最近在$ORACLE_HOME/network/admin/log 下的sqlnet.log文件里，不断发现异常error：

***********************************************************************
Fatal NI connect error 12170.

  VERSION INFORMATION:
        TNS for Solaris: Version 10.2.0.1.0 – Production
        Oracle Bequeath NT Protocol Adapter for Solaris: Version 10.2.0.1.0 – Production
        TCP/IP NT Protocol Adapter for Solaris: Version 10.2.0.1.0 – Production
  Time: 1-MAR-2009 18:42:58
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12535
    TNS-12535: TNS:operation timed out
    ns secondary err code: 12560
    nt main err code: 505
    TNS-00505: Operation timed out
    nt secondary err code: 145
    (Look up error number 145 in /usr/include/sys/errno.h )
    nt OS err code: 0
  Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=10.221.70.83)(PORT=4032))
***********************************************************************

通过client address可以查询到时应用服务器的地址，从listener.log里可以查到此session的链接时间，大部分都是4-6个小时左右，断开。客户端没有人报告异常，也没有客户反应链接不到数据库，那为什么会出现这个timed out的异常告警呢？如果没有问题的话，不应该会平白无故的告警的？

在网上查询到相关资料，怀疑是防火墙设置问题：

This is indicative of a client connection being dropped. This can be the result of firewall dropping connections per the firewall policy of idle sessions.  The workaround for this can be done as follows:

“For Oracle databases, SQLNET.EXPIRE_TIME can be used.  Although SQLNET.EXPIRE_TIME was intended as a dead connection detection (DCD) method, a side effect of sending probe packets between the database server and client/application server is that the connection remains active.  Set the SQLNET.EXPIRE_TIME to less than the firewall timeout and the problem is solved.”

参考链接： 1. http://scatteredreads.wordpress.com/2009/11/03/tns-12535-tnsoperation-timed-out/

                      2. http://hpdba.wordpress.com/tag/keep-alive/