While looking for the ftp server (sitting inside NAT network) setup, I have reached the following URL.
When I had following line suggested in above URL,

# Set the range of port number which will be used for the passive data port
portrange all 50000 50010

I got following line in /var/log/xferlog,

May 21 10:50:33 kurobsd ftpd[18536]: /etc/ftpd.conf line 5: maxport `50010 ': illegal number

So now it looks like:

# Set the range of port number which will be used for the passive data port
portrange all 50000 50009

And I have setup following URL. It seems to be working well. It is the same thing as but in case HTTP is not available and FTP only environment, it may help. (who believes no HTTP but FTP only ? ) Be carefull for the security, I have not checked anything about that.