一、发现TNS-12525错误
检查公司服务器listener日志中周期性发现如下错误提示：

08-DEC-2011 02:25:34 * (CONNECT_DATA=(SID=ora9i)) * (ADDRESS=(PROTOCOL=tcp)(HOST=211.155.236.234)(PORT=4877)) * establish * ora9i * 0
08-DEC-2011 02:25:35 * (CONNECT_DATA=(SID=ora9i)) * (ADDRESS=(PROTOCOL=tcp)(HOST=211.155.236.234)(PORT=4879)) * establish * ora9i * 0
08-DEC-2011 02:25:43 * <unknown connect data> * (ADDRESS=(PROTOCOL=tcp)(HOST=211.155.236.234)(PORT=2486)) * establish * <unknown sid> * 12525
TNS-12525: TNS:listener has not received client's request in time allowed
 TNS-12535: TNS:operation timed out
  TNS-12606: TNS: Application timeout occurred
08-DEC-2011 02:26:02 * <unknown connect data> * (ADDRESS=(PROTOCOL=tcp)(HOST=211.155.236.234)(PORT=4876)) * establish * <unknown sid> * 12525
TNS-12525: TNS:listener has not received client's request in time allowed
 TNS-12535: TNS:operation timed out
  TNS-12606: TNS: Application timeout occurred

继续检查日志发现，只有211.155.236.234是外网ip，其他都是通过内网ip访问，都未报类此错误。初步怀疑是公司和运营商网络之间的防火墙导致

二、数据库版本

SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE    10.2.0.4.0      Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production

三、MOS说明

Symptoms
At regular intervals (every three hours), you see new connections rejected with TNS-12525 "listener has not received client's request in time allowed".
The listener log shows the following error stack :
10-APR-2008 17:39:15 * <unknown connect data> * (ADDRESS=(PROTOCOL=tcp)(HOST=10.221.50.31)(PORT=4184)) * establish * <unknown sid> * 12525
  TNS-12525: TNS:listener has not received client's request in time allowed
    TNS-12535: TNS:operation timed out
      TNS-12606: TNS: Application timeout occurred
At the same time the firewall log reports :
"TCP packet out of state: First packet isn't SYN; tcp_flags: PUSH-ACK"
Wireshark (Ethereal) traces show the TCP 3-way handshake never completes for those connections.
Changes
Value of idle timer was recently changed at firewall level.
Cause
Firewall closes ports not allowing the 3-way handshake to complete at TCP level.
Solution
Increase the firewall's idle timeout interval.

New Connections Intermittently Rejected with TNS-12525