BRev Port 9090 — Prometheus and Cockpit
Two common services share port 9090, and mistaking one for the other is a meaningful error. Prometheus serves its query UI and API there, and Cockpit — the web-based Linux server manager shipped with Red Hat and Fedora — serves its console on the same number. One exposes your monitoring data; the other exposes a root terminal in a browser.
Prometheus deliberately has no authentication. Upstream’s position is that authentication and TLS belong in a reverse proxy, so a default installation answers any query from anyone who reaches the port. That matters more than it sounds: metric labels routinely carry internal hostnames, service names, database identifiers, version numbers and endpoint paths — a detailed internal map — and the admin API can delete time series outright if it is enabled.
Cockpit is a different order of exposure. It authenticates with the system’s own accounts and gives a terminal, service control, storage management and package installation — so a valid login is the host. Our reference marks 9090 never for the pair, and the correct shape for both is the same: bind to localhost or a management network and put an authenticating reverse proxy or a VPN in front.
Port 9090 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 9090 | Registered |
| Service | Prometheus / Cockpit | Prometheus UI, or the Cockpit server manager. (TCP) |
| Category | Infrastructure | the plumbing: DNS, directory, routing, monitoring |
| Exposure | Never | Never expose to the internet |
Should port 9090 face the internet? Prometheus has no authentication at all; Cockpit is root on the box.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
You cannot tell which service is on 9090
Fetch the root path: Prometheus redirects to its query UI and exposes /-/healthy and /metrics, while Cockpit presents a system login page. The local commands on this page name the process, which settles it immediately — prometheus versus cockpit-ws.
Prometheus is behind a proxy and still reachable directly
The usual gap. Prometheus binds 0.0.0.0 by default, so adding an authenticating proxy on 443 does nothing until 9090 itself is restricted. Use --web.listen-address=127.0.0.1:9090 so the proxy is the only path, and remember exporters (node_exporter on 9100 and friends) need the same treatment.
Check port 9090 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 9090 — swap example.com for the host you are actually testing.
Is port 9090 reachable on another host?
nc -vz example.com 9090"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 9090 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 9090Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 9090A 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:9090Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 9090?
sudo ss -tlnp | grep :9090Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :9090The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :9090The 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 | |
|---|---|---|---|---|---|
| 9090 | tcp | Prometheus / Cockpit | Prometheus UI, or the Cockpit server manager.Prometheus has no authentication at all; Cockpit is root on the box. | 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
Does Prometheus have authentication?
Not built in by design — upstream expects a reverse proxy to provide TLS and authentication. A default installation answers any query from anyone who can reach port 9090.
What can someone learn from an exposed Prometheus?
A great deal: metric labels carry internal hostnames, service and job names, endpoint paths, database identifiers and software versions. That is a map of the internal estate, plus enough version detail to pick an exploit.
Is Cockpit on 9090 safe to expose?
No. It authenticates with system accounts and grants a terminal, service control and package management — a successful login is root-equivalent on the host. Reach it through a VPN or an SSH tunnel.