some ftp behavior over firewall

[bpxs]

I encounter some ftp access problems recently. I've a server-U behind a linksys NAT router in my home providing file sharing with my friends. My router forword port 20 and 21 to the ftp server(this settings are from linksys support page).

I find those users who have routerable ip can use flashfxp access my ftp server by no pasv mode only, and some users behind a firewall can not access my ftp server at all. I tried to connect to connect to my ftp server from a pc using ms proxy, only dos prompt ftp can perform upload and download, the other ways always timeout when listing directory after successful login.

Anyone can explain the reasons for me and how to access ftp server with two firewalls at each side?

[MxxCon]

either side must fordward some port above 1024 range to their computer...
probably server side so there's less work for clients.
consider reading http://www.dslreports.com/forum/remark,713373~root=equip,16~mode=flat
http://www.dslreports.com/forum/remark%2C1381832%3Broot=equip%2C16%3Bmode=flat
http://www.dslreports.com/faq/linksys

[bpxs]

Thanks so much for your suggestion. I got the point!