Firewalls close connections between Elasticsearch nodes
Firewalls disconnect the Elasticsearch nodes.
Elasticsearch uses long-running connections to maintain contact between the Elasticsearch nodes. Some on-premises deployments may have firewalls that automatically close connections between the nodes, what leads to the timeout of Elasticsearch.
Resolving issues with closed connections between nodes
You can configure the Elasticsearch to ping each node to refrain firewalls from closing the connections between the nodes. On each Linux virtual machine that runs the Elasticsearch nodes, adjust the TCP keepalive settings according to your requirements.