Testing Oracle Client Connectivity

Oracle's TNS (Transparent Network Substrate) names resolution method relies on a configuration file known as tnsnames.ora. This file, typically located in the $ORACLE_HOME/network/admin directory, provides the necessary details for connecting to Oracle databases across a network.
However, if you encounter issues connecting to an Oracle database, the tnsnames.ora file could be one of the potential causes. Here are several troubleshooting techniques you can utilize to resolve errors in this file:

1. Syntax Verification: The tnsnames.ora file follows a specific syntax. Misplaced parentheses, missing parameters, or typos can prevent the file from working correctly. Use a text editor to open the file and carefully verify the syntax against Oracle's official documentation. It is a common practice to verify the correct format using a working tnsnames.ora file from another system.
2. Correct Naming: Confirm the naming conventions used in your tnsnames.ora file match those expected by your Oracle clients. This includes both the names of the databases and the names of the connection parameters.
3. Location of the File: Ensure the tnsnames.ora file is in the correct directory. The default location is $ORACLE_HOME/network/admin, but this can be changed by setting the TNS_ADMIN environment variable to point to a different directory. Check the TNS_ADMIN variable and ensure it points to the correct directory containing your tnsnames.ora file.
4. Permission Issues: Verify that the Oracle client has the necessary permissions to read the tnsnames.ora file. The file must have read permissions for the user running the Oracle client software.
5. Connect Descriptor: Make sure the connect descriptor in the tnsnames.ora file contains valid hostnames or IP addresses, and that the port number matches the listener port on the database server.
6. Testing Connection: Use Oracle's tnsping utility to test whether the Oracle client can connect to the database using the entries in the tnsnames.ora file. This utility helps you determine if the tnsnames.ora file is functioning correctly.
7. Oracle Net Manager: Use the Oracle Net Manager (a GUI tool) to validate the tnsnames.ora file. It helps in managing and enabling detailed tracing for the client and the listener.

Remember, if you make any changes to the tnsnames.ora file, you should always attempt a new connection afterwards to ensure the changes have taken effect. Oracle's TNS resolution does not require a server or service restart after modifying the tnsnames.ora file.

Sometimes, you setup a Oracle Net connection, and it does not work. This is more likely to happen when you edit your tnsnames.ora file manually than it is when you use Oracle Net Configuration Assistant. Partly that is because Oracle Net Configuration Assistant tests the connection capability when you create it, and partly that's because using Oracle Net Configuration Assistant reduces your chance of making a mistake.
If you are faced with a Oracle Net connection that is not working, here are several things that you should check:

1. Whether your listener.ora file on the server is correct
2. The syntax of your tnsnames.ora entry
3. The underlying network connectivity
4. The Oracle Net connectivity
5. The default domain in your sqlnet.ora file

The first item, the entry for the database in your listener.ora file, is normally an issue only when you have recently edited that file. You have already seen how to do that. The next section in this lesson talks about item 2. In the following two lessons, you will learn how to check the other three items.

A failed to resolve service name error might result from a mistake in the syntax used for the tnsnames.ora entry that you are testing. Compare the entry you are testing against other entries that work. Watch the parentheses closely. You may be able to spot the error and fix it. If not, you may find it easier to delete the entry, and recreate it using Oracle Net Configuration Assistant.