[[discovery-settings]]
[discrete]
=== Discovery and cluster formation settings

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

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

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

When you want to form a cluster with nodes on other hosts, use the
<<static-cluster-setting, static>> `discovery.seed_hosts` setting. This setting
provides a list of other nodes in the cluster
that are master-eligible and likely to be live and contactable to seed
the <<modules-discovery-hosts-providers,discovery process>>. This setting
accepts a YAML sequence or array of the addresses of all the master-eligible
nodes in the cluster. Each address can be either an IP address or a hostname
that resolves to one or more IP addresses via DNS.

[source,yaml]
----
discovery.seed_hosts:
   - 192.168.1.10:9300
   - 192.168.1.11 <1>
   - seeds.mydomain.com <2>
   - [0:0:0:0:0:ffff:c0a8:10c]:9301 <3>
----
<1> The port is optional and defaults to `9300`, but can
    be <<built-in-hosts-providers,overridden>>.
<2> If a hostname resolves to multiple IP addresses, the node will attempt to
    discover other nodes at all resolved addresses.
<3> IPv6 addresses must be enclosed in square brackets.

If your master-eligible nodes do not have fixed names or addresses, use an
<<built-in-hosts-providers,alternative hosts provider>> to find their addresses
dynamically.

[discrete]
[[initial_master_nodes]]
==== `cluster.initial_master_nodes`

When you start an {es} cluster for the first time, a
<<modules-discovery-bootstrap-cluster,cluster bootstrapping>> step
determines the set of master-eligible nodes whose votes are counted in the
first election. In <<dev-vs-prod-mode,development mode>>, with no discovery
settings configured, this step is performed automatically by the nodes
themselves.

Because auto-bootstrapping is <<modules-discovery-quorums,inherently
unsafe>>, when starting a new cluster in production
mode, you must explicitly list the master-eligible nodes whose votes should be
counted in the very first election. You set this list using the
`cluster.initial_master_nodes` setting.

IMPORTANT: After the cluster forms successfully for the first time, remove the `cluster.initial_master_nodes` setting from each nodes'
configuration. Do not use this setting when
restarting a cluster or adding a new node to an existing cluster.

[source,yaml]
--------------------------------------------------
discovery.seed_hosts:
   - 192.168.1.10:9300
   - 192.168.1.11
   - seeds.mydomain.com
   - [0:0:0:0:0:ffff:c0a8:10c]:9301
cluster.initial_master_nodes: <1>
   - master-node-a
   - master-node-b
   - master-node-c
--------------------------------------------------
<1> Identify the initial master nodes by their <<node-name,`node.name`>>, which
defaults to their hostname. Ensure that the value in
`cluster.initial_master_nodes` matches the `node.name` exactly. If you use a
fully-qualified domain name (FQDN) such as `master-node-a.example.com` for your
node names, then you must use the FQDN in this list. Conversely, if `node.name`
is a bare hostname without any trailing qualifiers, you must also omit the
trailing qualifiers in `cluster.initial_master_nodes`.

See <<modules-discovery-bootstrap-cluster,bootstrapping a cluster>> and
<<modules-discovery-settings,discovery and cluster formation settings>>.
