OS Ubuntu 18
ES version 8.10.1

Since 8.0, Elasticsearch requires HTTPS for inter-node cluster communication, so an SSL certificate has to be generated first.

You can start a single-node ES instance and use its command line to generate the certificate files.

Single-node ES startup command: 

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docker run -d \
    --name elasticsearch \
    -e "discovery.type=single-node"
    -e ELASTIC_PASSWORD=elastic \
    elasticsearch:8.10.1

Enter the container and generate the certificates.

CA certificate: 

elasticsearch-certutil ca		# press enter for every prompt; this produces elastic-stack-ca.p12

Private key certificate: 

elasticsearch-certutil cert --ca elastic-stack-ca.p12		# press enter for every prompt

Copy the certificate files out of the container and delete them:

ES configuration file — copy it to the other nodes as well: 

cluster.name: elasticsearch-cluster
node.name: node1					# remember to change the node name
network.host: 0.0.0.0
http.port: 9200
transport.port: 9300

discovery.seed_hosts:
    - 10.1.1.10:9300
	- 10.1.1.20:9300
	- 10.1.1.30:9300

## Bootstrap the cluster using an initial set of master-eligible nodes:
cluster.initial_master_nodes:
	- node1
	- node2
	- node3

# Whether cross-origin requests are allowed
http.cors.enabled: true

# "*" means any origin is allowed; you can also list specific domains or use a regex.
http.cors.allow-origin: "*"

# Headers allowed in cross-origin requests
http.cors.allow-headers: Authorization

# Whether to return the Access-Control-Allow-Credentials header
http.cors.allow-credentials: true

# Enable x-pack
xpack.security.enabled: true
# Enable SSL
xpack.security.transport.ssl.enabled: true
xpack.security.transport.ssl.verification_mode: certificate
xpack.security.transport.ssl.client_authentication: required

# Point at the CA certificate you generated
xpack.security.transport.ssl.keystore.path: certs/elastic-certificates.p12
xpack.security.transport.ssl.truststore.path: certs/elastic-certificates.p12
xpack.security.authc.api_key.enabled: true

Startup command: 

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docker run -d \
    --name elasticsearch \
    --restart=always \
	--network=host \
    -e ES_JAVA_OPTS="-Xms2g -Xmx2g" \
    -e ELASTIC_PASSWORD=elastic \
    -p 9200:9200 \
    -p 9300:9300 \
    -v /data/config/es/certs/:/usr/share/elasticsearch/config/certs/ \
    -v /data/config/es/elasticsearch.yml:/usr/share/elasticsearch/config/elasticsearch.yml \
    elasticsearch:8.10.1

Test command: curl -k --user elastic:elastic -XGET http://127.0.0.1:9200/_cat/indices?v

Reference: https://www.zsjweblog.com/2022/03/09/elasticsearch8-1-0%e9%9b%86%e7%be%a4%e6%90%ad%e5%bb%ba/