BRev Port 5173 — Vite dev server
Port 5173 is Vite’s default, which makes it the dev-server port for a large share of current front-end work: Vue, React, Svelte, SvelteKit, Astro and anything scaffolded with create-vite. Unlike 3000 it is fairly unambiguous — a listener on 5173 is almost certainly Vite — and by default it binds 127.0.0.1 only, which is the right choice.
The flag that changes the risk is --host (or server.host in the config), commonly added to test on a phone. It binds all interfaces, so the dev server becomes reachable by everyone on the network — and a Vite dev server is not a hardened thing. It serves source files, source maps, environment variables exposed to the client bundle, and a websocket for hot module replacement. On a café, co-working or conference network that is your unbuilt source published to strangers.
The concrete demonstration was CVE-2025-30208 in March 2025: a path-traversal flaw in the dev server’s file-serving allowed reading arbitrary files from the filesystem — not just project files — from any client that could reach the port. Related CVEs followed the same pattern. Vite patched quickly and the lesson stands: our reference marks 5173 never, because a dev server is a tool for your machine, not a service.
Port 5173 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 5173 | Registered |
| Service | Vite dev server | The Vite default — Vue, React, Svelte and SvelteKit. (TCP) |
| Category | Dev & tooling | dev servers, build tools, admin consoles |
| Exposure | Never | Never expose to the internet |
Should port 5173 face the internet? A dev server with HMR; CVE-2025-30208 read arbitrary files through it. 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
Vite starts on 5174 or higher instead
Vite increments past a busy port automatically, so a number you did not choose means 5173 is held — usually a previous run that did not exit, or a second project. Find the PID with the local commands on this page. Setting strictPort: true makes Vite fail loudly instead of quietly moving, which is better for scripts and Docker port mappings.
The site loads on your phone but hot reload never fires
HMR runs over a separate websocket, and with --host it advertises a host the phone cannot reach — or the page is on https while the socket is ws. Set server.hmr.host to the address the device actually uses. Remember that --host also exposed the server to the whole network while you debug this.
Check port 5173 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 5173 — swap example.com for the host you are actually testing.
What on this machine is holding port 5173?
sudo ss -tlnp | grep :5173Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :5173The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :5173The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
Is port 5173 reachable on another host?
nc -vz example.com 5173"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 5173 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 5173Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 5173A 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:5173Handy 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 | |
|---|---|---|---|---|---|
| 5173 | tcp | Vite dev server | The Vite default — Vue, React, Svelte and SvelteKit.A dev server with HMR; CVE-2025-30208 read arbitrary files through it. 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
Is it safe to run Vite with --host?
Only on a network you trust. It publishes the dev server to every device on the segment: source files, source maps, client-side environment variables and the HMR socket — and dev servers have had arbitrary-file-read vulnerabilities. Prefer a tunnel for sharing a preview.
How do I change the Vite port?
vite --port 3001 on the command line, or server.port in vite.config. Add strictPort: true so it fails when the port is taken instead of silently choosing another — which is what makes Docker mappings and scripts unreliable.
Why is something already on 5173?
Almost always another Vite instance: a second project, or a previous run whose process survived the terminal. Identify it with lsof, ss or netstat rather than moving to the next port.