Privileged host, trigger script via netfilter?

[John Nilsson]

I wonder, what is the best way to have netfilter change some settings after 
a specifik packet is traversing the chains.

What I want is to allow my host to send a ping or similar to the firewall 
with the result of setting a special rule to route all unknown (i.e. Failed 
connection tracking) trafik to that host.

A special packet (ping?) comes from 192.168.0.3

triggers sh /etc/init.d/firewall 192.168.0.3
this script has theese lines (among others):
iptables -X
iptables -F
iptables -Z
[ ".$1" != "." ] && iptables -A FORWARD -d $1 -j ACCEPT
[ ".$1" != "." ] && iptables -A PREROUTING -d $IPFW -j DNAT --to $1

Question 1: Will the script do the intended?
Question 2: What is the best way to trigger this script form the INPUT 
chain?
Note that security is not a big issue. My main intrest is NAT.

/John Nilsson

P.S. Please CC me, of some reason I cannot read the digests D.S.

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