BRev Port 5900 — VNC
VNC shares a screen and its input devices over the network, and unlike RDP it is a family of independent implementations rather than one product — RealVNC, TightVNC, TigerVNC, UltraVNC, x11vnc and the VNC server built into macOS Screen Sharing all speak the RFB protocol on 5900. What they do not share is a security model, which is the root of most of the trouble.
The classic RFB authentication is a DES challenge-response with the password truncated to eight characters, so even a “strong” VNC password has a small keyspace, and it protects the session only — the pixels afterwards are unencrypted in the base protocol. Worse, a large share of internet-exposed instances are configured with no authentication whatsoever, which is why exposed VNC has been catalogued repeatedly as open desktops, some of them industrial control panels.
The numbering trips people: VNC ports are 5900 plus the X display number, so display :0 is 5900, :1 is 5901, :2 is 5902. A machine running several user sessions listens on a run of ports, and “VNC is not on 5900” usually means the server started on the next free display rather than that it failed. The right deployment is to bind the server to localhost and reach it through an SSH tunnel, which supplies the authentication and encryption the protocol lacks.
Port 5900 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 5900 | Registered |
| Service | VNC | Virtual Network Computing remote desktop. (TCP) |
| Category | Remote access | shells, desktops and VPNs |
| Exposure | Never | Never expose to the internet |
Should port 5900 face the internet? Weak 8-character password limit, and many instances have no password at all. Tunnel over SSH or a VPN.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
The viewer connects and then drops, or refuses the password
Check the eight-character truncation first — several servers silently cut the password, so a 12-character password set in one tool may not match what another sends. Then check which display you are on: connecting to 5900 when the session is on :1 gives a refusal that looks like an auth failure. Point the viewer at host:1 (or host:5901) explicitly.
You want it reachable from outside and are about to forward 5900
Tunnel it instead: ssh -L 5900:localhost:5900 user@host, then point the viewer at localhost:5900. The VNC server stays bound to loopback, SSH provides the encryption and the real authentication, and nothing about the VNC configuration has to be trusted. This is a one-line change with a much better outcome than any VNC-side hardening.
Check port 5900 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 5900 — swap example.com for the host you are actually testing.
Is port 5900 reachable on another host?
nc -vz example.com 5900"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 5900 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 5900Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 5900A 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:5900Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 5900?
sudo ss -tlnp | grep :5900Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :5900The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :5900The 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 | |
|---|---|---|---|---|---|
| 5900 | tcp | VNC | Virtual Network Computing remote desktop.Weak 8-character password limit, and many instances have no password at all. Tunnel over SSH or a VPN. | 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 VNC encrypted?
Not in the base RFB protocol — the session content travels in the clear. Some implementations add TLS or their own encryption layer, but you cannot assume it and viewers rarely warn. Tunnelling over SSH or a VPN is the reliable answer.
Why is VNC on port 5901 instead of 5900?
Because the port is 5900 + the display number. Display :1 listens on 5901, :2 on 5902, and so on. A server started for a second user picks the next display, which moves the port.
VNC or RDP?
RDP is faster on Windows because it sends drawing operations rather than pixels, and it has real authentication. VNC is cross-platform and shares the physical console — which is what you want for a headless Linux box or to see what is on a screen someone is already using. Neither should be exposed directly.