Toolman 100% local

Port 1433 — Microsoft SQL Server

TCP registered

What runs on port 1433

The default port for MS SQL Server.

Microsoft SQL Server. Named instances complicate this: the SQL Browser service on UDP 1434 tells clients which port a named instance is actually using, so blocking 1434 breaks named-instance connections while the default instance on 1433 keeps working. Like every database port, this should be reachable only from application servers, never from the internet — exposed SQL Server instances are scanned for continuously.

Security considerations

Never expose a database directly to the internet. Put it behind a VPN or a private network.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :1433
sudo ss -lntp | grep :1433

# Windows
netstat -ano | findstr :1433
Get-NetTCPConnection -LocalPort 1433

# is it reachable from outside?
nc -zv example.com 1433
curl -v telnet://example.com:1433

Freeing the port

# find the process, then stop it
sudo lsof -ti :1433 | xargs kill        # Linux / macOS
netstat -ano | findstr :1433            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Should this port be open to the internet?

No. Port 1433 should never be reachable from a public address. Bind it to localhost or a private network, and reach it through a VPN or bastion host if remote access is genuinely needed. Internet-wide scanners find newly exposed instances of this service within minutes.

Opening or blocking port 1433

There is no blanket-allow rule on this page, because there is no case for one. Microsoft SQL Server on port 1433 should be reachable from named addresses or not at all, so what follows restricts rather than opens:

# ufw — Debian, Ubuntu
sudo ufw allow from 10.0.0.0/8 to any port 1433 proto tcp
sudo ufw deny 1433/tcp                    # everything else

# firewalld — RHEL, Fedora, CentOS
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1433 protocol=tcp accept'
sudo firewall-cmd --reload

# iptables, if you are still there
sudo iptables -A INPUT -p tcp --dport 1433 -s 10.0.0.0/8 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 1433 -j DROP

# Windows
New-NetFirewallRule -DisplayName "Microsoft SQL Server 1433" -Direction Inbound -Protocol TCP -LocalPort 1433 -RemoteAddress 10.0.0.0/8 -Action Allow

Change 10.0.0.0/8 to whatever your private range actually is. A firewall rule is the second line: the first is binding the service to a private interface, because a service that only listens on 127.0.0.1 cannot be reached however the firewall is configured.

Quick reference

Port1433
ProtocolTCP
ServiceMicrosoft SQL Server
RangeRegistered (1024–49151) — any user process may bind

Frequently asked questions

What is port 1433 used for?

The default port for MS SQL Server.

Is it safe to open port 1433?

Never expose a database directly to the internet. Put it behind a VPN or a private network.

How do I check if port 1433 is open?

Locally, sudo lsof -i :1433 on macOS or Linux, or netstat -ano | findstr :1433 on Windows. From outside, nc -zv host 1433 tells you whether anything answers.

Why do I get "address already in use" on port 1433?

Another process is bound to it — often a previous run of your own program that did not exit cleanly. Find it with lsof -ti :1433 and stop it, or configure your application to use a different port.

Can I change the port this service uses?

Almost always yes, in the service's configuration. Moving off a default port reduces automated scan noise, but it is obfuscation rather than security — a real attacker scans all 65,535.

Other common ports

All port numbers