MASQ without forwarding?

[Mark Carey]

Hi,
I am running MASQUERADING for my local class C subnet using a Linux box, 
with the following command;
$IPT -t nat -A POSTROUTING -o ppp+ --source $MY_NET -destination $ANYWHERE 
-j MASQUERADE

However I want to run MASQUERADING without having to have ip_forwarding 
enabled.  Is there a simple way to do this?
I have tried changing the above rule to;
$IPT -t nat -A PREROUTING -i eth0 --source $MY_NET --destination ! 
$FW_BOX,$MY_NET -j MASQUERADE
However this does not work.

I also note that the first rule above, requires the machines being MASQ'ED 
to have the ability to send a recieve traffic through the FORWARD table.  
Why is this so?  Does IPTables start the new connection via the MASQUERADE 
rule with all subsequent data travelling through the FORWARD rule?

If this is all covered in a tutorial somewhere please point me in that 
direction, I have been unsucessful in locating it thus far.

Thank You,

Mark Carey

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