Port triggering is a new feature supported by Vigor router that allows a host machine to dynamically and automatically forward a specific port back to itself.
Trigger Port is the outgoing (destination) port that the application uses. The Incoming Ports are the ports used by the application for it to work. The Incoming Ports are not opened until the corresponding Trigger Port is triggered (your computer is using a specified Trigger Port for specific traffic).
How it works?
We will take an example to explain it. There are many applications such as IP phones, games and IM applications that might need Port Triggering function to solve NAT problem. An example is connecting to an IRC server that needs authenticating a username with the identification protocol via port 113.
When you sign on with IRC, you typically connect to the IRC server at a port in the range 6660-6670. To verify your username, the Internet Relay Chat (IRC) server makes a new incoming connection to your computer on port 113. This doesn’t work if you are behind a NAT router. Because it doesn’t know what computer that data is for, so it drops this connection.
With Port Forwarding function, you can open port 113 for this computer. But this setup limits that only one computer behind the NAT router can use IRC application because Port Forwarding is a static forwarding that requires you to enter an internal IP address to forward the port 113 to. Furthermore, with Port Forwarding function the port 113 is open all the time.
Port Triggering can overcome the two disadvantages mentioned above. The below figures illustrate it.
Please note that port 113 can only be forwarded to one computer at a time. If A and B were trying to send data out at the same time, there would be a conflict. Port 113 would not be forwarded to both computers.
How to setup Port Triggering for IRC?
1. Take Vigor2920 as an example. Access into the web configurator of Vigor2920.
2. Go to the NAT>> Port Trigger page and add an entry as the following figure.
3. Click index number link for configuring the detailed settings.
| Item |
Description |
| Service |
There are User Defined and some predefined services available. The
configuration for predefined services is just for reference and can also
be modified.
For IRC with IDENT protocol, select User Defined |
| Comment |
Enter the name (e,g., IRC IDENT) of the application in the field
provided. |
| Trigger Protocol |
TCP |
| Trigger Port |
Enter the range (e.g., 6660 – 6670) for triggering the incoming ports. |
| Incoming Protocol |
TCP. |
| Incoming Port |
Enter the range (e.g., 113) of the ports which should be triggered by
trigger port. |
4. Click OK to save the settings.
Advantages (Compared with Port Forwarding)
Ports are not open to the Internet all the time. So it is a little more secure than port forwarding. Ports can be forwarded to different computers automatically.
Disadvantages
Port triggering only allows one client on the network to uses a particular incoming port at a time. If two or more clients run the same application at the same time, only one will work.
The router will close the incoming port after some period of time if there is no data pass through. This might break your application down.