[ale] SNAT issues

[Chris Fowler]

I'm not even sure we'll ever use SNAT.

Here is how the system works:

Device running 2.4.24 is installed on network a.  It is a
192.168.5.0/24.
Another device is installed on network b.  It is also 192.168.5.0/24.

Both device's VPN into central server using a method that uses pppd. On
both devices masquerading is turned on where "eth0" is considered the
"public" interface.  

We want to access 192.168.5.1 on site A and 192.168.5.1 on site b.  I
can use host based routing and route back through the VPN to remote
device but I can not add two routes for the same address.   Now comes
DNAT.

On the device I use DNAT and assign the remote network a "virtual
network".  Maybe 10.0.1.0/24 for net A and 10.0.2.0/24 for net b.  Then
on the device I dnat 10.0.1.5 -> 192.168.5.1 and on net b I dnat
10.0.2.5 -> 192.168.5.1.  

This solves the problem of remote networks I have no control over having
identical subnets and identical addresses for what I need to access.

I can not think of a reason I would want to SNAT back from the remote
network to the central office.  I never ever go there.