Chapter 17 Network Address Translation (NAT)

Table 119 ADVANCED > NAT > Port Forwarding

LABEL

DESCRIPTION

Apply

Click Apply to save your changes back to the ZyWALL.

 

 

Reset

Click Reset to begin configuring this screen afresh.

 

 

17.7 Port Triggering

Some services use a dedicated range of ports on the client side and a dedicated range of ports on the server side. With regular port forwarding you set a forwarding port in NAT to forward a service (coming in from the server on the WAN) to the IP address of a computer on the client side (LAN). The problem is that port forwarding only forwards a service to a single LAN IP address. In order to use the same service on a different LAN computer, you have to manually replace the LAN computer's IP address in the forwarding port with another LAN computer's IP address.

Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically take turns using the service. The ZyWALL records the IP address of a LAN computer that sends traffic to the WAN to request a service with a specific port number and protocol (a "trigger" port). When the ZyWALL's WAN port receives a response with a specific port number and protocol ("incoming" port), the ZyWALL forwards the traffic to the LAN IP address of the computer that sent the request. After that computer’s connection for that service closes, another computer on the LAN can use the service in the same manner. This way you do not need to configure a new IP address each time you want a different LAN computer to use the application.

For example:

Figure 250 Trigger Port Forwarding Process: Example

1Jane (A) requests a file from the Real Audio server (port 7070).

2Port 7070 is a “trigger” port and causes the ZyWALL to record Jane’s computer IP address. The ZyWALL associates Jane's computer IP address with the "incoming" port range of 6970-7170.

3The Real Audio server responds using a port number ranging between 6970-7170.

4The ZyWALL forwards the traffic to Jane’s computer IP address.

5Only Jane can connect to the Real Audio server until the connection is closed or times out. The ZyWALL times out in three minutes with UDP (User Datagram Protocol) or two hours with TCP/IP (Transfer Control Protocol/Internet Protocol).

 

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