FTP Backup Disturbing NAT Server Connections

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http://askubuntu.com – It's a very strange problem. I have a Ubuntu 12.04 server A with KVM (Kernel Virtual Machine), proftpd and MySQL installed and UFW configured. An guest Ubuntu 12.04 B with OpenVPN and phpMyAdmin installed and another guest Ubunutu 12.04 C with Tomcat7 installed. I'm using my laptop D which is connected to B with OpenVPN and D is connecting to A through B (iptables masquerade). And here is the problem: I've written a Java program using commons-net-3.1-ftp to backup the FTP storage from A. And when this backup program is running on D, B cannot connect to any of the services running on A (ftp, M (HowTos)