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Oracle Network Configuration (listener.ora , tnsnames.ora , sqlnet.ora)

In its most basic form, Oracle uses three files (listener.ora, tnsnames.ora & sqlnet.ora) for network configuration. This article gives an example of each file as a starting point for simple network configuration.

Assumptions

The example files below are relevant for an Oracle installation and instance with the following values.

listener.ora

The "listerner.ora" file contains server side network configuration parameters. It can be found in the "$ORACLE_HOME/network/admin" directory on the server. Here is an example of a basic "listener.ora" file from Linux. We can see the listener has the default name of "LISTENER" and is listening for TCP connections on port 1521. Notice the reference to the hostname "myserver.example.com". If this is incorrect, the listener will not function correctly.

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1)) 
      (ADDRESS = (PROTOCOL = TCP)(HOST = myserver.example.com)(PORT = 1521))
    )
  )

After the "listener.ora" file is amended the listener should be restarted or reloaded to allow the new configuration to take effect.

$ # Restart
$ lsnrctl stop
$ lsnrctl start

$ # Or Reload.
$ lsnrctl reload

The listener defined above doesn't have any services defined. These are created when database instances auto-register with it. In some cases you may want to manually configure services, so they are still visible even when the database instance is down. If this is the case, you may use a "listener.ora" file like the following.

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1)) 
      (ADDRESS = (PROTOCOL = TCP)(HOST = myserver.example.com)(PORT = 1521))
    )
  )

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (GLOBAL_DBNAME = orcl.example.com)
      (ORACLE_HOME = /u01/app/oracle/product/11.2.0.4/db_1)
      (SID_NAME = orcl)
    )
  )

If there are multiple database instances on the server, you can added multiple SID_DESC entries inside the SID_LIST section.

tnsnames.ora

The "tnsnames.ora" file contains client side network configuration parameters. It can be found in the "$ORACLE_HOME/network/admin" directory on the client. This file will also be present on the server if client style connections are used on the server itself. Here is an example of a "tnsnames.ora" file.

LISTENER = (ADDRESS = (PROTOCOL = TCP)(HOST = myserver.example.com)(PORT = 1521))

orcl.example.com =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = myserver.example.com)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = orcl)
    )
  )

The alias used at the start of the entry can be whatever you want. It doesn't have to match the name of the instance or service. Notice the PROTOCOL, HOST and PORT match that of the listener. The SERVICE_NAME can be any valid service presented by the listener. You can check the available services by issuing the lsnrctl status or lsnrctl service commands on the database server. Typically there is at least one service matching the ORACLE_SID of the instance, but you can create more.

sqlnet.ora

The "sqlnet.ora" file contains client side network configuration parameters. It can be found in the "$ORACLE_HOME/network/admin" directory on the client. This file will also be present on the server if client style connections are used on the server itself, or if some additional server connection configuration is required. Here is an example of an "sqlnet.ora" file.

NAMES.DIRECTORY_PATH= (TNSNAMES, ONAMES, HOSTNAME)
NAMES.DEFAULT_DOMAIN = example.com

# The following entry is necessary on Windows if OS authentication is required.
SQLNET.AUTHENTICATION_SERVICES= (NTS)

There are lots of parameters that can be added to control tracing, encryption, wallet locations etc. These are out of the scope of this article.

Testing

Once the files are present in the correct location and amended as necessary the configuration can be tested using SQL*Plus by attempting to connect to the database using the appropriate username (SCOTT), password (TIGER) and service (orcl.example.com).

$ sqlplus scott/tiger@orcl.example.com

Common Problems

The OS hostname command must return the correct hostname of your database server. If not, fix it so it does.

$ hostname
myserver.example.com
$

The server must have a correct entry in the "/etc/hosts" file matching the hostname and IP address of the server, as well as the loopback entry for localhost. For example in this case the values are as follows.

127.0.0.1      localhost localhost.localdomain localhost4 localhost4.localdomain4
192.168.0.123  myserver.example.com  myserver

If these are provided by DNS, that is fine also.

If you are using the ORACLE_HOSTNAME environment variable, possibly set in your "/home/oracle/.bash_profile", it must be set to the correct value.

export ORACLE_HOSTNAME=myserver.example.com

The HOST entry in the "listener.ora" file must point to an active network adapter, either real or loopback. If not, the listener will fail to start.

If the HOST entry in the "listener.ora" file is set to "localhost", the listener will start and accept connections from the local server, but not from other clients. You would typically expect this to be set to the hostname of the database server, although some people use the IP address instead.

For the client to make a connection via the listener, there must be a clear route through the network between the two machines. If you are struggling to connect, check network firewalls and the local firewall (iptables, firewalld, Windows Firewall) on the database server.

For more information see:

Hope this helps. Regards Tim...

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