squid as a semi transparent

Hi
i have set up my squid 2.5 stable as a transparnet Proxy

with iptables redirection to port 3128 and squid.conf

httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on


My questions is how do i make the squid use it's own ip address
when it goes out to the web server .

Tia
Tal