doc: Add docs about looking up Monitors through DNS

Signed-off-by: Wido den Hollander <wido@42on.com>

doc/rados/configuration/mon-config-ref.rst

@@ -228,6 +228,7 @@ the monitors. However, if you decide to change the monitor's IP address, you
 must follow a specific procedure. See `Changing a Monitor's IP Address`_ for
 details.
 
+Monitors can also be found by clients using DNS SRV records. See `Monitor lookup through DNS`_ for details.
 
 Cluster ID
 ----------
@@ -854,6 +855,7 @@ Miscellaneous
 .. _Monitor Keyrings: ../../../dev/mon-bootstrap#secret-keys
 .. _Ceph configuration file: ../ceph-conf/#monitors
 .. _Network Configuration Reference: ../network-config-ref
+.. _Monitor lookup through DNS: ../mon-lookup-dns
 .. _ACID: http://en.wikipedia.org/wiki/ACID
 .. _Adding/Removing a Monitor: ../../operations/add-or-rm-mons
 .. _Add/Remove a Monitor (ceph-deploy): ../../deployment/ceph-deploy-mon

doc/rados/configuration/mon-lookup-dns.rst

@@ -0,0 +1,46 @@
+===============================
+Looking op Monitors through DNS
+===============================
+
+Since version 11.0.0 RADOS supports looking up Monitors through DNS.
+
+This way daemons and clients do not require a *mon host* configuration directive in their ceph.conf configuration file.
+
+Using DNS SRV TCP records clients are able to look up the monitors.
+
+This allows for less configuration on clients and monitors. Using a DNS update clients and daemons can be made aware of changes in the monitor topology.
+
+By default clients and daemons will look for the TCP service called *ceph-mon* which is configured by the *mon_dns_srv_name* configuration directive.
+
+Example
+-------
+When the DNS search domain is set to *example.com* a DNS zone file might contain the following elements.
+
+First, create records for the Monitors, either IPv4 (A) or IPv6 (AAAA).
+
+::
+
+    mon1.example.com. AAAA 2001:db8::100
+    mon2.example.com. AAAA 2001:db8::200
+    mon3.example.com. AAAA 2001:db8::300
+
+::
+
+    mon1.example.com. A 192.168.0.1
+    mon2.example.com. A 192.168.0.2
+    mon3.example.com. A 192.168.0.3
+
+
+With those records now existing we can create the SRV TCP records with the name *ceph-mon* pointing to the three Monitors.
+
+::
+
+    _ceph-mon._tcp.example.com. 60 IN SRV 10 60 6789 mon1.example.com.
+    _ceph-mon._tcp.example.com. 60 IN SRV 10 60 6789 mon2.example.com.
+    _ceph-mon._tcp.example.com. 60 IN SRV 10 60 6789 mon3.example.com.
+
+In this case the Monitors are running on port *6789*.
+
+The current implementation in clients and daemons does *not* honor nor respect the weight or priority set in SRV records.
+
+All records returned will be treated equally in a Round Robin fashion.