现象：
使用第三方工具PL/SQL Developer和Toad均无法远程连接数据库，报错均为：ORA-12170:TNS:Connect timeout occurred
[oracle@localhost dbs]$ lsnrctl status
LSNRCTL for Linux: Version 11.2.0.1.0 - Production on 12-JUN-2014 11:42:52
Copyright (c) 1991, 2009, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias LISTENER
Version TNSLSNR for Linux: Version 11.2.0.1.0 - Production
Start Date 12-JUN-2014 11:22:22
Uptime 0 days 0 hr. 20 min. 29 sec
Trace Level off
Security ON: Local OS Authentication
SNMP OFF
Listener Parameter File /oracle/product/11.2.0/network/admin/listener.ora
Listener Log File /oracle/diag/tnslsnr/localhost/listener/alert/log.xml
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=localhost)(PORT=1521)))
Services Summary...
Service "VCDB" has 3 instance(s).
Instance "VCDB", status UNKNOWN, has 1 handler(s) for this service...
Instance "VCDB", status BLOCKED, has 1 handler(s) for this service...
Instance "VCDB", status READY, has 1 handler(s) for this service...
Service "VCDBXDB" has 1 instance(s).
Instance "VCDB", status READY, has 1 handler(s) for this service...
The command completed successfully
[oracle@localhost dbs]$
既有动态注册也有静态注册信息。
原因：
数据库服务器的防火墙导致的无法连接问题，应该是在系统安装的时候没有关掉防火墙。
解决方法：
执行命令service iptables stop 之后便可以正常连接。