Forcing source port with NAT

[Jason White <jdwhite@jdwhite.org>]

Greetings,
  I have an application, Zephyr Messaging Service, which uses a client
application that sends out UDP packets on port 2104.  The Zephyr server
will refuse packets from the client if the source address is not 2104.
I want to run this application behind a NAT on one given machine.  I
need a way to ensure that packets from this one machine with a source
address of 2104 go through the NAT that they emerge on port 2104 of the
external ip address.  To diagram a packet:

[Intenal machine-10.0.0.2:2104] --> [NAT internal: 10.0.0.1] -->
  [NAT external: 200.200.200.200: 2104] --> ///internet/// ...

Basically, I need to ensure that anything coming in on 200.200.200.200:2104
goes to 10.0.0.2:2104 and anything going out from 10.0.0.2:2104 goes out
200.200.200.200:2104.

I know how to map the external to internal, but internal to external
isn't immediately obvious

Thanks,
-Jason

-- 
Jason White (jdwhite@jdwhite.org)  http://www.jdwhite.org/~jdwhite
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