I have an internal server, which uses a certain service. This service listens on a port, and speaks on a different port. The problem with the service is that it can't listen and speak on the same IP address, so I have configured 2 IP addresses for that NIC, and so I "solved" the problem with the listening and speaking.

I have a problem though...

I need that server to be NATed, with a public IP address, and that server needs to be available from the outside (and as only one IP)...

The question is, how do I solve the situation here?

If I do a NAT for one IP address (the listening port), then he will be able to get requests from the outside, but won't be able to send out traffic (because the other IP won't have NAT). If I do NAT on both of the IPs, then when traffic comes in for the listening port, it won't necessarily arrive to the listening IP, but rather to the speaking one.

I hope I made myself clear and that there is a sensible solution here that I am missing.