Try this...
Squid versions 2.6 or newer
With Squid installed on your Unix/Linux box, set the following:
http_port 192.168.1.10:3128 transparent (you can try http_port 0.0.0.0:3128 transparent)
substituting the IP address you're listening on, and the port you wish to use in the example, making sure they match the variables at the top of the router setup script below.
Important for Debian users!
The Squid3 (squid3_3.0.PRE5-5) package from Debian Etch isn't working with this kind of transparent proxy. Try using Squid3 from Debian Lenny or downgrade to Squid-2.6 in Etch.
Source: http://www.dd-wrt.com/wiki/index.php/Squid_Transparent_Proxy
**I know it`s an one+ year old post