I think this link could help you
Re: [FW-1] R: [FW-1] Routing.... SecurePlatform and NAT
In SecurePlatform, you need to do the following changes to your system. First, you need to add the following entries to /etc/sysctl.conf:
net.ipv4.conf.all.proxy_arp = 1
net.ipv4.conf.default.proxy_arp = 1
Second, to /etc/rc.local, you need to add a routing entry and a proxy ARP entry for each address you want. For example, if your external address for your mail server is 1.2.3.4 and your internal address is 192.168.3.4, then you need to add the following entries to /etc/rc.local:
route add -host 1.2.3.4 gw 192.168.3.4
arp -s 1.2.3.4 00:c0:aa:bb:cc:dd pub
Note, "00:c0:aa:bb:cc:dd" needs to be replaced with the MAC address of the external interface of your firewall
when u use the command "sysctl -w" then you don't have to reboot after setting the correct values
Regards
Eduard