How do I map a external port to a different internal port?

Hi there!
I need to map a external port to an different internal port number.
This is the situation: My ISP does not allow incoming connection on Port 21. I have got a new NAS which also has a FTP Server on Port 21 and you cant change the Port number. So if I input ftp://www.myftp.dyndns.org:1234 the following rule should exist:
incoming data @ Port 1234 -> 192.168.1.103 @ Port 21

In the Webinterface a can only apply the rule if both ports are the same. In my internal network I am able to connect to the ftp server (21) but from outside it is not working because of my ISP

Thx in advance!

 

What router do you have?

 

Sorry, of course this information would be helpful ;-)
If have got the WRT54G

 

And you have tried port triggering, can you not enter start and end range as the same the forward range as 21? then try adding a port forward entry perhaps?

 

try port forward range instead of port forward

 

These suggestions did not work :-(

 

What firmware are you running? Also, what revision is your router? I suggest running Thibors HyperWRT v15c or DD-WRT v23 SP1. Both will allow you to forward and map ports.

www.linksysinfo.co.uk/thibor/

www.dd-wrt.com (the generic micro version can now be flashed to the v5)