It sounds like the device (unwisely) puts its IP address in the data stream. That's the only reason I can think of why it might need to be mangled. A la FTP, SIP, etc. Usually such protocols require application-layer gateways to fix up their traffic.

At least, that's my inference from their request.

On 2011-05-06 5:11 PM, Scott Reed wrote:
That is how portmap works. You map a port on device A to point to device B. All communication to the outside appears to come from the device doing the map.

Can you create a VPN between the controller side and the outside service so it looks like it is on the same network?


On 5/6/2011 4:41 PM, Bill Prince wrote:

We have a client that has a new HVAC system (Delta Controls). It uses a controller that can only talk L2. The HVAC guys for the client asked me to set up a portmap for port 47808.

I did this, but it appears that the MT portmap substitutes the original (public) source address with the router's internal gateway address.

So the device replies with it's own private address, which gets sent back to their monitoring software, and when they reply to the private IP, it gets lost.

So they are asking me to mangle the portmapped packets to stick in the original public IP, to fool their controller.

I have no clue how to do this.



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