How to set Elasticsearch discovery seed hosts

Setting discovery.seed_hosts tells a self-managed Elasticsearch node which master-eligible transport addresses to contact when it starts or rejoins a cluster. A complete seed list helps nodes find the same cluster after restarts, node additions, DNS changes, or maintenance windows.

Elasticsearch reads discovery.seed_hosts as a static node setting from /etc/elasticsearch/elasticsearch.yml on package-managed Linux nodes. The entries are transport-layer addresses, not HTTP URLs, so each name or IP address must lead to a reachable 9300 endpoint unless the cluster uses a different configured transport port.

Peer discovery still depends on matching cluster.name values, routable transport addresses, and compatible security material between nodes. For a brand-new multi-node cluster, cluster.initial_master_nodes is used only for the first election and should be removed after the cluster forms; joining or restarting nodes should use the seed host list without a bootstrap list.

Steps to set Elasticsearch discovery seed hosts:

Back up the current Elasticsearch configuration file.

$ sudo cp --archive /etc/elasticsearch/elasticsearch.yml /etc/elasticsearch/elasticsearch.yml.bak

Open /etc/elasticsearch/elasticsearch.yml on the node that needs the discovery update.
```
$ sudoedit /etc/elasticsearch/elasticsearch.yml
```
Set the shared cluster.name value if it is not already defined.

Nodes with different cluster.name values never join the same cluster.
```
cluster.name: search-cluster
```
Set discovery.seed_hosts to the transport addresses of every master-eligible node in the cluster.
```
discovery.seed_hosts:
  - es-master-a:9300
  - es-master-b:9300
  - es-master-c:9300
```
Use fixed DNS names or IP addresses that resolve to reachable transport endpoints. Each address may be host or host:port; if the port is omitted, Elasticsearch checks transport.profiles.default.port, then transport.port, and otherwise uses 9300.
Remove cluster.initial_master_nodes from joining or restarting nodes.
```
cluster.initial_master_nodes:
  - es-master-a
  - es-master-b
  - es-master-c
```
cluster.initial_master_nodes is only for the first bootstrap of a brand-new cluster. Do not leave it on nodes joining an existing cluster, nodes that are restarting, or nodes in a full-cluster restart.
Save the file.
Apply the same discovery.seed_hosts list on the remaining nodes that should be able to rejoin the cluster.

The first installed node in a multi-node package deployment also needs a complete discovery.seed_hosts list, or later restarts can fail discovery bootstrap checks.
Restart Elasticsearch on one changed node at a time.
```
$ sudo systemctl restart elasticsearch
```
Wait for each restarted node to rejoin before restarting the next one so the cluster keeps quorum and shard movement stays controlled.

Related: How to manage the Elasticsearch service with systemctl in Linux
Confirm the service returned to the running state.
```
$ systemctl is-active elasticsearch
active
```
Use sudo journalctl --unit=elasticsearch.service --no-pager --lines=80 when the unit does not return active.
Wait for the expected node count to appear through the local HTTP API.
```
$ curl --silent --show-error --fail "http://localhost:9200/_cluster/health?wait_for_nodes=>=3&timeout=60s&filter_path=cluster_name,status,timed_out,number_of_nodes&pretty"
{
  "cluster_name" : "search-cluster",
  "status" : "green",
  "timed_out" : false,
  "number_of_nodes" : 3
}
```
number_of_nodes and timed_out confirm discovery more directly than the health color alone.

Self-managed package installs with security enabled typically require HTTPS, a trusted CA, and authentication. Use the same scheme and credentials that operators use for the cluster HTTP API.
List cluster nodes and confirm that the node rejoined the expected cluster state.
```
$ curl --silent --show-error --fail "http://localhost:9200/_cat/nodes?v&s=name&h=ip,name,node.role,master"
ip         name        node.role   master
192.0.2.40 es-master-a cdfhilmrstw *
192.0.2.41 es-master-b cdfhilmrstw -
192.0.2.42 es-master-c cdfhilmrstw -
```
The master column must show exactly one *. If the node is missing from the list, recheck transport reachability, DNS resolution, cluster.name, and whether the seed list targets master-eligible nodes.

Related: How to check Elasticsearch cluster formation