answer on wrong network interface

Dear all, on my server, i have two network interfaces. The first setup with IP 192.168.3.4 (gateway 192.168.3.1) for internal filetransfer etc. pluged in a firewall dmz#1. The second interface with IP 192.168.5.6 (gateway 192.168.5.1) only for FTP-transfer pluged in a firewall dmz#2. The Firewall routes the static external-IP to the dmz#2, so that our clients are able to connect to the FTP-program. The PROBLEM: If external or internal clients try to connect to the FTP-Programm, the contact (or ping) goes through the dmz#2 to IP 192.168.5.6 and replays the answer on the default gateway 192.168.3.1 to the dmz#1. Now the firewallrouter droped the connection. Can anybody can explain, how I am able to force the server to answer on the same and right gateway, he received data from, and not use always the default (wrong) gateway? Why am I not able to use the two interfaces separat and by its own. To have a cross answer on the wrong interfaces is realy not what i want. Thanks in advance! Spousi :cry: