proxy questions ??

i have proxy 2.0 on MS back office server 4.5. THIS PROXY SERVER IS RUNNING BEHIND CISCO PIX FIREWALL.
I have configured 2 NIC IP of first NIC is 192.168.1.9 & 192.168.1.10 for for second NIC which is an External Network Interface.

now i have given IP of External interface i.e. 192.168.1.10 as gateway for my first NIC.
and i have given IP of my firewall (192.168.1.7) as a gateway to second NIC.

but when i click on webproxy > properties>security i still get error “packet filtering cannot be enabled on this server because there is no external external interface

whatz the soultion ??