BRev Port 8443 — HTTPS alternate
Port 8443 is to 443 what 8080 is to 80: TLS on a port an unprivileged process can bind. Tomcat’s default TLS connector uses it, and so do a very large number of appliances and management interfaces — firewalls, storage arrays, hypervisor consoles, printers, Kubernetes ingress controllers and VMware components all commonly publish their web administration here.
That population is the whole risk profile. The traffic is encrypted, which people read as “secured”, but what is on the far end is usually an administrative console with full control of a device — and appliance web UIs have a long history of pre-authentication vulnerabilities, precisely because they are written by hardware vendors rather than web teams. Our reference marks it care for exactly this reason: encrypted, but usually an admin interface.
A second detail worth knowing: an appliance on 8443 almost always presents a self-signed certificate, so the browser warning is expected and everyone learns to click through it — which destroys the warning’s value on the day it is a real interception. If the interface is important enough to reach, it is worth a real certificate from an internal CA, or restricting it to a management network where the question does not arise.
Port 8443 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 8443 | Registered |
| Service | HTTPS alt | Alternate TLS port — Tomcat, appliances, admin UIs. (TCP) |
| Category | Web | HTTP(S) and things browsers speak |
| Exposure | Care | Expose with care |
Should port 8443 face the internet? Encrypted, but usually an admin interface. Restrict and patch.
Legitimately public in some setups, but only hardened — current version, real authentication, rate limits, and ideally an allow-list.
What commonly goes wrong
The browser refuses to connect and the port is definitely open
Usually a scheme mismatch — visiting http://host:8443 against a TLS listener produces a confusing failure rather than a redirect, and some servers respond with binary that the browser rejects. Try https:// explicitly. openssl s_client -connect host:8443 settles it: if it completes a handshake, TLS is there and the problem is the client.
Certificate warnings that everyone clicks through
Standard on appliances, which ship self-signed certificates keyed to a hostname you do not use. Install a certificate from an internal CA that your machines already trust, or restrict the interface to a management VLAN. Training people to bypass certificate warnings on an admin console is the worst of the available outcomes.
Check port 8443 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 8443 — swap example.com for the host you are actually testing.
Is port 8443 reachable on another host?
nc -vz example.com 8443"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 8443 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 8443Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 8443A 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:8443Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 8443?
sudo ss -tlnp | grep :8443Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :8443The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :8443The 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 | |
|---|---|---|---|---|---|
| 8443 | tcp | HTTPS alt | Alternate TLS port — Tomcat, appliances, admin UIs.Encrypted, but usually an admin interface. Restrict and patch. | Care |
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 port 8443 secure because it is HTTPS?
Encryption protects the traffic, not the application. Most things listening on 8443 are administrative interfaces with a poor patch history, so the practical exposure is higher than a public website on 443, not lower.
Why 8443 rather than 443?
Because 443 is privileged on Unix and often already taken by the main web server. 8443 lets a second TLS service run as an ordinary user on the same host — which is why application servers and appliances default to it.
Should 8443 be reachable from the internet?
Only when it is a real public service. When it is an admin console — the usual case — restrict it by source address or put it behind a VPN. Encrypted access to a management interface is still access to a management interface.