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Oracle Net

Oracle Net allows a client to connect to an Oracle instance or instance to connect among themselves.

Connect descriptor

A connect descriptor consists of

tnsnames.ora

The tnsnames.ora contains the mappings
Oracle Net tries to find the tnsnames.ora in the following locations:
If the directory pointed at with the $TNS_ADMIN environment variable exists, but does contain have a tnsnames.ora file, the file is assumed to be missing and Oracle NET will not try to find it in other directories.
On Windows, Oracle also checks for a stated location in the registry.

Format of entries in tnsnames.ora

tnsnames.ora contains one or more entries in the following format:
NET_SERVICE_NAME = (
   DESCRIPTION = (
      ADDRESS=( protocol_address )
   )
   (  CONNECT_DATA = (
        SERVICE_NAME=service_name
      )
   )
) 
The value if protocol_address is used to locate and contact the listener.
After the listener was contacted, the client passes the information in service_name to the listener which in turn will then try to locate and contact the specified Oracle service
The value of SERVICE_NAME corresponds to the global database name specified when the database was created.
The global database name is a combination of the database name and a domain name. For example, the global database name foodb.somewhere.xyz corresponds to the database name foodb and the domain name somewhere.xyz.

Leading blank spaces

MOS Note 730067.1 points out that ORA-01031: insufficient privileges may be thrown if the definition of SQLNET.AUTHENTICATION_SERVICES has leading blank spaces.

TODO: extproc_connection_data

Is extproc_connection_data used to call external procedures?

Error messages

TNS-03505: Failed to resolve name Unable to contact the listener specified in the (DESCRIPTION…) section of the connect descriptor.
ORA-12154: TNS: could not resolve the connect identifier specified The listener could be contacted but it was unable to contect the service specified in the (CONNECT_DATA…) section
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor The listener could be contacted but it is not configured to handle the service specified in the (CONNECT_DATA …) seciton
ORA-12518: TNS:listener could not hand off client connection
ORA-12560: TNS:protocol adapter error
ORA-03135: connection lost contact

See also

trcasst: the trace assistant for Oracle Net Services trace files.

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