BRev Port 3306 — MySQL / MariaDB
Port 3306 carries the MySQL client/server protocol, shared by MySQL, MariaDB and Percona. A database is not a public service: it holds everything at once, its authentication is a single credential pair, and there is no rate limiting in front of it by default. Our reference marks it never, and exposed instances are found by internet-wide scans and ransomed — the pattern is a dropped database and a note demanding payment for its return.
The setting that decides your exposure is bind-address in my.cnf. Set to 127.0.0.1, the server accepts only local connections and the port is unreachable from anywhere else no matter what the firewall does. Set to 0.0.0.0 — which several packages and most container images do — it listens on every interface, and only a firewall stands between the database and the internet. Check the value rather than assuming the default.
Application access should come from the private network the application is on, and human access through an SSH tunnel: ssh -L 3306:localhost:3306 user@dbhost, then point a client at 127.0.0.1. That gives you SSH’s key-based authentication and encryption in front of the database credential, with no port exposed at all.
Port 3306 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 3306 | Registered |
| Service | MySQL / MariaDB | The MySQL wire protocol. (TCP) |
| Category | Databases | data stores and their wire protocols |
| Exposure | Never | Never expose to the internet |
Should port 3306 face the internet? Never public — bind to 127.0.0.1 or a private network. Exposed instances are brute-forced and ransomed within days.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
“Host ‘x.x.x.x’ is not allowed to connect to this MySQL server”
The network reached the database — the port is open and listening — and MySQL’s own account host-matching rejected you. That is a permission question (user@host grants), not a firewall question. Worth noticing: if you can see this message from outside, so can everyone else, and the port should not have been reachable in the first place.
“Can’t connect to MySQL server on …” with error 2003
Nothing answered on 3306: the server is stopped, bound to 127.0.0.1 only, or a firewall dropped the packet. Distinguish them from the host itself — the local commands on this page show whether mysqld is listening and on which address. A listener on 127.0.0.1 with a client on another machine is the most common cause.
Check port 3306 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 3306 — swap example.com for the host you are actually testing.
Is port 3306 reachable on another host?
nc -vz example.com 3306"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 3306 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 3306Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 3306A 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:3306Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 3306?
sudo ss -tlnp | grep :3306Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :3306The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :3306The 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 | |
|---|---|---|---|---|---|
| 3306 | tcp | MySQL / MariaDB | The MySQL wire protocol.Never public — bind to 127.0.0.1 or a private network. Exposed instances are brute-forced and ransomed within days. | Never | |
| 33060 | tcp | MySQL X protocol | MySQL’s document-store protocol.A second door into the same database. | 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 open port 3306 to the internet?
No. Exposed MySQL instances are brute-forced continuously and ransomed routinely. Bind to 127.0.0.1 or a private subnet, and reach it through an SSH tunnel, a VPN or the application server — never directly.
How do I connect to a remote MySQL database safely?
An SSH tunnel is the simplest correct answer: ssh -L 3306:localhost:3306 user@dbhost, then connect your client to 127.0.0.1:3306. SSH supplies key authentication and encryption, and the database port stays closed to the world.
Does MariaDB use a different port?
No, MariaDB uses 3306 as well and speaks a compatible protocol, so nothing about the port or the exposure advice changes. Running both on one host means one of them has to move.