[Serusers] Private IP address on public interface

Hello, I have set up SER with two ethernet interfaces, one with private IP address behind the firewall and one with public address. I want to separate internal traffic from external traffic, where NAT traversal issues are important. As long as I have both parties on the internal interface, everything works fine. When I establish a connection from the internal interface to the external one, SER sends the private IP address as source IP address on the public interface! Therefore the destination receives an unroutable source IP address. I do not use DNS, I have /etc/hosts. There I provided the mapping between public IP address and host name in the first line. The mapping between private IP address and the same host name is in the line below. Does anybody have a hint to solve this issue? Thank you for your support. Best regards, Peter