BRev Port 9200 — Elasticsearch
Port 9200 serves the Elasticsearch HTTP REST API — the interface for querying, indexing, and administering the cluster. It is plain HTTP with JSON, which means an exposed cluster does not need a client or a protocol tool: a browser and a URL are enough to list every index and page through every document. That accessibility is exactly why exposed Elasticsearch has produced so many of the largest recorded data leaks.
For most of its history security was a paid feature, so the free distribution shipped with no authentication and, for a long period, bound all interfaces. Elasticsearch 8 changed that — TLS and authentication are on by default now — but the deployed population and a decade of tutorials still reflect the old defaults, and every Docker Compose file that sets xpack.security.enabled=false is recreating them deliberately.
Port 9300 is the other half: the binary transport protocol nodes use to talk to each other and form a cluster. It should never be reachable from outside the cluster network at all — a node that can join can read everything. And an exposed Kibana on 5601 is not a separate problem; it is a user interface onto the same data, so finding one implies the other.
Port 9200 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 9200 | Registered |
| Service | Elasticsearch HTTP | The Elasticsearch REST API. (TCP) |
| Category | Databases | data stores and their wire protocols |
| Exposure | Never | Never expose to the internet |
Should port 9200 face the internet? The canonical open-database leak — thousands of exposed clusters have been indexed and ransomed.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
A cluster is reachable and you want to know what was exposed
Assume everything. A GET on /_cat/indices lists every index, and the search API returns documents to anyone who can reach the port — no authentication step to fail, no rate limit, no audit trail on the free tier. Treat the whole dataset as disclosed for the period the port was open, and check the index list for anything an attacker added.
The cluster stays yellow or red after locking the network down
Cluster health depends on nodes reaching each other on 9300, not on 9200. A firewall change that closes the transport port between nodes leaves shards unassigned while the REST API still answers — which looks like a data problem and is a networking one. Check node-to-node connectivity on 9300 first.
Check port 9200 yourself
No browser can open a raw TCP socket, so no web page can honestly tell you whether this port is open on a remote host. These are the real commands, generated for port 9200 — swap example.com for the host you are actually testing.
Is port 9200 reachable on another host?
nc -vz example.com 9200"succeeded!" means the TCP handshake completed — the port is open. "Connection refused" is closed; a hang until timeout means filtered (a firewall dropped it silently).
nmap -Pn -p 9200 example.comThe only common tool that distinguishes open / closed / filtered. -Pn skips the ping sweep, so it still works against hosts that ignore ICMP.
Test-NetConnection example.com -Port 9200Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 9200A blank screen or a banner means it connected. "Connection refused" means closed. Quit with Ctrl-] then `quit`.
curl -v --max-time 5 telnet://example.com:9200Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 9200?
sudo ss -tlnp | grep :9200Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :9200The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :9200The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
No web page can tell you whether a port is open on a remote host
A browser cannot open a raw TCP socket — there is no API for it, by design. So a site that reports “port 3389 is OPEN on 203.0.113.10” is not checking from your browser; it is asking its own server to scan on your behalf. That works, but it means their server sees the host you asked about, scans from their address rather than yours, and answers a question about their network path, not yours.
We don't have a server here and won't pretend to. So this page does three honest things instead:
| Port | Proto | Service | What it is | Exposure | |
|---|---|---|---|---|---|
| 9200 | tcp | Elasticsearch HTTP | The Elasticsearch REST API.The canonical open-database leak — thousands of exposed clusters have been indexed and ransomed. | Never |
http://host:port/ and times it. A connection that is established proves a socket is open; an instant refusal usually means nothing is listening; anything else is honestly reported as “can't tell”. Each attempt gives up after 2.5s.nc -vz example.com 443
nmap -Pn -p 443 example.com
telnet example.com 443
curl -v --max-time 5 telnet://example.com:443
This page cannot answer that, and neither can any other page — from your browser. Opening a TCP connection to an arbitrary host and port is not something JavaScript is allowed to do. Every “open port checker” that gives you a green tick is running the scan on their server. Which is fine, as long as you know what you are agreeing to: they learn which host and port you were worried about, and the answer they give you is about the route from their data centre — not from the internet as your users see it.
What to do instead
Run the check from outside your own network. Copy an nmap or nc line from the card above and run it from a machine that is not behind the same router — a phone on mobile data with a terminal app, a cheap VPS, a colleague's laptop. Running it from inside the LAN tests the LAN, not the internet, and will happily say “open” while the world sees nothing.
Then check the thing that is actually blocking it, in this order: the service is bound to0.0.0.0 and not 127.0.0.1 · the host firewall (ufw, firewalld, Windows Defender Firewall) · the cloud security group or the router's port-forward rule · and finally your ISP, which commonly blocks 25, 80 and 445 on residential lines whatever you configure.
What a browser genuinely can verify
One thing: that an HTTPS service answers. If what you actually want to know is “is my site up and how fast is it”, the latency test measures real round trips to a CORS-open URL. If the name doesn't resolve, or resolves somewhere unexpected, that is a DNS problem, not a port problem — and it is the more common cause of “my port is closed” than an actual firewall.
An IP address finds the machine; a port picks which program on it gets the traffic. TCP sets up a connection first — a handshake, ordered delivery, retransmission — which is why the web, email and SSH use it. UDP just sends the packet with no handshake and no guarantee it arrives, which is why DNS, video calls and games use it: late data is worse than lost data.
That difference is why a UDP port is so much harder to test. TCP either completes a handshake or it doesn't, so a tool can tell you. A UDP service that has nothing to say to an unexpected packet stays silent — and silence looks identical to a firewall dropping it.
The three ranges
0–1023Assigned by IANA to core services. On Unix, binding one needs root (or CAP_NET_BIND_SERVICE).1024–49151Registered by vendors for their applications. Any user can bind these — which is why every dev server lives here.49152–65535Never assigned. The OS hands these out as the SOURCE port of your outgoing connections. (Linux actually uses 32768–60999 by default.)Closed is not the same as filtered
Closed means the host answered: the packet arrived, nothing is listening, and you got an immediate refusal (a TCP RST). Filtered means nothing answered at all — a firewall dropped your packet in silence, so you wait for a timeout and learn nothing. The difference matters: closed proves the host is reachable and the port is free; filtered tells you only that something between you and it is discarding traffic.nmap reports the two separately. A browser cannot distinguish them, which is why this page reports a timeout as “can't tell” rather than guessing.
The one habit worth keeping
Bind services to 127.0.0.1 instead of 0.0.0.0 unless something outside the machine genuinely needs them, and reach them over SSH or a VPN. Every database on the never-expose list above ships with that advice, and the leaks all begin the same way: a default bind address, a cloud instance with a public IP, and a scanner that found it within the hour.
The blocked-port list this page uses is the WHATWG Fetch Standard's “bad port” table — the ports every browser refuses to connect to (Chrome calls it ERR_UNSAFE_PORT). Local probing is additionally gated by Chrome's Local Network Access permission, which replaced Private Network Access and ships in Chrome 142.
FAQ
Is it safe to expose Elasticsearch on port 9200?
No. It is an HTTP API over your entire index set, historically without authentication. Bind it to a private network, enable security and TLS, and let only the application tier reach it.
What is the difference between ports 9200 and 9300?
9200 is the HTTP REST API used by clients. 9300 is the internal transport protocol nodes use to form the cluster and replicate data. 9300 must stay inside the cluster network — a host that can join it can read everything.
Does Elasticsearch have authentication by default?
Since version 8, yes — TLS and authentication are enabled on a new installation. Earlier versions did not on the free tier, which is why so many deployments and tutorials disable security explicitly.