BRev Port 3000 — the default dev-server port
Port 3000 is the most contested number on a developer’s machine. create-react-app, Express, Next.js, Rails, NestJS, Meteor and Grafana all default to it, which means two projects open at once collide immediately — and the collision is the single most common thing anyone searches about this port.
The important half of the fix is finding what is holding it rather than moving to 3001, because the usual culprit is a process you thought had exited: a dev server whose terminal was closed but whose child survived, a Docker container still publishing the port, or a debugger attached to a stopped run. Moving to 3001 leaves it running, and by Friday there are four. The local commands on this page name the PID directly.
The security note is short but real. Our reference marks 3000 never for exposure: a development server ships verbose error pages with source context, runs an unminified bundle with source maps, often exposes a hot-reload websocket and sometimes a debug endpoint, and has none of the hardening a production build gets. Bind it to 127.0.0.1 — several frameworks now bind all interfaces by default, which on a shared café or office network publishes your working state to everyone on it.
Port 3000 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 3000 | Registered |
| Service | Node / React / Rails dev | The default of create-react-app, Express, Rails, Grafana and half the tutorials. (TCP) |
| Category | Dev & tooling | dev servers, build tools, admin consoles |
| Exposure | Never | Never expose to the internet |
Should port 3000 face the internet? A development server: verbose errors, no hardening, often a debug endpoint. Localhost only.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
“EADDRINUSE: address already in use :::3000”
Find the owner before changing the port. On macOS and Linux: lsof -i :3000 (or ss -tlnp on Linux) prints the PID and program; kill it, and use kill -9 only if it ignores the first signal. On Windows: netstat -ano | findstr :3000 gives the PID and tasklist /FI "PID eq <pid>" names it. All three commands are generated on this page.
The port is free according to the tools and the app still refuses
Check the address family. A process bound to ::1 (IPv6 loopback) and one binding 0.0.0.0 can look like different sockets in a quick read of the output, and Docker Desktop on Windows and macOS proxies ports through a helper process, so the PID you find is com.docker.backend rather than the container. Stop the container, not the helper.
Check port 3000 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 3000 — swap example.com for the host you are actually testing.
What on this machine is holding port 3000?
sudo ss -tlnp | grep :3000Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :3000The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :3000The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
Is port 3000 reachable on another host?
nc -vz example.com 3000"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 3000 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 3000Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 3000A 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:3000Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
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 | |
|---|---|---|---|---|---|
| 3000 | tcp | Node / React / Rails dev | The default of create-react-app, Express, Rails, Grafana and half the tutorials.A development server: verbose errors, no hardening, often a debug endpoint. Localhost only. | 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
How do I find what is using port 3000?
lsof -i :3000 on macOS, sudo ss -tlnp | grep :3000 on Linux, netstat -ano | findstr :3000 on Windows followed by tasklist to name the PID. Each returns the process holding the socket — copy-ready versions are on this page.
Is it safe to expose a dev server on port 3000?
No. Development builds serve full stack traces, source maps and hot-reload endpoints with no hardening, and often connect to a real database. Bind to 127.0.0.1 and use a tunnel if someone else needs to see it.
Why do so many tools default to 3000?
It is above the privileged 1024 boundary, memorable, and Node’s early ecosystem settled on it — after which every framework that wanted to feel familiar copied it. There is nothing special about the number itself.