当我们安装并新建数据库的时候，往往都会需要配置监听（LISTENER），但有时我们会碰到数据库无法连接的情况。其中一个经典的错误就是：
    12154, 00000, "TNS:could not resolve service name"
// *Cause:  The service name specified is not defined correctly in the
// TNSNAMES.ORA file.
// *Action:  Make the following checks and correct the error:
//           - Verify that a TNSNAMES.ORA file exists and is in the proper
//             place and accessible. See the operating system specific manual
//             for details on the required name and location.
//           - Check to see that the service name exists in one of the
//             TNSNAMES.ORA files and add it if necessary.
//           - Make sure there are no syntax errors anywhere in the file.
//             Particularly look for unmatched parentheses or stray characters.
//             Any error in a TNSNAMES.ORA file makes it unusable. See
//             Chapter 4 in the SQL*Net V2 Administrator's Guide. If
//             possible, regenerate the configuration files using the Oracle
//             Network Manager.
    我们一般都是先检查CLIENT端的配置是否出现问题。如果我们检测客户端没有问题，就需要考虑SERVER端LISTENER的配置了。
    以下为一个LISTENER.ORA的例子：
    LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = TCP)(HOST = localhost.localdomain)(PORT = 1521))
      )
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC))
      )
    )
  )
   或
   LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.11.100)(PORT = 1521))
      )
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC))
      )
    )
  )
   这两个配置在大多数情况下是没有问题的，但是一旦当网络情况复杂的时候，我们的连接就无法进行了。
    我曾处理过这样的一些情况，象对于使用VPN或者防火墙进行地址映射进行连接等情况，这样的配置就有可能出现问题。
    原因就在于HOST的配置。localhost.localdomain或者192.168.11.100的配置都会造成LISTENER只是监听指定的IP地址，而复杂的网络环境或者是数据库主机修改IP地址等都会直接造成IP变化而无法正常监听连接要求。所以还是非常建议使用主机名称的方式。例如：HOST=XXDB1，这样就会减少12154出错的可能了。

    附赠：当然在LINUX环境下，IPTABLES会自动开启，便会造成12533的错误出现。
    12533, 00000, "TNS:illegal ADDRESS parameters"
// *Cause: An illegal set of protocol adapter parameters was specified. In
// some cases, this error is returned when a connection cannot be made to the
// protocol transport.
// *Action: Verify that the destination can be reached using the specified
// protocol. Check the parameters within the ADDRESS section of
// TNSNAMES.ORA.  Legal ADDRESS parameter formats may be found in the
// Oracle operating system specific documentation for your platform.
// Protocols that resolve names at the transport layer (such as DECnet object
// names) are vulnerable to this error if not properly configured or names are
// misspelled.
   如果可以设定IPTABLES最好，当情急之下，还是一关了之来的痛快 :-)
   1) 即时生效，重启后失效
   开启： service iptables start
   关闭： service iptables stop

   2) 重启后生效
   开启： chkconfig iptables on
   关闭： chkconfig iptables off

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