Forcing source port with NAT

[Jason White <jdw-netfilter@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 (jdw-netfilter@jdwhite.org)  http://www.jdwhite.org/~jdwhite
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