restricting ip address with iptables

[wuboy]

hi all,

i have a configuration where i have a box doing NAT and firewalling two DNS boxes. now i want to restrict ip addresses to access one or the other, depending on which ip address the request is made from, it will resolve queries from either one DNS or the other.

i have something in the form of:

$IPTABLES -t nat -A PREROUTING -p TCP -i $EXTERNAL1 -d $EXT_IP1 --dport 53 -j DNAT --to-destination 192.168.1.2
for universal IPs

and

$IPTABLES -t nat -A PREROUTING -p TCP -s 1.2.3.4/32 -i $EXTERNAL1 -d $EXT_IP1 --dport 53 -j DNAT --to-destination 192.168.1.3
for the 1.2.3.4 IP address

of course i have for TCP and UDP as well.

however, this doesnt seem to work, so when i try to test with two different computers, i get directed to the same DNS. am i missing something?

thanks in advance