I'm on to this setup this very moment. I'm still having some problems with the passive port range on LAN and WAN. I already have a server working with passive mode with pure-ftpd, but it's hard to figure out what the problem is when I've already copied all the options from this server.
You know that the easiest way to configure your pure-ftpd is to use the conf/ folder and save files here?
try these options:
-p 41000:41100 -- > specifies the portrange. Save the file as /etc/pure-ftpd/conf/PassivePortRange
It is possible you'll need to specify you LAN IP for your router if u use one. I've set mine to:
u simply save the value "192.168.1.2" in the file /etc/pure-ftpd/conf/ForcePassiveIP
Remember the "-p" and "-P" is NOT to be included in the configfiles (if u use them, u need them in your startupscript ofcourse)
Add-on: I do remember when troubling with these settings, that resetting my Ubuntu server worked out. Not sure why, but I remember it did.
/etc/init.d/pure-ftpd restart didn't update my settings for some reason. Worth a try