In Oracle, a listener is a process that runs on the database server computer. It manages the traffic of incoming connect requests from clients to the database server, and therefore, is very important. Each time a client requests a network session with a server, it is the listener that receives this client request. The listener will check to see that the client information matches the listener information, and thereby grant (or deny) the request. To learn more about listeners, click here.
Generally, the listener.ora file can be found in $ORACLE_HOME/network/admin or ORACLE_BASE\ORACLE_HOME\network\admin on Unix and Windows platforms respectively.
When there is any problem with the listener, the user will likely run into ORA-12541.
The ORA-12541 message reads, “TNS no listener.” This happens if the listener is either down as it attempts to make a connection or if it has not been started or improperly configured. In addition, it may be caused by a connectivity problem with the network.
To figure out what may be causing the problem, follow the solution steps outlined below.
First, make sure that the listener is up and running. To do this, go to the Control Panel into Services under NT or listener control program (LSNRCTL).
C:\Documents and Settings\ME>lsnrctl stat
This should show the current status of the listener. If the listener is currently not running, re-start the listener service by running the “net start” command by filling in “XX” with the version being used:
C:\Documents and Settings\ME>net start OracleOraDbXXg_home1TNSListener
If the listener is up and running, the problem may lie with the listener not having been associated with the correct instance or protocol. To make sure all instances are correct, you can obtain basic information about the listener and the configuration settings by simple commands, such as the STATUS command. The status command will give basic information about the listener as well as the configured protocol addresses, summary of the services registered, and the service handlers allocated to each service. There are 4 types of status: ready, blocked, ready/secondary, and unknown. Below are descriptions of each status signals.
- Ready: Instance can accept connections
- Blocked: Instance cannot accept connections
- READY/SECONDARY: A secondary instance in an Oracle9i Real Application Clusters primary/secondary configuration that is ready to accept connections
- UNKNOWN: Status unknown because instance registered statically in the listener.ora file rather than dynamically with service registration
Below is a partial example by Oracle of the STATUS command:
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=net))) STATUS of the LISTENER ———————— Alias LISTENER Version
TNSLSNR for Solaris: Version 126.96.36.199.0 – Development Start Date…
Services Summary… Service “sales.us.acme.com” has 1 instance(s). Instance “sales”, status READY, has 3 handler(s) for this service… Service “hr.us.acme.com” has 1 instance(s). Instance “hr”, status READY, has 2 handler(s) for this service… The command completed successfully
If you are still unable to get the listener up and running, you may want to delete the listener and create a new one in the network configuration assistant.
The last possible cause may be a network problem. Check to see that there is a proper network connection if you find that the listener has been configured and is running correctly.
Following proper configuration steps is key to successfully establishing a connection with the listener. For more information, review the Oracle documentation on “configuring and administering the listener” here.
To avoid future problems with the listener, Oracle users should actively monitor and ensure security. One of the first and easiest ways to protect the listener is to encrypt it with a password to ensure that only authorized users can view and change settings.
In addition to security, it is always a good idea to consistently monitor and review the listener log file for any of the following conditions, which can cause problems with the listener:
- Irregular connection establishment times
- Connectivity problems and refusals
- Unexpected shutdown of listener which can lead to a DOS attack
If you continue to have trouble with ORA-12541 or any other listener problems, contact your system DBA or a licensed Oracle consultant for professional help.