system: add system dns page

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+.. _system-dns:
+
+##########
+System DNS
+##########
+
+
+This section describes configuring DNS on the system, namely:
+
+ * DNS name servers
+ * Domain search order
+
+
+DNS name servers
+================
+
+.. cfgcmd:: set system name-server <address>
+
+   Use this command to specify a DNS server for the system to be used
+   for DNS lookups. More than one DNS server can be added, configuring
+   one at a time. Both IPv4 and IPv6 addresses are supported.
+
+
+
+Example
+-------
+
+In this example, some *OpenNIC* servers are used, two IPv4 addresses
+and two IPv6 addresses:
+
+
+.. code-block:: none
+
+   set system name-server 176.9.37.132
+   set system name-server 195.10.195.195
+   set system name-server 2a01:4f8:161:3441::1
+   set system name-server 2a00:f826:8:2::195
+
+
+Domain search order
+===================
+
+In order for the system to use and complete unqualified host names, a
+list can be defined which will be used for domain searches.
+
+
+.. cfgcmd:: set system domain-search domain <domain>
+
+   Use this command to define domains, one at a time, so that the system
+   uses them to complete unqualified host names. Maximum: 6 entries.
+
+
+.. note:: Domain names can include letters, numbers, hyphens and periods
+   with a maximum length of 253 characters.
+
+
+Example
+-------
+
+The system is configured to attempt domain completion in the following
+order: vyos.io (first), vyos.net (second) and vyos.network (last):
+
+
+.. code-block:: none
+
+   set system domain-search domain vyos.io
+   set system domain-search domain vyos.net
+   set system domain-search domain vyos.network
+