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Configuring Port Triggering

Port triggering is a dynamic extension of port forwarding that is useful in these cases:

When port triggering is enabled, the router monitors outbound traffic looking for a specified outbound "trigger" port. When the router detects outbound traffic on that port, it remembers the IP address of the local computer that sent the data. The router then temporarily opens the specified incoming port or ports, and forwards incoming traffic on the triggered ports to the triggering computer.

While port forwarding creates a static mapping of a port number or range to a single local computer, port triggering can dynamically open ports to any computer that needs them and can close the ports when they are no longer needed.

Note: If you use applications such as multiplayer gaming, peer-to-peer connections, real- time communications such as instant messaging, or remote assistance (a feature in Windows XP), you should also enable Universal Plug and Play (UPnP) according to the instructions in Using Universal Plug and Play.

To configure port triggering, you need to know which inbound ports the application needs. Also, you need to know the number of the outbound port that will trigger the opening of the inbound ports. You can usually determine this information by contacting the publisher of the application or user groups or newsgroups.

To set up port triggering:

  1. Select Port Forwarding/Port Triggering under Advanced in the main menu. The Forwarding/Port Triggering screen displays.
  2. Select the Port Triggering radio button. The port triggering information displays.

    Figure 5-3

  3. Clear the Disable Port Triggering check box.

    Note: If the Disable Port Triggering check box is selected after you configure port triggering, port triggering is disabled. However, any port triggering configuration information you added to the router is retained even though it is not used.
  4. In the Port Triggering Timeout field, enter a value up to 9999 minutes. This value controls the inactivity timer for the designated inbound ports. The inbound ports close when the inactivity time expires. This is required because the router cannot be sure when the application has terminated.
  5. Click Add Service.

    Figure 5-4

  6. In the Service Name field, type a descriptive service name.
  7. In the Service User field, select Any (the default) to allow this service to be used by any computer on the Internet. Otherwise, select Single address, and enter the IP address of one computer to restrict the service to a particular computer.
  8. Select the service type, either TCP or UDP or both (TCP/UDP). If you are not sure, select TCP/UDP.
  9. In the Triggering Port field, enter the number of the outbound traffic port that will cause the inbound ports to be opened.
  10. Enter the inbound connection port information in the Connection Type, Starting Port, and Ending Port fields.
  11. Click Apply. The service appears in the Port Triggering Portmap table.

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