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Shared Server: Port 1521 Open on Firewall But Unable to Connect Due to Errors: ORA-12535,TNS-12203 (Doc ID 361284.1)

Last updated on AUGUST 15, 2018

Applies to:

Oracle Net Services - Version to [Release 8.1.7 to 11.2]
Information in this document applies to any platform.

This document describes a situation where a recent change to SHARED SERVER has occurred the firewall is now blocking the redirected connections to the dispatcher processes.
The dispatchers will spawn on different ports unless a specific port is configured in the DISPATCHERS configuration.
It's important to note that the shared server client will need access to both the Listener port AND the dispatcher port to which the connection is redirected.


Firewall is present between the Oracle database and the clients. Port 1521 is open in the firewall.
Client connections time out or fail to complete giving errors like ORA-12535 or TNS-12203.


  1. Firewall service has been installed between database and clients.
  2. One of these:
  • Database has been switched from dedicated mode to shared mode or client connections were restricted to shared mode.
  • Database has been migrated to RAC setup.


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In this Document
 Oracle dedicated databases on Windows platforms
 Oracle shared-mode databases
 Oracle 10g direct handoff feature on Windows platforms
 Oracle RAC clusters

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