Just Learned: About listener.ora

Recently I have been planning to upgrade a 10.2.0.4 Oracle database to 12.1.0.2. For that, I created a test db called minidb to play with upgrade steps. At the end, I need to bring up a listener for that db. This is what I've just learned: do you know we can start a listener without listener.ora? The listener.ora is a file that stores listener configuration information. Because the configuration parameters have default values, it is possible to start and use a listener with no configuration. The below screenshot shows I don't have a listener.ora file at the normal location ($ORACLE_HOME/nework/admin), and I don't have listener up and running:

exxxxxspd02 /apps/opt/oracle/product/12.1.0/network/admin [minidb] > ls -l
total 7
drwxr-xr-x   2 oracle   dba            6 Mar  6 08:41 samples
-rw-r--r--   1 oracle   dba          373 Mar 16  2014 shrept.lst
-rw-r--r--   1 oracle   dba          127 Mar  6 08:18 tnsnames.ora
exxxxxspd02 /apps/opt/oracle/product/12.1.0/network/admin [minidb] > lsnrctl status

LSNRCTL for Solaris: Version 12.1.0.2.0 - Production on 06-MAR-2015 08:42:20

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
TNS-12541: TNS:no listener
 TNS-12560: TNS:protocol adapter error
  TNS-00511: No listener
   Solaris Error: 146: Connection refused

Now I started up a listener:

exxxxxspd02 /apps/opt/oracle/product/12.1.0/network/admin [minidb] > lsnrctl start

LSNRCTL for Solaris: Version 12.1.0.2.0 - Production on 06-MAR-2015 08:44:59

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Starting /apps/opt/oracle/product/12.1.0/bin/tnslsnr: please wait...

TNSLSNR for Solaris: Version 12.1.0.2.0 - Production
Log messages written to /apps/opt/oracle/diag/tnslsnr/exxxxxspd02/listener/alert/log.xml
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=exxxxxspd02)(PORT=1521)))

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Solaris: Version 12.1.0.2.0 - Production
Start Date                06-MAR-2015 08:44:59
Uptime                    0 days 0 hr. 0 min. 0 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /apps/opt/oracle/diag/tnslsnr/exxxxxspd02/listener/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=exxxxxspd02)(PORT=1521)))
The listener supports no services
The command completed successfully

Notice by default this listener is listening on port 1521 and supports no services. I can register my minidb with this local listener through a mechanism called "Dynamic Service Registration". For that to happen, I need to configure an initialization parameter called "LOCAL_LISTENER":

exxxxxspd02 /apps/opt/oracle/product/12.1.0/network/admin [minidb] > sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Fri Mar 6 08:50:13 2015

Copyright (c) 1982, 2014, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> show parameter local_list

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
local_listener                       string
SQL> alter system set local_listener='(ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))' scope=both;

System altered.

SQL> ho lsnrctl status

LSNRCTL for Solaris: Version 12.1.0.2.0 - Production on 06-MAR-2015 08:51:47

Copyright (c) 1991, 2014, Oracle.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Solaris: Version 12.1.0.2.0 - Production
Start Date                06-MAR-2015 08:44:59
Uptime                    0 days 0 hr. 6 min. 48 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /apps/opt/oracle/diag/tnslsnr/exxxxxspd02/listener/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=exxxxxspd02)(PORT=1521)))
Services Summary...
Service "minidb" has 1 instance(s).
  Instance "minidb", status READY, has 1 handler(s) for this service...
The command completed successfully

What happened at background is that by default, the LREG background process can register service information with its local listener on the default local address of TCP/IP, port 1521.

exxxxxspd02 /apps/opt/oracle/product/12.1.0/network/admin [minidb] > ps -ef |grep lreg |grep -v grep
  oracle 28186     1   0 08:24:15 ?           0:00 ora_lreg_minidb

To ensure service registration works properly, the initialization parameter file should contain the following parameters:

• SERVICE_NAMES for the database service name 

•  INSTANCE_NAME for the instance name 

•   LOCAL_LISTENER for the local listener 

Note the SERVICE_NAMES defaults to the global database name, a name comprising the DB_NAME and DB_DOMAIN parameters in the initialization parameter file. The value for the INSTANCE_NAME parameter defaults to the Oracle system identifier (SID).