Looking up 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.

mon dns srv name

  • Description

  • the service name used querying the DNS for the monitor hosts/addresses

  • Type

  • String

  • Default

  • ceph-mon

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).

  1. mon1.example.com. AAAA 2001:db8::100
  2. mon2.example.com. AAAA 2001:db8::200
  3. mon3.example.com. AAAA 2001:db8::300
  1. mon1.example.com. A 192.168.0.1
  2. mon2.example.com. A 192.168.0.2
  3. 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.

  1. _ceph-mon._tcp.example.com. 60 IN SRV 10 20 6789 mon1.example.com.
  2. _ceph-mon._tcp.example.com. 60 IN SRV 10 30 6789 mon2.example.com.
  3. _ceph-mon._tcp.example.com. 60 IN SRV 20 50 6789 mon3.example.com.

Now all Monitors are running on port 6789, with priorities 10, 10, 20 and weights 20, 30, 50 respectively.

Monitor clients choose monitor by referencing the SRV records. If a cluster has multiple Monitor SRV recordswith the same priority value, clients and daemons will load balance the connections to Monitors in proportionto the values of the SRV weight fields.

For the above example, this will result in approximate 40% of the clients and daemons connecting to mon1,60% of them connecting to mon2. However, if neither of them is reachable, then mon3 will be reconsidered as a fallback.