Change outbound ICMP source

[Nathaniel Hall <nathaniel.d.hall@xxxxxxxxx>]

I have been trying to figure out how to change the source IP address of 
an ICMP packet that originates from the firewall.  Here is my application.

Instead of dropping a packet I reject it with ICMP host unreachable 
messages.  I would like to make it appear that the firewall isn't there, 
so I would like to change the source IP address to be that of our 
upstream router.  How would I go about doing this?

--
Nathaniel Hall, GSEC GCFW GCIA