When your names are provided from LDAP server then you don't need tnsnames.ora
file anymore.
In order to use LDAP server you have to change attribute NAMES.DIRECTORY_PATH
in your sqlnet.ora
file like this:
NAMES.DIRECTORY_PATH= (LDAP, TNSNAMES)
This will first search in LDAP and - if nothing found - then search tnsnames.ora
file.
Then you have to create a file called ldap.ora
in the same directory with content similar to this:
DEFAULT_ADMIN_CONTEXT = "dc=domain,dc=net"
DIRECTORY_SERVERS = (ldap-server1.domain.net:389:636,ldap-server2.domain.net:389:636)
DIRECTORY_SERVER_TYPE = OID
Check documentation for details:
Directory Usage Parameters (ldap.ora)
Parameters for the sqlnet.ora File