$ORACLE_HOME/network/admin/sqlnet.ora
如果使用了SQLNET.AUTHENTICATION_SERVICES=(NTS)
则说明可以使用OS认证就，只要conn / as sysdba 就可以登陆
但如果注释掉或SQLNET.AUTHENTICATION_SERVICES=(none)
必须要使用conn sys/password@oracle as sysdba才能登陆
1、在windows下，SQLNET.AUTHENTICATION_SERVICES必须设置为NTS或者 ALL才能使用OS认证；不设置或者设置为其他任何值都不能使用OS认证。
2、在linux下，在SQLNET.AUTHENTICATION_SERVICES的值设置为ALL，或者不设置的情况下，OS验证才能成功；设置为 其他任何值都不能使用OS认证。
我们看看ORACLE对这个设置是怎么解释的：
SQLNET.AUTHENTICATION_SERVICES
Purpose
Use the parameter SQLNET.AUTHENTICATION_SERVICES to enable one or more authentication services. If authentication has been installed, it is recommended that this parameter be set to either none or to one of the authentication methods.
Default
None
Values
Authentication Methods Available with Oracle Net Services:
* none for no authentication methods. A valid username and password can be used to access the database.
* all for all authentication methods
* nts for Windows NT native authentication
Windows NT native authentication
An authentication method that enables a client single login access to a Windows NT server and a database running on the server.
【原文】https://blog.csdn.net/ppp_10001/article/details/5845802