IPFW and FTP client behind NAT

[wishmaster]

Hi, colleagues.

I have the main server/router and Samba server behind this one. This Samba server at every night sends some data via FTP to another server on the Internet.
The first remote server is under my power and use about the same configuration as main plus FTPD (port 2112) daemon.
The second remote server is not in my power and we use is as backup storage and as I know OS is f...ing Linux.

When I connect to the first server and transmit a very big file with transmission duration > 300 sec, the control channel (port pair 36313 <-> 2112) always "recreated" when the expiration timer aim to zero.

root at xxx: ipfw -d show|grep '111.222.230.62'
15150       69        5255 (29s) STATE tcp 111.222.230.62 36313 <-> 111.222.13.195 2112 :nts
15150   320423   321696704 (300s) STATE tcp 111.222.230.62 60759 <-> 111.222.13.195 49758 :nts

The issue is with the second remote server. When I transmit a very big file, the control channel does not "recreated" and transmitting this file and all the next is always fails.

root at xxx: ipfw -d show|grep '111.222.0.7'
03200  2985778  2299927348 (300s) STATE tcp 111.222.0.253 63307 <-> 111.222.0.7 44678 :nts
03200       59        4622 (6s) STATE tcp 111.222.0.253 63623 <-> 111.222.0.7 21 :nts

root at xxx: ipfw -d show|grep '111.222.0.7'
03200  3137837  2414765852 (300s) STATE tcp 111.222.0.253 63307 <-> 111.222.0.7 44678 :nts

The main server/router uses IPFW and in most places dynamic rules. Is workaround I have added one rule on external interface:

$cmd 5153 allow log tcp from any 21 to any 1024-65535 # ipfw - ftp issue

But I want find the problem.

Thanks,
Vitaly