A search cluster's HTTP API — hit / and it hands you its name, version, and cluster.
Elasticsearch binds its REST API to the first free port in 9200-9300, which in practice is 9200 (node-to-node transport uses 9300); OpenSearch, the fork, keeps the same default. A plain GET of the root returns a JSON banner with the cluster name and version, and every index, document, and admin operation is available over the same interface. IANA's registration for the port is the long-obsolete WAP connectionless session service, which you will never actually meet.
Do not expose
Exposed clusters with no authentication are a standing cause of mass data leaks and ransom-note indices — the API can read, rewrite, and delete every document. Enable security and put it behind a proxy or a private network.
$ curl -sS -m 5 http://TARGET:9200/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.