Listener.ora & tnsnames.ora

Oracle networking is very important when you want to connect a particular database. To setup network, we must have listener running on the server and tnsnames.ora configured on the client side.

Configure listener.ora
Configure listener.ora in Multi-Tenant
Configure tnsnames.ora

Make sure you open the listener port before configuration

firewall-cmd --zone=public --add-port=1521/tcp --permanent

Configure 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. Below is the default listener configuration

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.7)(PORT = 1521))
    )
  )

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

Whenever you make changes to the listener.ora file, restart listener

lsnrctl stop
lsnrctl start

Configure listener.ora in Multi-Tenant

In multi-tenant architecture, each PDB can be considered as an individual databases. You can either configure one listener that accepts connections for all PDBs or you can create separate listener for each PDB.

Method 1: One listener for all PDBs

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.7)(PORT = 1521))
    )
  )

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/19.3/db_home)
      (SID_NAME = CDB)
    )
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/19.3/db_home)
      (SID_NAME = PDB1)
    )
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/19.3/db_home)
      (SID_NAME = PDB2)
    )
  )

Method 2: Multiple listeners for individual PDBs

You must configure multiple listeners for each individual PDB in real-time

CDB_LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.9)(PORT = 1521))
    )
  )

SID_LIST_CDB_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/19.3/db_home)
      (SID_NAME = CDB)
    )
  )

PDB1_LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.9)(PORT = 1522))
    )
  )

SID_LIST_PDB1_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/19.3/db_home)
      (SID_NAME = PDB1)
    )
  )

PDB2_LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.9)(PORT = 1523))
    )
  )

SID_LIST_PDB2_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/19.3/db_home)
      (SID_NAME = PDB2)
    )
  )

Starting all the listeners

lsnrctl start cdb_listener
lsnrctl start pdb1_listener
lsnrctl start pdb2_listener

Stop a specific listener

lsnrctl stop pdb1_listener

Configure tnsnames.ora

tnsnames.ora file helps you connect from one database (or client) to another database. The tns file resides under $ORACLE_HOME/network/admin location.

Make sure listener is running on the database server that you want to connect

tnsnames.ora file contains the connection details of the remote database that you want to connect. First the request is sent to listener running on the target database and then the connection is established. On target server (that you want to connect), both listener and database must be up and running. Else, connection will not establish.

Below is a completed tnsnames.ora file where we are trying to connect ORA12C database running on 192.168.1.9 host

ORA12C =
 (DESCRIPTION = 
   (ADDRESS_LIST =
     (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.9)(PORT = 1521))
   )
 (CONNECT_DATA =
   (SERVICE_NAME = ORA12C)
 )
)

There is not restriction as to how many connections you can add to tnsnames.ora file. For example if you want to add multiple database connection entries, you file will look like below

ORA12C =
 (DESCRIPTION = 
   (ADDRESS_LIST =
     (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.0.9)(PORT = 1521))
   )
 (CONNECT_DATA =
   (SERVICE_NAME = ORA12C)
 )
)

ORA11G =
 (DESCRIPTION = 
   (ADDRESS_LIST =
     (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.0.9)(PORT = 1521))
   )
 (CONNECT_DATA =
   (SERVICE_NAME = ORA11G)
 )
)

B6P =
 (DESCRIPTION = 
   (ADDRESS_LIST =
     (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.77)(PORT = 1521))
   )
 (CONNECT_DATA =
   (SERVICE_NAME = B6P)
 )
)

Now that you have added tns entries to the file, let's connect remote database

sqlplus scott@ORA12c

Further Read

Local naming parameters (tnsnames.ora)