http://www.itpub.net/482483.html
几个原因，第1个可能与名字解析有关。
设置 TRACE_LEVEL_CLIENT=16 在sqlnet.ora文件,发现
cli.trc出现如下内容:
nioqper: error from nscall
nioqper: nr err code: 0
nioqper: ns main err code: 12560
nioqper: ns (2) err code: 0
nioqper: nt main err code: 530
nioqper: nt (2) err code: 126
nioqper: nt OS err code: 0
niomapnserror: entry
niqme: entry
niqme: reporting NS-12560 error as ORA-12560
niqme: exit
niomapnserror: returning error 12560
niomapnserror: exit
niotns: Couldn't connect, returning 12560
niotns: exit
nsbfrfl: entry
nsbfrfl: normal exit
nigtrm: Count in the NI global area is now 0
nnfgdei: entry
nrigbd: entry
nrigbd: exit
nigtrm: Count in the NL global area is now 0
