安装好oracle 12c后，通过工具PLSQL Developer连接报如下错误:
解决办法,添加红色部分:
[oracle@localhost admin]$ more sqlnet.ora
# sqlnet.ora Network Configuration File: /u01/app/oracle/product/12.2.0.1/db_1/network/admin/sqlnet.ora
# Generated by Oracle configuration tools.
NAMES.DIRECTORY_PATH= (TNSNAMES, EZCONNECT)
SQLNET.ALLOWED_LOGON_VERSION_CLIENT=8
SQLNET.ALLOWED_LOGON_VERSION_SERVER=8