用客户端连接oracle12c服务器端时，经常会遇到ORA-28040的报错提示。
ORACLE官方对ORA-28040的报错提示的解释：
oracle12c@oracle-ora12c$oerr ora 28040
28040, 0000, "No matching authentication protocol"
// *Cause: There was no acceptable authentication protocol for
// either client or server.
// *Action: The administrator should set the values of the
// SQLNET.ALLOWED_LOGON_VERSION_SERVER and
// SQLNET.ALLOWED_LOGON_VERSION_CLIENT parameters, on both the
// client and on the server, to values that match the minimum
// version software supported in the system.
// This error is also raised when the client is authenticating to
// a user account which was created without a verifier suitable for
// the client software version. In this situation, that account‘s
// password must be reset, in order for the required verifier to
// be generated and allow authentication to proceed successfully.
在oracle12c版本中在监听中引入了允许连接的客户端和服务器端的最低版本信息，可以在$ORACLE_HOME/network/admin/sqlnet.ora文件中添加以下信息解决该问题：
SQLNET.ALLOWED_LOGON_VERSION=8
