
+* DNS search domains
+
+Read [DNS Settings](<http://library.averesystems.com/ops_guide/4_7/gui_admin_network.html#gui-dns>) for more details about using this page.