[[discovery-settings]]
=== Important discovery and cluster formation settings
++++
<titleabbrev>Discovery and cluster formation settings</titleabbrev>
++++

There are two important discovery and cluster formation settings that should be
configured before going to production so that nodes in the cluster can discover
each other and elect a master node.

[float]
[[unicast.hosts]]
==== `discovery.seed_hosts`

Out of the box, without any network configuration, Elasticsearch will bind to
the available loopback addresses and will scan local ports 9300 to 9305 to try
to connect to other nodes running on the same server. This provides an auto-
clustering experience without having to do any configuration.

When the moment comes to form a cluster with nodes on other servers, you must
use the `discovery.seed_hosts` setting to provide a seed list of
other nodes in the cluster that are master-eligible and likely to be live and
contactable.  This setting should normally contain the addresses of all the
master-eligible nodes in the cluster.
This setting contains either an array of hosts or a comma-delimited string. Each
value should be in the form of `host:port` or `host` (where `port` defaults to
the setting `transport.profiles.default.port` falling back to `transport.port`
if not set). Note that IPv6 hosts must be bracketed. The default for this
setting is `127.0.0.1, [::1]`
[float]
[[initial_master_nodes]]
==== `cluster.initial_master_nodes`

When you start a brand new Elasticsearch cluster for the very first time, there
is a <<modules-discovery-bootstrap-cluster,cluster bootstrapping>> step, which
determines the set of master-eligible nodes whose votes are counted in the very
first election. In <<dev-vs-prod-mode,development mode>>, with no discovery
settings configured, this step is automatically performed by the nodes
themselves. As this auto-bootstrapping is <<modules-discovery-quorums,inherently
unsafe>>, when you start a brand new cluster in <<dev-vs-prod-mode,production
mode>>, you must explicitly list the names or IP addresses of the
master-eligible nodes whose votes should be counted in the very first election.
This list is set using the `cluster.initial_master_nodes` setting.

[source,yaml]
--------------------------------------------------
discovery.seed_hosts:
   - 192.168.1.10:9300
   - 192.168.1.11 <1>
   - seeds.mydomain.com <2>
cluster.initial_master_nodes:
   - master-node-a <3>
   - 192.168.1.12 <4>
   - 192.168.1.13:9301 <5>
--------------------------------------------------
<1> The port will default to `transport.profiles.default.port` and fallback to
    `transport.port` if not specified.
<2> If a hostname resolves to multiple IP addresses then the node will attempt to
    discover other nodes at all resolved addresses.
<3> Initial master nodes can be identified by their <<node.name,node name>>.
<4> Initial master nodes can also be identified by their IP address.
<5> If multiple master nodes share an IP address then the port must be used to
    disambiguate them.

For more information, see <<modules-discovery-settings>>.
