UDP DNAT Problem

["Craig Packard" <cpackard@zoominternet.net>]

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From what I understand if I setup a DNAT rule, when a packet matching the rule comes in it is sent to the specified host, obviously this is done by changing the destination field to be the system 'behind' the firewall.  It was however my understanding--and what I've seen in practice--that the source field would not be changed.  So in other words the source of the packet would still be the host out on the internet that actually sent the original packet.  However I have made a set of DNAT rules that I couldn't get to work. So a setup a packet sniffer at several points.  Here is what I noticed and has me confused.

The inside computer (A) sends out a UDP packet to the internet connected computer (B), of course this packet goes through firewall (FW).

The packet goes out as expect srcA dstB
But the weird part is the response comes back in srcFW dst(A)   Where I would like it would be src(B) dst(A)!!!

I think this is screwing up the communications.  Can anyone help me understand what is happening?

-    Craig

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