What is Port 1434?

by Erik Mikac | Published on February 18, 2025

Anyone familiar with Microsoft SQL will have a passing knowledge of port 1434. Unlike your well-known ports like 80 and 443, this one is a little less known. So, let's cover what port 1434 is and its primary application. But first, let's go over what a port is.

Before delving into the many uses of port 1434, let's discuss what a computer port actually is. A computer port is a logical endpoint for communication. Software applications use it to send and receive data. Any application that needs network access will communicate via a port. For example, HTTPS requests typically use port 443 for secure internet communication.

Port 1434 is one of many network ports available on your computer. In total, there are 65,536 network ports (ranging from 0 to 65535). Most ports are not reserved for a specific purpose. This allows different applications to be used as needed. However, in this case, a specific use case reserves port 1434—so let's go over that.

Port 1434 and a Technical Breakdown

Microsoft SQL Server Browser Service has reserved port 1434 for exclusive use. It's official, too. Microsoft registered port 1434 with the venerable IANA (Internet Assigned Numbers Authority). The IANA is an officially recognized organization that manages and standardizes internet communication. They reserve domains, IP addresses, MIME types, and even ports. So, while port 1434 can technically be used for other purposes, it is inadvisable to do so.

Port 1434 in Networking

Microsoft SQL Server Browser Service uses UDP to communicate via port 1434. Let's break down the difference between UDP and TCP usage for port 1434.

As illustrated in the table, port 1434 is almost always used in conjunction with UDP.

Security Implications of Port 1434

Since port 1434 has a well-known use, it makes it a popular vector of attack for hackers. If left exposed, hackers can use it to discover valuable metadata about your SQL servers. However, the biggest exploit to port 1434 came from a virus called SQL Slammer. In 2003, the SQL Slammer virus hit vulnerable servers. This caused internet traffic to slow down all over the world.

SQL Slammer worked by sending malicious UDP packets to port 1434. These packets contained random data, including shellcode. Because SQL Server did not check the size of incoming packets, the buffer overflowed.

When the buffer overflowed, it changed the return address. This address tells the CPU where to go after the function ends. This caused the server to execute the shellcode instead of the normal program code.

In other words, the server didn't keep running as usual. Instead, it was redirected to execute the malicious shellcode. The shellcode instructed the server to send out more infected packets, and this cycle continued, thus enabling self-propagation.

This rapid, self-propagating behavior allowed SQL Slammer to infect large numbers of systems in a very short amount of time. SQL Slammer caused network congestion and service disruptions all across the Internet.

Since this was over twenty years ago, you can bet that developers have put plenty of patches into place to mitigate this attack. However, it's always good to be aware of these types of attacks, and why it's important to always monitor your ports.

Testing and Troubleshooting

There are several different ways to troubleshoot and test port 1434. Troubleshooting falls into three domains: network connectivity, service availability, and firewall rules. Let's go over a checklist of possible issues.

1. Check if SQL Server Browser Service is Running. This can be checked in the SQL Server Configuration Manager.

2. Use telnet or PowerShell to ensure connectivity. For example, in PowerShell you can do this: Test-NetConnection -ComputerName <server-ip> -Port 1434 -Udp. If you're using Linux, instead utilize netcat like this: nc -vz -u <server-ip> 1434

3. Check firewall and security configurations. Port 1434 is generally closed since it isn't often used. Zero-trust security architecture mandates all ports not in use are closed by default. First, on the server itself, verify port 1434 is authorized for inbound traffic. Next, check on the firewall itself. Verify port 1434 is on the ACL for inbound traffic. Next, check whether dynamic or static ports are used for the SQL instance. If dynamic ports are in use, verify whether UDP 1434 is pointing to the actual SQL Server instance port.

4. Use WireShark to monitor packet transmissions to the SQL server. Capture the packets by using the following filter: udp.port == 1434. If you don't see any packets, it indicates a firewall issue, network misconfiguration, or a service is down.

FAQs

What Is the Default Protocol For Port 1434?

The default protocol for port 1434 is UDP. SQL Server Browser Service uses port 1434 to help clients locate SQL Server instances running on dynamic ports. While TCP 1434 exists, it is rarely used by SQL Server.

How Can I Check If Port 1434 Is Open On My Network?

There are a couple different ways to do that, depending on the OS you're using. If you're using PowerShell, this command will do the trick: Test-NetConnection -ComputerName <server-ip> -Port 1434 -Udp. If you're on Linux, try this one: nc -vz -u <server-ip> 1434.

In some cases, it'll be necessary to use WireShark to properly troubleshoot port 1434. In that case, use udp.port == 1434 filter.

Often, firewall accessibility is the culprit. Verify the port is open on Windows with this command: netstat -an | findstr 1434. On Linux or Mac, use this one: sudo netstat -tulnp | grep 1434.

What are Common Security Threats Related To Port 1434?

Here are the four most common security related threats to port 1434:

• SQL Slammer Worm (2003) – A self replicating worm that crashed SQL servers. We touched on it earlier on in this article. 

• Port Scanning Attacks – Attackers look for open SQL Server instances to exploit.

• Denial-of-Service (DoS) Attacks – Flooding UDP 1434 with requests can disrupt SQL Server discovery.

• Information Leakage – Exposing UDP 1434 shows SQL Server instance details. This makes targeted attacks easier. After all, knowledge is power.

How Can I Secure Port 1434 Effectively?

There are several steps you can take to mitigate attacks through port 1434. Here are a couple of tips:

• If you don't need SQL Server Browser Service, just disable the port.

• Restrict UDP access via your firewall.

• Enable SQL Server Authentication and Encryption. 

• Monitor all traffic to and from the power using WireShark or an IDS.

Is Port 1434 Only Used by SQL Server?

Technically, no. Like all ports, 1434 can be used with anything. However, using it for non-SQL Server purposes is not recommended. Doing so could cause potential conflicts and security risks regarding SQL server.

What Is the Role Of Port 1434 in SQL Server?

Port 1434 (UDP) allows SQL Server clients to discover which port number an instance is running on. It is handy at finding named instances using dynamic ports.

Can Closing Port 1434 Impact My Database Performance?

Closing port 1434 will not impact performance, per se. However, it will affect connectivity to the database. If you're using dynamic ports like most people do, then the clients will be unable to find the database. (Clients here, meaning any application that needs access to the database.)

If you're using a static port (e.g., TCP 1433), closing UDP 1434 will have no impact. In fact, it's even recommended for security reasons.

Final Thoughts

Port 1434 plays a crucial role in SQL Server communications. It's especially true for clients connecting to named instances using dynamic ports. Port 1434's function within Microsoft SQL Server makes it an important consideration for database administrators. However, its well-known use also makes it a potential attack vector. One such example is the infamous SQL Slammer worm.

To maintain security, organizations should carefully manage access to this port. Remember to restrict it when not needed and ensure proper firewall configurations. If dynamic ports are in use, administrators should monitor network traffic. Make sure to implement best practices to prevent unauthorized access. 

Hopefully, this article has put you on solid footing to tackle any problem related to port 1434.

Interested in becoming a network engineer? The CCNA is the perfect place to start.