BRev Port 1433 — Microsoft SQL Server
Port 1433 is the default TCP listener for the default SQL Server instance. Named instances are different and this is the detail that confuses most firewall work: by default they take a dynamic port assigned at service start, and clients discover it by asking the SQL Server Browser service on UDP 1434. That means a firewall rule for TCP 1433 alone does not reach a named instance, and opening UDP 1434 hands out an inventory of your instances to anyone who asks.
The exposure verdict is never. SQL Server is brute-forced continuously, and the account being attacked is almost always sa — a well-known, high-privilege login that exists on every installation. When xp_cmdshell is enabled, a successful login is command execution on the host, which is why exposed instances turn into cryptominers and ransomware footholds rather than just data breaches.
The right configuration is a fixed TCP port for each named instance, the SQL Server Browser disabled, and the listener reachable only from the application subnet — plus Windows Authentication rather than SQL logins where the domain allows it, and the sa account disabled or renamed. In Azure SQL the equivalent is the firewall rule list, and “allow Azure services” is broader than most people intend.
Port 1433 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 1433 | Registered |
| Service | Microsoft SQL Server | The MSSQL wire protocol. (TCP) |
| Category | Databases | data stores and their wire protocols |
| Exposure | Never | Never expose to the internet |
Should port 1433 face the internet? Never public — bind to localhost or a private subnet. It is brute-forced around the clock.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
A named instance connects by name locally and not remotely
The client resolved the dynamic port through SQL Server Browser on UDP 1434, which is available locally and blocked across the firewall. Assign the instance a static TCP port in SQL Server Configuration Manager, open exactly that port, and connect with host,port (a comma, not a colon) to skip the browser entirely.
“A network-related or instance-specific error occurred”
The generic client message covers several distinct faults: the service stopped, TCP/IP disabled in the network protocols (it is off by default in some editions), the wrong instance name, or a dropped packet. Test the port itself with the commands on this page first — that separates “nothing is listening” from “SQL Server rejected me”.
Check port 1433 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 1433 — swap example.com for the host you are actually testing.
Is port 1433 reachable on another host?
nc -vz example.com 1433"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 1433 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 1433Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 1433A 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:1433Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 1433?
sudo ss -tlnp | grep :1433Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :1433The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :1433The 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 | |
|---|---|---|---|---|---|
| 1433 | tcp | Microsoft SQL Server | The MSSQL wire protocol.Never public — bind to localhost or a private subnet. It is brute-forced around the clock. | 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
Should port 1433 be open to the internet?
No. It is one of the most consistently brute-forced ports there is, the sa account is a known target, and a compromise can mean code execution on the host. Restrict it to the application subnet or a VPN.
What is UDP 1434 for?
The SQL Server Browser service, which tells clients which port a named instance is using. It is also how an attacker enumerates every instance on a host, so disable it and use static ports instead.
Why does my named instance not use 1433?
Only the default instance claims 1433. Named instances take a dynamic port at startup unless you configure a static one, which is why they need either the Browser service or an explicit port in the connection string.