vsftpd behind NAT - PASV error

Hi, behind a router a vsftpd servers only runs
if I try using PORT mode, wich is not a good way to
use vsftpd. I have found that the only way to make it
work is putting the pasv_address option with the
public IP that it has in that moment the ADSL connection

vsftpd demands me to put an IP number (I think a variable
will be good too) for make a valid IP entry on the server

Q: How to put a variable or a value that permit me
use PASV mode trough a router ?

settings:
pasv_promiscuous=YES
pasv_promiscuous=YES
pasv_min_port=60010
pasv_max_port=60020
pasv_address=200.40.x.x < the moments IP

Only using pasv_address with the updated Dyn IP
i can make it work.

the error happens when it says:
Connecting ftp data socket 192.168.1.250:60020...
and only works that way if I connect beeing on the same LAN

thanks in advanced for your help

best bet is to try active or open up a port range that pasv trys to open when trying to get the directory listing