What is Port 1521?

by Colin Cohen | Published on October 20, 2023

Port 1521 is dedicated to Oracle database connections. Oracle client applications communicate with Oracle database servers over this port using Oracle’s SQL*Net protocol.

What is Port 1521?

Port 1521 is dedicated to Oracle database servers and the clients connecting to them. You can also use it for Oracle Net Services.

Oracle Database Management System and Oracle Net Services

The Oracle database management system allows organizations to efficiently organize, store, and access large amounts of data in a relational database. To manage and use this data, Oracle client applications communicate with Oracle database servers over port 1521, passing commands and other instructions that the server processes.

Oracle Net Services is a layer that operates on top of both Oracle client applications and Oracle database servers. It provides network infrastructure for an Oracle environment that is both scalable and highly available. Due to its underlying dependence on the Oracle database management system, it, too, operates over port 1521.

Clients, Applications, and Database Servers Need Port 1521

For the purposes of efficiency and efficacy, you typically separate — on different computers — Oracle database servers. The database servers store data and process requests made to them and the Oracle client applications that make the requests. In order to communicate, an Oracle client application on one computer must make its requests to an Oracle database server on a different computer over port 1521.

Oracle client applications can include enterprise management systems such as Oracle Financials and database management tools like Oracle’s SQL Developer. They generally both communicate with Oracle database servers by sending commands to them over port 1521 in a language called SQL.

What is Port 1521 Used For?

You use port 1521 to query and manage data and connect to Oracle databases.

Why Would Oracle Apps Need to Query or Manage Data?

When an Oracle enterprise application such as Oracle Financials needs to display data to a user (or to print it), it must first query the data in an Oracle database server over port 1521. It does this by issuing SQL commands. For example, if an application wanted to retrieve all data stored in a database table called Company, it would issue the following query:


When database administrators need to manage data in an Oracle database server, they use tools such as SQL Developer over port 1521. Like enterprise applications, these tools manage the data in the database server by issuing SQL commands to it.

RELATED: Top 5 Tools for Managing MySQL Database Servers

What is the Port Used for Oracle Database Connections?

Before any Oracle client application can query or manage data in an Oracle database server, it must first connect to the server over port 1521 so that the server can authenticate it and understand what commands a particular client is authorized to perform.

Oracle database connections can be transient, meaning they close after running a set of SQL commands, or they can be persistent, meaning they remain open until specifically closed.

What are Port 1521 Vulnerabilities?

There are vulnerabilities associated with port 1521, such as brute-force attacks, but you can mitigate them.

Unauthorized Access Attempts with Port 1521

Attackers will often target port 1521 because of the value of data that typically resides behind it. So if you leave this port open, you may receive many unauthorized access attempts. Because of this, it is not unusual for organizations to block this port and use another. However, this will unlikely deter a sophisticated hacker.

Brute-Force Attacks Over Port 1521

Attackers will often use brute-force techniques when targeting port 1521. There are a number of tools, such as nmap, that will enumerate service identifiers (SIDs), which uniquely identify database instances. These same tools can also list account credentials. If an attacker obtains an SID and its credentials, it could potentially access sensitive data, which could have catastrophic consequences for an organization.

How to Mitigate Oracle Database Security Vulnerabilities

There are a number of ways to mitigate Oracle database security vulnerabilities, and organizations will usually employ a set of them. They include:

  1. Keeping critical software always up to date, especially security patches.

  2. Changing the Oracle database port from 1521 to another.

  3. Implementing security policies that will hinder attackers from easily enumerating SIDs and credentials.

  4. Implementing encryption on database servers.

Is TCP Port 1521 Encrypted?

You turn on encryption in an Oracle environment by enabling it on the database server. Once you do this, all communication between Oracle client applications and the database server over port 1521 must be encrypted.

Is Oracle Database Connection 1521 TCP or UDP?

You can use either TCP or UDP as a transport protocol on port 1521, but you most likely use TCP because of the need for data integrity.

This transport layer protocol explainer can help break down the differences between TCP and UDP even further.

Data Integrity Is Essential for Oracle Port 1521

Because data integrity is essential when using most Oracle applications, you will typically use TCP as the transport protocol. This is because with TCP, unlike with UDP, you don’t have to worry about data getting lost, damaged, delivered out of order, or duplicated during transmissions.

Port 1521 and Port Conflicts

Conflicts can arise when using port 1521, but it's possible to resolve them.

For instance, you may find that there are too many database management systems trying to access port 1521. Only one application can listen to a given port at one time, including port 1521. So if you are trying to start an Oracle database server on port 1521 and are unable to do so because another application is using this port, you must first close this application. You can also have the database server (and the clients that connect) use a different port.

How to Open Port 1521 for Oracle

You open port 1521 on your computer through a setting in your firewall. How you do this is dependent on your operating system and firewall.

Key Takeaways About Port 1521

When using Oracle database products, you will typically use port 1521 for communication between the client apps that make data requests and the database servers that process them.

This well-known port is the default for Oracle's SQL*Net protocol, and facilitates connections to Oracle databases. Sysadmins and data administrators will regularly encounter port 1521 when configuring and securing Oracle databases, making it a crucial element of database management. Understanding how to use and protect port 1521 is essential for maintaining the security and functionality of your data.

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