Port / TCP
3306MySQL / MariaDB
The default listener for MySQL and MariaDB client connections, replication, and admin tooling.
If it is internet-facing: Finding 3306 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Trusted network only
Transport
Encryption optional
Risk if public
High
Exposure verdict
Port 3306 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Encryption is available but negotiated or configured rather than guaranteed. A client that does not insist on it will silently fall back to plaintext.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other databases & caches ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Public MySQL attracts continuous credential stuffing, and ransom scripts that drop databases and leave a payment note.
- Bind to 127.0.0.1 for single-host applications; use a private network or tunnel for everything else.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 3306 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 3306 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 3306 -j DROPIf you do need it open to everyone
sudo ufw allow 3306/tcpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -tlpn 'sport = :3306'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iTCP:3306 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 3306 <host>A version probe confirms whether MySQL / MariaDB is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 3306The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
no port changeChange the configuration, not the port
An SSH tunnel on port 22 gives remote access without publishing the database at all.
The fix here lives in the service configuration rather than in a different port assignment.