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ProtocolPorts

Port reference

Port 1434 (UDP) – MS SQL Monitor

SQL Server Browser / Resolution Service — tells clients which TCP port each named SQL Server instance listens on.

udpRegistered

Quick facts

Transport
udp
Category
Registered
Risk level
Critical

Actively exploited and high-impact — keep it off the public internet.

Default state

Open when the SQL Server Browser service is running, especially with named or multiple instances. Should not face the internet.

What is port 1434 used for?

UDP 1434 is the Microsoft SQL Server Browser (Resolution Service). When a client tool such as SQL Server Management Studio or a database driver wants to connect to a named SQL Server instance, it sends a small UDP query here and the server replies with the dynamic TCP port that instance is listening on. It's only relevant on hosts running named or multiple SQL Server instances.

When would you open it?

You only need 1434 if remote clients connect to named SQL Server instances that use dynamic ports. A common alternative is to give each instance a fixed TCP port and leave the Browser service off, so you don't have to open 1434 at all.

Is it safe to open?

The Browser reveals your instance names and versions and has a notorious history (the SQL Slammer worm), so keep UDP 1434 off the internet and restrict it to a trusted private network. See the security notes below.

How to check if this port is open

Linux
ss -tulpn | grep :1434
nmap -sU -p 1434 <target>
Windows
netstat -ano | findstr :1434
Test-NetConnection <host> -Port 1434   # TCP only
macOS
lsof -i :1434
nmap -sU -p 1434 <target>

How to open this port on your router

To reach this service from outside your network, forward the port on your router:

  1. Open your router's admin page (usually http://192.168.1.1 or http://192.168.0.1) and sign in.
  2. Find the "Port Forwarding" section — it may be called NAT, Virtual Server, or Applications & Gaming.
  3. Add a rule forwarding external port 1434 to your device's local IP, internal port 1434, protocol UDP.
  4. Save and reboot the router if prompted, then test the port from outside your network.

Only forward ports you understand — it exposes that device to the public internet. For remote admin access, a VPN is safer than forwarding the port.

Allow this port through the firewall

Linux (ufw)
sudo ufw allow 1434/udp
Linux (firewalld)
sudo firewall-cmd --permanent --add-port=1434/udp
sudo firewall-cmd --reload
Linux (iptables)
sudo iptables -A INPUT -p udp --dport 1434 -j ACCEPT
Windows
New-NetFirewallRule -DisplayName "Allow 1434" -Direction Inbound -Protocol UDP -LocalPort 1434 -Action Allow

Security & risks

Common attacks

  • SQL Slammer worm buffer-overflow RCE (CVE-2002-0649)
  • Instance and version enumeration for follow-on attacks
  • UDP reflection / amplification abuse

CVE-2002-0649

Hardening

  • Block UDP 1434 at the firewall; never expose it to the internet
  • Disable the SQL Server Browser and use static instance ports
  • Keep SQL Server fully patched (Slammer-class bugs are wormable)
  • Restrict database access to trusted application hosts only

How to block this port

Linux (ufw)
sudo ufw deny 1434/udp
Linux (firewalld)
sudo firewall-cmd --permanent --remove-port=1434/udp
sudo firewall-cmd --reload
Linux (iptables)
sudo iptables -A INPUT -p udp --dport 1434 -j DROP
Windows
New-NetFirewallRule -DisplayName "Block 1434" -Direction Inbound -Protocol UDP -LocalPort 1434 -Action Block

nmap snippet

nmap -sU -p1434 --script ms-sql-info,ms-sql-dac <target>

Replace <target> with the host or range you're authorized to scan.

Related ports

Port 1433Port 3306Port 5432Port 53

Frequently asked questions

What is port 1434 used for?
UDP 1434 is the SQL Server Resolution Service (SQL Server Browser). Clients query it to learn the dynamic TCP port a named SQL Server instance is using.
What was the SQL Slammer worm?
SQL Slammer (2003) exploited a buffer overflow in the UDP 1434 resolution service (CVE-2002-0649) to spread worldwide in minutes, causing massive internet disruption.

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