Fixing the Issue: ORA-12518 TNS Listener Could Not Hand Off Client Connection - Comprehensive Guide


This guide will provide you with a comprehensive approach to resolve the ORA-12518 TNS Listener issue, which occurs due to the inability of the TNS Listener to hand off client connections. By following the step-by-step solutions provided, you can easily tackle this problem and ensure the smooth functioning of your Oracle database environment.

## Table of Contents
- [Understanding the ORA-12518 error](#understanding-the-ora-12518-error)
- [Possible reasons for the error](#possible-reasons-for-the-error)
- [Step-by-step guide to resolve the error](#step-by-step-guide-to-resolve-the-error)
- [FAQs](#faqs)
- [Related links](#related-links)

## Understanding the ORA-12518 error

The ORA-12518 error is a TNS Listener issue that occurs when the listener is unable to hand off client connections to the Oracle server. This error can happen due to various reasons, such as insufficient system resources, incorrect configuration, or other underlying issues. The error message displayed is as follows:

ORA-12518: TNS:listener could not hand off client connection

## Possible reasons for the error

Some of the possible reasons for the ORA-12518 error include:
1. Insufficient system resources
2. Incorrect `PROTOCOL_INFO` parameter configuration
3. Database server process limit reached
4. Incompatible Oracle client and server versions

## Step-by-step guide to resolve the error

### Step 1: Verify system resources

Check the availability of system resources, such as memory and CPU usage, to ensure that there is no shortage of resources causing the error.

- Use the `top` command on Linux or Task Manager on Windows to check system resources.

### Step 2: Check the PROTOCOL_INFO parameter

Verify that the `PROTOCOL_INFO` parameter is correctly set in the `listener.ora` file.

- Locate the `listener.ora` file, typically found in the `$ORACLE_HOME/network/admin` directory on Linux or `%ORACLE_HOME%\network\admin` on Windows.
- Ensure the `PROTOCOL_INFO` parameter is correctly set. If not, update the parameter and restart the listener using the `lsnrctl` utility:

On Linux

$ lsnrctl stop
$ lsnrctl start

On Windows

C:> lsnrctl stop
C:> lsnrctl start

### Step 3: Check the database server process limit

Ensure that the database server process limit has not been reached, which could prevent new connections from being established.

- Use the following SQL query to check the current number of processes and the maximum allowed processes:

SELECT current_utilization, limit_value
FROM v$resource_limit
WHERE resource_name = 'processes';
  • If the current utilization is close to the limit, consider increasing the processes parameter in the init.ora file and restarting the database.

Step 4: Verify Oracle client and server compatibility

Ensure that the Oracle client and server are compatible by checking their versions:

  • Use the following command to check the Oracle server version:
SELECT * FROM v$version WHERE banner LIKE 'Oracle%';
  • Check the Oracle client version by running the sqlplus utility with the -version option:
# On Linux
$ sqlplus -version

# On Windows
C:\> sqlplus -version
  • Ensure that the Oracle client and server versions are compatible. If not, update the client or server to a compatible version.


1. Can the ORA-12518 error be caused by a firewall blocking connections?

Yes, a firewall blocking connections can cause the ORA-12518 error. Ensure that the firewall is configured to allow connections to the Oracle server and listener.

2. How do I check the status of the TNS Listener?

You can check the status of the TNS Listener using the lsnrctl utility:

# On Linux
$ lsnrctl status

# On Windows
C:\> lsnrctl status

3. Can I increase the number of processes without restarting the database?

No, you cannot increase the number of processes without restarting the database. You need to update the init.ora file and restart the database for the changes to take effect.

4. How can I check if the listener is running on the default port (1521)?

You can check the listener's port by running the lsnrctl status command and looking for the "Port" information in the output.

5. Can I have multiple TNS Listeners running on the same server?

Yes, you can have multiple TNS Listeners running on the same server, but they must be configured to listen on different ports.

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