Multiple NIC's > FTP Servers

[JamesM]

Hi, bit of a newbie to network configuration, so if anything outside of the core issue in this post seems fundamentally wrong, please let me know, as it's probably not meant to be!

I have a VM implementation with the following interfaces:

LAN: 192.168.0.1/24
WAN: xxx.xxx.xxx.130
OPT1: xxx.xxx.xxx.140
OPT2: xxx.xxx.xxx.141
OPT3: xxx.xxx.xxx.142
OPT4: xxx.xxx.xxx.143

OPT1-4 provide unique IP's for 4 webservers which sit on the LAN.
I've been able to happily NAT all the usual services (HTTP(S),RDP, SMTP) to the servers, however I also need to set up FTP Servers on each.

I've successfully done this using the WAN IP and the FTP helper proxy for one server, but as soon as I try this on the OPT's things go wrong, as to my understanding the proxy helper only works with the WAN interface (I assume this is because the client is addressing the OPT IP, but the response then comes from a different (WAN) IP and things get confused).

The other option seems to be doing outbound NAT, but this seems like a bit of a clumsy way to do it in comparison.

Anyone have a suggestion as to how else this could be achieved?

Appreciate the help.

JM